Advanced Study Material for Class 10 Science: Chapter 05 Periodic Classification of Elements
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CBSE Class 10 Science Chapter 5 Periodic classification of elements MCQs
ASSIGNMENT QUESTIONS SET - 2
CHAPTER - 5
PERIODIC CLASSIFICATION OF ELEMENTS
1. Upto which element, the Law of Octaves was found to be applicable
(a) Oxygen
(b) Calcium
(c) Cobalt
(d) Potassium
Answer: (b) Calcium
1. Newlands arranged elements in increasing atomic masses.
2. The law of octaves worked only up to calcium, and the properties did not repeat in the heavier elements that followed.
3. Beyond calcium, the properties of the elements did not repeat every eighth element.
4. Hence, the correct option is (b).
Teacher's Note:
a) Newlands' law worked only for the first few elements.
b) Elements after calcium broke the pattern, so the law failed for heavier elements.
2. According to Mendeleev's Periodic Law, the elements were arranged in the periodic table in the order of
(a) increasing atomic number
(b) decreasing atomic number
(c) increasing atomic masses
(d) decreasing atomic masses
Answer: (c) increasing atomic masses
1. Mendeleev's Periodic Law stated that properties of elements are a periodic function of their atomic masses.
2. So, the elements were arranged in increasing order of atomic masses.
3. Hence, the correct option is (c).
Teacher's Note:
a) Atomic number was not known in Mendeleev's time.
b) Atomic number became the basis only in the modern periodic table.
3. In Mendeleev 's Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the perioidc table later
(a) Germanium
(b) Chlorine
(c) Oxygen
(d) Silicon
Answer: (a) Germanium
1. Mendeleev left gaps for elements he predicted, which he named eka-aluminium, eka-boron and eka-silicon.
2. Germanium was discovered later and fitted into the gap for eka-silicon.
3. Hence, the correct option is (a).
Teacher's Note:
a) Germanium was predicted as eka-silicon.
b) Chlorine, oxygen and silicon were already known before Mendeleev's table.
4. Which of the following statement (s) about the Modern Periodic Table are incorrect
(i) The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number
(ii) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses
(iii) Isotopes are placed in adjoining group (s) in the Periodic Table
(iv) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number
(a) (i) only (b) (i), (ii) and (iii)
(c) (i), (ii) and (iv) (d) (iv) only
Answer: (b) (i), (ii) and (iii)
1. Statement (i) is incorrect, because the modern table is arranged in increasing atomic number, not decreasing.
2. Statement (ii) is incorrect, because atomic mass is not the basis of the modern table.
3. Statement (iii) is incorrect, because isotopes have the same atomic number and are placed in the same position, not in adjoining groups.
4. Statement (iv) is correct.
5. Hence, the incorrect statements are (i), (ii) and (iii), and the correct option is (b).
Teacher's Note:
a) Atomic number is the basis of the modern periodic table.
b) Isotopes of an element occupy one single place in the table because their atomic numbers are the same.
c) Watch for the word "incorrect" in such questions, as it reverses the choice.
5. Which of the following statements about the Modern Periodic Table is correct:
(a) It has 18 horizontal rows known as Periods
(b) It has 7 vertical columns known as Periods
(c) It has 18 vertical columns known as Groups
(d) It has 7 horizontal rows known as Groups
Answer: (c) It has 18 vertical columns known as Groups
1. The vertical columns of the modern periodic table are called groups.
2. There are 18 groups in the modern periodic table.
3. Hence, the correct option is (c).
Teacher's Note:
a) Groups are vertical, and periods are horizontal rows.
b) There are 7 periods, but 18 groups.
6. Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E
Answer: (b) B, C, D
1. Atomic number 2 is helium (A), which is in period 1.
2. Atomic number 3 is lithium (B), atomic number 7 is nitrogen (C) and atomic number 10 is neon (D). All three lie in period 2.
3. Atomic number 30 is zinc (E), which lies in period 4.
4. Hence, the correct option is (b).
Teacher's Note:
a) The period is decided by the number of shells, which is the period number.
b) Lithium, nitrogen and neon each have electrons in two shells.
7. The elements A, B, C, D and E have atomic number 9, 11, 17, 12 and 13 respectively. Which pair of elements belong to the same group?
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Answer: (c) A and C
1. Atomic number 9 is fluorine (A) with configuration 2,7, so it is in group 17.
2. Atomic number 17 is chlorine (C) with configuration 2,8,7, so it is in group 17.
3. Sodium (B) is in group 1, magnesium (D) in group 2 and aluminium (E) in group 13.
4. Hence, A and C belong to the same group, and the correct option is (c).
Teacher's Note:
a) Elements in the same group have the same number of valence electrons.
b) Fluorine and chlorine are both halogens.
8. Where would you locate the element with electronic configuration 2,8 in the Modern Periodic Table?
(a) Group 8
(b) Group 2
(c) Group 18
(d) Group 10
Answer: (c) Group 18
1. The electronic configuration 2,8 has 10 electrons, so the element is neon (atomic number 10).
2. Neon has 8 electrons in its outermost shell, which is a complete octet.
3. Such elements are noble gases and lie in group 18 (period 2).
4. Hence, the correct option is (c).
Teacher's Note:
a) Add the electrons in all shells to get the atomic number.
b) A complete outer shell places an element in group 18.
9. An element which is an essential constituent of all organic compounds belongs to
(a) group 1
(b) group 14
(c) group 15
(d) group 16
Answer: (b) group 14
1. Carbon is the element present in all organic compounds.
2. Carbon has 4 valence electrons, so it lies in group 14.
3. Hence, the correct option is (b).
Teacher's Note:
a) Carbon forms four covalent bonds in most organic compounds.
b) Group 14 is the carbon family.
10. Which of the following is the outermost shell for elements of period 2?
(a) K shell
(b) L shell
(c) M shell
(d) N shell
Answer: (b) L shell
1. Elements of period 2 have electrons in two shells, K and L.
2. The L shell is the second shell and is the outermost shell for these elements.
3. Hence, the correct option is (b).
Teacher's Note:
a) The period number equals the number of shells.
b) The K shell is the innermost shell.
11. Which one of the following elements exhibit maximum number of valence electrons?
(a) Na
(b) Al
(c) Si
(d) P
Answer: (d) P
1. Sodium has 1 valence electron (2,8,1).
2. Aluminium has 3 valence electrons (2,8,3).
3. Silicon has 4 valence electrons (2,8,4).
4. Phosphorus has 5 valence electrons (2,8,5), which is the maximum.
5. Hence, the correct option is (d).
Teacher's Note:
a) Valence electrons increase by one from left to right across a period.
b) Phosphorus is in group 15.
12. Which of the following gives the correct increasing order of the atomic radii of O, F and N ?
(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N
Answer: (d) F, O, N
1. O, F and N belong to period 2, in the order N, O, F from left to right.
2. Atomic radius decreases from left to right across a period.
3. So, the atomic radii are N greater than O greater than F.
4. Increasing order: F, O, N. Hence, the correct option is (d).
Teacher's Note:
a) Atomic radius falls as nuclear charge rises across a period.
b) Fluorine is the smallest of the three.
13. Which among the following elements has the largest atomic radii?
(a) Na
(b) Mg
(c) K
(d) Ca
Answer: (c) K
1. Potassium is in period 4, while Na and Mg are in period 3 and Ca is in period 4 in group 2.
2. Potassium is in group 1, the leftmost group, so its atomic radius is the largest among these four.
3. Hence, the correct option is (c).
Teacher's Note:
a) Atomic radius increases down a group.
b) Atomic radius decreases across a period, so group 1 elements are the largest in their period.
14. Which of the following elements would lose an electron easily?
(a) Mg
(b) Na
(c) K
(d) Ca
Answer: (c) K
1. Metals lose electrons more easily when they have a larger atomic size and lower ionisation energy.
2. Potassium has the largest atomic size among these elements and is in group 1 of period 4.
3. So, K loses its electron most easily, and the correct option is (c).
Teacher's Note:
a) Electropositive character increases down group 1.
b) The outer electron of potassium is far from the nucleus, so it is held weakly.
15. Which of the following elements does not lose an electron easily?
(a) Na
(b) F
(c) Mg
(d) Al
Answer: (b) F
1. Sodium, magnesium and aluminium are metals, and they lose electrons easily.
2. Fluorine is a non-metal with 7 valence electrons, and it tends to gain one electron rather than lose one.
3. Hence, the correct option is (b).
Teacher's Note:
a) Non-metals have high ionisation energy.
b) Fluorine is the most electronegative element.
16. Which of the following are the characteristics of isotopes of an element?
(i) Isotopes of an element have same atomic masses
(ii) Isotopes of an element have same atomic number
(iii) Isotopes of an element show same physical properties
(iv) Isotopes of an element show same chemical properties
(a) (i), (iii) and (iv)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer: (d) (ii) and (iv)
1. Isotopes have the same atomic number, so statement (ii) is correct.
2. Isotopes have different mass numbers, so statement (i) is wrong.
3. Isotopes have the same chemical properties, so statement (iv) is correct.
4. Isotopes have different physical properties, such as density, so statement (iii) is wrong.
5. Hence, the correct option is (d).
Teacher's Note:
a) Chemical properties depend on the electron arrangement, which is the same in isotopes.
b) Physical properties depend on mass, which differs between isotopes.
17. Arrange the following elements in the order of their decreasing metallic character
Na, Si, Cl, Mg, Al
(a) Cl > Si >Al > Mg >Na
(b) Na >Mg >Al >Si > Cl
(c) Na > Al > Mg > Cl > Si
(d) Al > Na> Si > Ca> Mg
Answer: (b) Na >Mg >Al >Si > Cl
1. Metallic character decreases from left to right across a period.
2. Sodium and magnesium are metals in period 3, and sodium is further left, so Na is more metallic than Mg.
3. Al, Si and Cl follow in decreasing metallic character, with Cl the least metallic.
4. Hence, the correct option is (b).
Teacher's Note:
a) Metallic character falls across period 3 from sodium to chlorine.
b) Silicon is a metalloid, so it is less metallic than aluminium.
18. Arrange the following elements in the order of their increasing nonmetallic character
Li, O, C, Be, F
(a) F < O < C < Be < Li
(b) Li < Be < C < O< F
(c) F < O < C < Be < Li
(d) F < O < Be < C < Li
Answer: (b) Li < Be < C < O< F
1. Lithium and beryllium are metals, so they have the least nonmetallic character.
2. Carbon is a non-metal and oxygen and fluorine are more strongly non-metallic.
3. Non-metallic character increases from left to right across period 2.
4. Increasing order: Li < Be < C < O < F. Hence, the correct option is (b).
Teacher's Note:
a) Non-metallic character rises across a period.
b) Fluorine is the most non-metallic element in this list.
19. What type of oxide would Eka- aluminium form?
(a) EO3
(b) E3O2
(c) E2O3
(d) EO
Answer: (c) E2O3
1. Eka-aluminium is the element predicted below aluminium, so it belongs to group 13.
2. Elements of group 13 have a valency of 3.
3. Oxygen has a valency of 2, so the formula is E2O3.
4. Hence, the correct option is (c).
Teacher's Note:
a) Aluminium forms Al2O3, and eka-aluminium follows the same pattern.
b) Eka-aluminium was later found to be gallium.
20. Three elements B, Si and Ge are
(a) metals
(b) non-metals
(c) metalloids
(d) metal, non-metal and metalloid respectively
Answer: (c) metalloids
1. Boron, silicon and germanium lie along the zig-zag line that separates metals from non-metals.
2. Elements on this line show properties of both metals and non-metals.
3. Hence, all three are metalloids, and the correct option is (c).
Teacher's Note:
a) Metalloids are also called semi-metals.
b) Silicon and germanium are used in semiconductors.
21. Which of the following elements will form an acidic oxide?
(a) An element with atomic number 7
(b) An element with atomic number 3
(c) An element with atomic number 12
(d) An element with atomic number 19
Answer: (a) An element with atomic number 7
1. Atomic number 7 is nitrogen, a non-metal, which forms the acidic oxide N2O5.
2. Atomic numbers 3, 12 and 19 are lithium, magnesium and potassium, which are metals and form basic oxides.
3. Hence, the correct option is (a).
Teacher's Note:
a) Non-metal oxides are generally acidic in nature.
b) Metal oxides are generally basic in nature.
22. The element with atomic number 14 is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?
(a) Metal
(b) Metalloid
(c) Non-metal
(d) Left-hand side element
Answer: (b) Metalloid
1. Atomic number 14 is silicon.
2. Silicon is hard and forms an acidic oxide, SiO2, like a non-metal.
3. It also forms a covalent halide, SiCl4, and it has the properties of both metals and non-metals, so it is a metalloid.
4. Hence, the correct option is (b).
Teacher's Note:
a) Silicon is a semiconductor, which is typical of metalloids.
b) The hardness of silicon is a feature of its giant covalent structure.
23. Which one of the following does not increase while moving down the group of the periodic table?
(a) Atomic radius
(b) Metallic character
(c) Valence
(d) Number of shells in an element
Answer: (c) Valence
1. Atomic radius increases down a group because more shells are added.
2. Metallic character increases down a group because the outer electrons are easier to remove.
3. The number of shells increases down a group.
4. Valence, which is the number of valence electrons, remains the same within a group, so it does not increase.
5. Hence, the correct option is (c).
Teacher's Note:
a) Elements of a group have the same valence electrons.
b) Atomic radius and metallic character both rise going down a group.
24. On moving from left to right in a period in the periodic table, the size of the atom.
(a) increases
(b) decreases
(c) does not change appreciably
(d) first decreases and then increases
Answer: (b) decreases
1. Moving across a period, the number of shells stays the same.
2. The nuclear charge increases, so electrons are pulled closer to the nucleus.
3. Hence, the size of the atom decreases, and the correct option is (b).
Teacher's Note:
a) Atomic size is the distance from the nucleus to the outermost shell.
b) Greater nuclear pull shrinks the atom across a period.
25. Which of the following set of elements is written in order of their increasing metallic character?
(a) Be Mg Ca
(b) Na Li K
(c) Mg Al Si
(d) C O N
Answer: (a) Be Mg Ca
1. Metallic character increases down a group, so in group 2 it rises from Be to Mg to Ca.
2. Option (b) is written as Na, Li, K, which is not in the order of increasing metallic character.
3. Option (c) decreases across period 3, and option (d) contains non-metals.
4. Hence, the correct option is (a).
Teacher's Note:
a) Metallic character increases down a group.
b) Metallic character decreases from left to right across a period.
26. The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.
Answer:
1. Such an arrangement is called a Dobereiner's triad.
2. Example: lithium (7), sodium (23) and potassium (39).
3. The average of the atomic masses of lithium and potassium is (7 + 39) / 2 = 23, which is the atomic mass of sodium.
Teacher's Note:
a) In a triad, the middle element's atomic mass is roughly the mean of the other two.
b) Calcium, strontium and barium form another well-known triad.
27. Elements have been arranged in the following sequence on the basis of their increasing atomic masses.
F, Na, Mg, Al, Si, P, S, Cl, Ar, K
(a) Pick two sets of elements which have similar properties.
(b) The given sequence represents which law of classification of elements?
Answer:
(a) 1. F and Cl are halogens with similar properties.
2. Na and K are alkali metals with similar properties.
(b) 1. The sequence follows increasing atomic masses, and every eighth element shows similar properties.
2. Hence, the sequence represents Newlands' Law of Octaves.
Teacher's Note:
a) Halogens are in group 17, and alkali metals are in group 1.
b) Newlands' law was based on increasing atomic masses, and it failed for heavier elements.
c) Argon is a noble gas, so it does not match F or Cl.
28. Can the following groups of elements be classified as Dobereiner's triad ?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40
Explain by giving reason.
Answer:
(a) 1. The mean of the atomic masses of Na and Cl is (23 + 35) / 2 = 29, which is not the atomic mass of Si (28).
2. Na, Si and Cl do not have similar properties, so they do not form a Dobereiner's triad.
(b) 1. Be, Mg and Ca are all group 2 metals with similar properties.
2. The mean of the atomic masses of Be and Ca is (9 + 40) / 2 = 24.5, which is approximately equal to the atomic mass of Mg (24).
3. So, Be, Mg and Ca can be classified as a Dobereiner's triad.
Teacher's Note:
a) A triad needs both similar properties and a middle atomic mass close to the mean.
b) Silicon and chlorine differ widely in properties, which breaks the triad.
c) The mean rule is approximate, so 24.5 is accepted as roughly 24.
29. In Mendeleev 's Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reason for the same.
Answer:
1. Mendeleev gave priority to the chemical properties of elements over their atomic masses.
2. Cobalt has properties similar to the elements of group 9 (rhodium, iridium), while nickel has properties similar to the elements of group 10 (palladium, platinum).
3. So, cobalt was placed before nickel to keep each element in the group having similar properties.
Teacher's Note:
a) Mendeleev's rule of atomic mass had this exception.
b) The correct ordering was later explained by atomic number, which is an integer.
30. "Hydrogen occupies a unique position in Modern Periodic Table". Justify the statement.
Answer:
1. Hydrogen has one valence electron, like the alkali metals of group 1, and forms H+ ions.
2. Hydrogen needs one electron to complete its duplet, like the halogens of group 17, and forms H- ions and covalent compounds.
3. Hydrogen exists as a diatomic gas, H2, which is a non-metal, unlike the metals of group 1.
4. Since it resembles two different groups, hydrogen cannot be placed in group 1 or group 17 alone, so it has a unique position.
Teacher's Note:
a) Hydrogen shows the properties of both group 1 and group 17.
b) It is therefore placed at the top of group 1 or group 17 by some tables.
31. Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev.
Answer:
1. Eka-silicon is in group 14, so its valency is 4, and its chloride is ECl4.
2. Eka-aluminium is in group 13, so its valency is 3, and its chloride is ECl3.
Teacher's Note:
a) Eka-silicon was later discovered as germanium, with the formula GeCl4.
b) Eka-aluminium was later discovered as gallium, with the formula GaCl3.
32. Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.
Answer:
1. Element A has 3 valence electrons, so it belongs to group 13 and its valency is 3.
2. Element B has 4 valence electrons, so it belongs to group 14 and its valency is 4.
3. Element C has 2 valence electrons, so it belongs to group 2 and its valency is 2.
Teacher's Note:
a) An element with 1 to 3 valence electrons usually has a valency equal to that number.
b) Valency is the combining capacity of an element.
33. If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?
Answer:
1. An element in group 14 has valency 4, and chlorine has valency 1.
2. So, the formula of its chloride is XCl4.
3. The bond is covalent, because the element shares electrons with chlorine.
Teacher's Note:
a) Carbon forms the covalent compound CCl4, which is a good example.
b) Covalent compounds have low melting points compared with ionic compounds.
34. Compare the radii of two species X and Y. Give reasons for your answer.
(a) X has 12 protons and 12 electrons
(b) Y has 12 protons and 10 electrons
Answer:
1. X is a neutral magnesium atom with 12 electrons, and Y is a magnesium ion, Mg2+, with 10 electrons.
2. The radius of X is greater than the radius of Y.
3. Y has lost two electrons, so it has fewer electrons while the nuclear charge is the same, so the nuclear pull on the remaining electrons increases and the ion shrinks.
Teacher's Note:
a) A cation is always smaller than its parent atom.
b) Mg2+ has only two shells, while the magnesium atom has three shells.
35. Arrange the following elements in increasing order of their atomic radii.
(a) Li, Be, F, N
(b) Cl, At, Br I
Answer:
(a) 1. Li, Be, N and F are all in period 2, where atomic radius decreases from left to right.
2. Increasing order of atomic radius: F < N < Be < Li.
(b) 1. Cl, Br, I and At are all in group 17, where atomic radius increases down the group.
2. Increasing order of atomic radius: Cl < Br < I < At.
Teacher's Note:
a) Across period 2, the atomic radius decreases from lithium to fluorine.
b) Down group 17, each new shell makes the atom bigger.
c) Lithium is the largest element in the first set.
36. Identify and name the metals out of the following elements whose electronic configurations are given below.
(a) 2, 8, 2
(b) 2, 8, 1
(c) 2, 8, 7
(d) 2, 1
Answer:
1. (a) 2, 8, 2 is magnesium, a metal.
2. (b) 2, 8, 1 is sodium, a metal.
3. (c) 2, 8, 7 is chlorine, a non-metal.
4. (d) 2, 1 is lithium, a metal.
5. Hence, the metals are magnesium (a), sodium (b) and lithium (d).
Teacher's Note:
a) Metals generally have 1 to 3 valence electrons.
b) Chlorine has 7 valence electrons, so it is a non-metal.
37. Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?
Answer:
1. Element A (atomic number 19) is potassium, with configuration 2,8,8,1. Element B (atomic number 17) is chlorine, with configuration 2,8,7.
2. Potassium gives one electron to chlorine, forming K+ and Cl-.
3. The formula of the product is KCl.
4. Electron dot structure: K gives its one outer electron, and Cl receives it, so Cl- has 8 electrons around it in the form [ :Cl: ]-, and K+ has none.
5. The bond formed is ionic, because the compound is formed by transfer of electrons from a metal to a non-metal.
Teacher's Note:
a) Ionic compounds are formed between metals and non-metals.
b) Potassium chloride conducts electricity when dissolved in water.
38. Arrange the following elements in the increasing order of their metallic character
Mg, Ca, K, Ge, Ga
Answer:
1. Germanium is a metalloid, so it is the least metallic element in this list.
2. Gallium is a metal in group 13 of period 4, and magnesium is a metal in group 2 of period 3.
3. Calcium is below magnesium in group 2, so calcium is more metallic than magnesium.
4. Potassium is in group 1 of period 4, and it is the most metallic element in this list.
5. Increasing order of metallic character: Ge < Ga < Mg < Ca < K.
Teacher's Note:
a) Metallic character increases down a group and decreases across a period.
b) Potassium reacts vigorously with water, showing its high metallic character.
39. Identify the elements with the following property and arrange them in increasing order of their reactivity
(a) An element which is a soft and reactive metal
(b) The metal which is an important constituent of limestone
(c) The metal which exists in liquid state at room temperature
Answer:
1. (a) Sodium (Na) is a soft and highly reactive metal.
2. (b) Calcium (Ca) is the metal that is an important constituent of limestone (CaCO3).
3. (c) Mercury (Hg) is the metal that is liquid at room temperature.
4. Increasing order of reactivity: Hg < Ca < Na.
Teacher's Note:
a) In the reactivity series, sodium is above calcium, and calcium is above mercury.
b) Mercury is the only metal that is liquid at ordinary room temperature.
c) Potassium is also soft and reactive, and it is more reactive than sodium.
40. Properties of the elements are given below. Where would you locate the following elements in the periodic table?
(a) A soft metal stored under kerosene
(b) An element with variable (more than one) valency stored under water.
(c) An element which is tetravalent and forms the basis of organic chemistry
(d) An element which is an inert gas with atomic number 2
(e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of " anodising"
Answer:
1. (a) Sodium: group 1, period 3.
2. (b) White phosphorus: group 15, period 3.
3. (c) Carbon: group 14, period 2.
4. (d) Helium: group 18, period 1.
5. (e) Aluminium: group 13, period 3.
Teacher's Note:
a) Sodium reacts with air and water, so it is stored under kerosene.
b) White phosphorus catches fire in air, so it is kept under water.
c) Anodising builds a protective layer of aluminium oxide on the metal surface.
41. An element is placed in 2nd Group and 3rd Period of the Periodic Table, burns in presence of oxygen to form a basic oxide.
(a) Identify the element
(b) Write the electronic configuration
(c) Write the balanced equation when it burns in the presence of air
(d) Write a balanced equation when this oxide is dissolved in water
(e) Draw the electron dot structure for the formation of this oxide
Answer:
(a) 1. The element is magnesium (Mg), which is in group 2 and period 3.
(b) 1. Electronic configuration of magnesium is 2, 8, 2.
(c) 1. 2Mg + O2 → 2MgO
(d) 1. MgO + H2O → Mg(OH)2
(e) 1. A magnesium atom loses its 2 outer electrons, Mg → Mg2+ + 2e-.
2. An oxygen atom gains these 2 electrons, so O has 8 electrons around it in the form [ :O: ]2-.
3. The oxide is MgO, with Mg2+ and O2- held by an ionic bond.
Teacher's Note:
a) Magnesium burns with a bright white flame.
b) Magnesium oxide dissolves in water to form magnesium hydroxide, which is basic.
c) Magnesium oxide is an ionic compound.
42. An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.
(a) Where in the periodic table are elements X and Y placed?
(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s)
(c) What will be the nature of oxide of element Y? Identify the nature of bonding in the compound formed
(d) Draw the electron dot structure of the divalent halide
Answer:
(a) 1. X (chlorine) is in group 17, period 3.
2. Y (calcium) is in group 2, period 4.
(b) 1. X is a non-metal.
2. Y is a metal.
(c) 1. The oxide of calcium, CaO, is basic in nature.
2. The bond in the compound CaCl2 is ionic.
(d) 1. Ca gives its two outer electrons, Ca → Ca2+ + 2e-.
2. Each chlorine atom accepts one electron, so each Cl has 8 electrons in the form [ :Cl: ]-.
3. The electron dot structure of the divalent halide is Ca2+ with two [ :Cl: ]- ions.
Teacher's Note:
a) Calcium chloride is a divalent halide because calcium has valency 2.
b) Calcium oxide reacts with water to form calcium hydroxide, which is a base.
c) Calcium chloride is ionic and dissolves in water.
43. Atomic number of a few elements are given below 10, 20, 7, 14
(a) Identify the elements
(b) Identify the Group number of these elements in the Periodic Table
(c) Identify the Periods of these elements in the Periodic Table
(d) What would be the electronic configuration for each of these elements?
(e) Determine the valency of these elements
Answer:
Atomic number 10, 20, 7 and 14
| Atomic Number | Element | Group | Period | Electronic Configuration | Valency |
|---|---|---|---|---|---|
| 10 | Neon (Ne) | 18 | 2 | 2, 8 | 0 |
| 20 | Calcium (Ca) | 2 | 4 | 2, 8, 8, 2 | 2 |
| 7 | Nitrogen (N) | 15 | 2 | 2, 5 | 3 |
| 14 | Silicon (Si) | 14 | 3 | 2, 8, 4 | 4 |
Teacher's Note:
a) Neon is a noble gas, so its valency is zero.
b) Nitrogen needs three electrons to complete its octet, so its valency is 3.
c) The group of an element can be found from its valence electrons, and the period from its number of shells.
44. In which form matter is present around us?
Answer: Matter is present around us in three physical states: solid, liquid and gas.
Teacher's Note:
a) Each state has a definite set of properties.
b) Gases are the most compressible of the three states.
45. At present, how many elements are known to us?
Answer: 118 elements are known at present, of which 94 occur naturally.
Teacher's Note:
a) The remaining elements have been made artificially in laboratories.
b) Many of the heaviest elements are radioactive and have very short half-lives.
46. The earliest attempt in classifying elements resulted in the formation of two groups of elements. What are they?
Answer: The two groups are metals and non-metals.
Teacher's Note:
a) The earliest classification divided elements based on their physical properties.
b) Metals and non-metals remain the broad basis of classification today.
47. Who made the first attempt of classifying elements?
Answer: Lavoisier made the first attempt, by dividing elements into metals and non-metals.
Teacher's Note:
a) Dobereiner's triads, proposed later, were the first systematic attempt to use atomic masses.
b) Lavoisier's classification was based on physical properties.
48. On what basis Dobereiner classified elements?
Answer: Dobereiner classified elements into groups of three (triads) with similar properties, arranged in increasing order of their atomic masses.
Teacher's Note:
a) The middle element of a triad had an atomic mass close to the mean of the other two.
b) This idea worked only for a few sets of elements.
49. Dobereiner classified elements into how many groups?
Answer: Dobereiner classified the elements into three groups, called triads.
Teacher's Note:
a) The three triads were Li, Na, K; Ca, Sr, Ba; and Cl, Br, I.
b) Each group had three elements.
50. What name was given to Dobereiner groups?
Answer: The groups were called Dobereiner's triads.
Teacher's Note:
a) A triad always has three elements.
b) Triads were the first attempt to use atomic masses for classification.
51. What is the total number of elements in Dobereiner groups?
Answer: There are 9 elements in the three Dobereiner triads, as each triad has 3 elements.
Teacher's Note:
a) Multiply the number of triads by three.
b) Only a few elements were known in Dobereiner's time.
52. How did John Newlands classify elements?
Answer:
1. Newlands arranged the elements in increasing order of their atomic masses.
2. He observed that every eighth element had properties similar to the first element, like the notes of an octave in music.
Teacher's Note:
a) The arrangement was in increasing atomic masses.
b) The pattern was compared to musical notes.
53. Name the first element of Newland's octaves.
Answer: Hydrogen is the first element of Newlands' octaves.
Teacher's Note:
a) Hydrogen has the lowest atomic mass among the elements.
b) Newlands' table began with hydrogen and then followed the increasing order of atomic masses.
54. Name the last element of Newland's octaves.
Answer: Thorium (Th) is the last element in Newlands' table.
Teacher's Note:
a) Newlands' table contained the elements known at that time.
b) Thorium is a heavy radioactive metal.
55. What is your observation from Newland's octaves?
Answer:
1. When elements are arranged in increasing order of atomic masses, every eighth element has properties similar to the first.
2. This pattern worked only up to calcium, so the law failed for heavier elements.
Teacher's Note:
a) The repetition of properties was seen only up to calcium.
b) Newlands' idea of octaves was a useful first step toward periodicity.
56. What is Newland's Law of octaves?
Answer: According to Newlands' Law of Octaves, when elements are arranged in order of increasing atomic masses, every eighth element has properties similar to the first element.
Teacher's Note:
a) The law is named after its comparison with the eight notes of a musical octave.
b) It did not hold for the elements heavier than calcium.
57. Besides atomic masses, on what other basis were the elements arranged in the Mendleev's periodic table?
Answer: The elements were also arranged on the basis of their chemical properties, mainly their valency, which is shown by the formulae of their oxides and hydrides.
Teacher's Note:
a) Valency decides the formula of the hydride and oxide of an element.
b) Elements with the same valency were placed in the same group.
58. Which chemical property of an element was treated as one of the basic property for classifying elements and why?
Answer: Valency, the combining capacity of an element with hydrogen or oxygen, was treated as a basic property. Elements with the same valency show similar chemical properties, so they were placed in the same group.
Teacher's Note:
a) Valency is the number of bonds an atom can form.
b) Similar valency explains similar chemical behaviour.
59. What name is given to vertical columns in Mendleev's periodic table?
Answer: The vertical columns are called groups.
Teacher's Note:
a) Mendeleev's table had eight groups.
b) Elements in the same group had similar properties.
60. What name is given to horizontal rows in Mendleev's periodic table?
Answer: The horizontal rows are called periods.
Teacher's Note:
a) Each period had elements with gradually changing properties.
b) The modern table also uses periods and groups.
61. While developing the Periodic table, at few places Mendleev inverted the sequence of some elements i.e. he placed an element with slightly greater atomic mass before the element of lower atomic mass. Why did he do so?
Answer: Mendeleev placed such elements in the order that kept them with other elements having similar chemical properties. For example, tellurium (127.6) was placed before iodine (126.9) so that iodine could be placed with the halogens.
Teacher's Note:
a) Chemical properties were given more importance than atomic masses.
b) These inversions were one reason why atomic number replaced atomic mass in the modern table.
62. Though the atomic mass of cobalt (58.9) is greater than nickel (58.7) yet Co is placed before Ni in Mendleev's periodic table. Why?
Answer: Cobalt was placed before nickel so that each element stayed with others of similar properties. Cobalt resembles rhodium and iridium, while nickel resembles palladium and platinum.
Teacher's Note:
a) Mendeleev's placement followed chemical properties, not strict atomic mass order.
b) The anomaly is removed when elements are arranged by atomic number.
63. Which elements did not exist at the time of Mendleev's periodic classification? What name was given to these elements?
Answer: The noble gases did not exist at the time. They were called inert gases, or the zero group, when they were discovered later.
Teacher's Note:
a) Noble gases were discovered in the late nineteenth century.
b) They have complete outer shells, so they are chemically unreactive.
64. In what way hydrogen resembles alkali metals?
Answer:
1. Hydrogen has one valence electron, like the alkali metals.
2. Hydrogen forms a unipositive ion, H+, like the alkali metals.
Teacher's Note:
a) Both are placed in the first position in a period.
b) Hydrogen is not a metal, unlike the alkali metals.
65. In what way hydrogen resembles halogens?
Answer:
1. Hydrogen needs one electron to complete its outer shell, like the halogens.
2. Hydrogen exists as a diatomic molecule, H2, like the halogens.
3. Hydrogen forms covalent compounds, like the halogens.
Teacher's Note:
a) Hydrogen can also form a hydride ion, H-.
b) This shared property is one reason hydrogen has no fixed group.
66. Why hydrogen cannot be given a fixed position in periodic table?
Answer: Hydrogen resembles both the alkali metals of group 1 and the halogens of group 17. It cannot be placed in either group alone, so it has no fixed position.
Teacher's Note:
a) Its resemblance to two groups makes it unique.
b) Some tables place it at the top of group 1 and others at the top of group 17.
67. What is the first limitation of Mendleev's periodic table?
Answer: The first limitation is that the position of hydrogen was not fixed.
Teacher's Note:
a) Hydrogen resembles both group 1 and group 17.
b) Mendeleev's table also had problems with some elements placed out of order.
68. How isotopes of all the elements posed a challenge to Mendleev's periodic table?
Answer: Isotopes of an element have different atomic masses but the same chemical properties. Mendeleev's table arranged elements by atomic mass, so the isotopes would need separate places, which was not possible in the table.
Teacher's Note:
a) Isotopes were discovered after Mendeleev's time.
b) Atomic number solved this problem because isotopes have the same atomic number.
69. Who proposed that atomic number is the more fundamental property for classifying elements?
Answer: Henry Moseley proposed that atomic number is the more fundamental property for classifying elements.
Teacher's Note:
a) Moseley worked on X-ray spectra of elements in 1913.
b) The modern periodic law is based on atomic number.
70. In Modern periodic table, How do elements belonging to the same group resemble each other? Write two points.
Answer:
1. Elements of the same group have the same number of valence electrons.
2. Elements of the same group have the same valency and show similar chemical properties.
Teacher's Note:
a) Valence electrons decide the chemical behaviour of an element.
b) Physical properties like density may still differ within a group.
71. Different elements have same number of shells, in group or in period?
Answer: Elements in the same period have the same number of shells.
Teacher's Note:
a) The period number is the number of shells.
b) Elements in the same group have the same number of valence electrons instead.
72. First period of the Modern periodic table contains only two elements. Justify.
Answer:
1. The first period has only one shell, the K shell.
2. The K shell can hold a maximum of 2 electrons.
3. So, only two elements, hydrogen (1 electron) and helium (2 electrons), can be placed in the first period.
Teacher's Note:
a) The maximum capacity of a shell is 2n², where n is the shell number.
b) The second period has eight elements because the L shell holds 8 electrons.
73. How many elements are present in second group of the periodic table? Justify.
Answer:
1. Group 2 has six elements: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra).
2. All of them have 2 valence electrons, so they are placed in group 2.
Teacher's Note:
a) Group 2 elements are called alkaline earth metals.
b) Radium is radioactive, so it is rarely used in classroom examples.
74. "The valence electrons determine the kind and number of bonds formed by an element". Justify.
Answer:
1. Valence electrons are the electrons taking part in bonding.
2. Sodium has 1 valence electron, which it loses to form an ionic bond in NaCl.
3. Carbon has 4 valence electrons, which it shares to form 4 covalent bonds in CH4.
4. So, the number of valence electrons decides the number of bonds and whether the bond is ionic or covalent.
Teacher's Note:
a) Metals with few valence electrons usually form ionic bonds.
b) Non-metals with four or more valence electrons usually share electrons.
75. An element belongs to the first group and third period of the periodic table. What conclusion can you draw from its position?
Answer:
1. The element is sodium (Na), with atomic number 11 and electronic configuration 2, 8, 1.
2. It has 1 valence electron and 3 shells, and its valency is 1.
3. It is a metal, highly reactive, and it forms a basic oxide.
Teacher's Note:
a) Group 1 elements are alkali metals.
b) Sodium reacts vigorously with water.
76. A metal M forms an oxide having the formula M2O3. It belongs to the third period and thirteenth group of the Modern periodic table. Write the atomic number and valency of the element.
Answer:
1. The element is in period 3 and group 13, so it is aluminium (Al).
2. The atomic number of aluminium is 13.
3. The valency of aluminium is 3.
Teacher's Note:
a) Group 13 elements have 3 valence electrons.
b) The electronic configuration of aluminium is 2, 8, 3.
77. What were the two major shortcomings of Mendleev's periodic table? How have these been removed in the modern periodic table?
Answer:
1. Mendeleev's table arranged some elements by atomic mass, which placed pairs such as cobalt and nickel, and tellurium and iodine, out of their proper groups.
2. The position of hydrogen was not fixed.
3. In the modern periodic table, elements are arranged in increasing order of atomic number, which removes the anomalies in the pairs.
4. The modern table also explains the position of isotopes, which have the same atomic number and are placed in the same position.
Teacher's Note:
a) Atomic number is a whole number and does not show such anomalies.
b) Hydrogen still has a special position, but it is explained using valence electrons.
78. Two elements X and Y have atomic numbers 12 and 16 respectively. Write the electronic configuration for these elements. To which period of the modern periodic table do these two elements belong? What type of bond will be formed between them and Why?
Answer:
1. X (atomic number 12) is magnesium, with electronic configuration 2, 8, 2.
2. Y (atomic number 16) is sulphur, with electronic configuration 2, 8, 6.
3. Both elements belong to period 3, because they have three shells.
4. The bond formed is ionic, because magnesium, a metal, loses 2 electrons and sulphur, a non-metal, gains 2 electrons to form Mg2+ and S2-.
5. The compound formed is magnesium sulphide, MgS.
Teacher's Note:
a) Metals with 1 to 3 valence electrons tend to lose them.
b) Non-metals with 5 to 7 valence electrons tend to gain electrons.
79. What were the two achievements of Mendleev's periodic table? What was the basis of classification of elements in it?
Answer:
1. Achievement 1: The elements were arranged in a systematic way, with elements of similar properties placed in the same group.
2. Achievement 2: Mendeleev predicted the properties of undiscovered elements, such as eka-aluminium, eka-boron and eka-silicon, and left gaps for them.
3. Basis of classification: atomic masses, along with the chemical properties of the elements, such as valency and the formulae of their oxides and hydrides.
Teacher's Note:
a) Predictions were later confirmed when these elements were discovered.
b) The periodic table brought order to a large number of known elements.
80. Atomic radius decreases in moving from left to right in a period. Why?
Answer:
1. Across a period, the electrons are added to the same shell.
2. The number of protons in the nucleus increases, so the nuclear charge increases.
3. The greater nuclear attraction pulls the electrons closer to the nucleus, so the atomic radius decreases.
Teacher's Note:
a) Shielding by inner electrons is almost the same across a period.
b) The effect is strongest in the short periods.
81. Atomic radius increases down the group. Why?
Answer:
1. Moving down a group, a new shell is added with each period.
2. The outer electrons are therefore farther from the nucleus.
3. Inner shells also shield the outer electrons more, so the atomic radius increases.
Teacher's Note:
a) The number of shells equals the period number.
b) Greater distance from the nucleus means weaker attraction.
82. In the modern periodic table a zig-zag line separates metals from non-metals. What are these elements called and why?
Answer:
1. The elements lying along the zig-zag line are called metalloids, or semi-metals.
2. They are called so because they show properties of both metals and non-metals.
Teacher's Note:
a) Boron, silicon, germanium, arsenic, antimony and tellurium are metalloids.
b) Silicon and germanium are used in semiconductors.
83. X, Y and Z are the elements of a doberniers triad. If the atomic mass of X is 7 and that of Z is 39, what should be the atomic mass of Y?
Answer:
1. In a Dobereiner's triad, the atomic mass of the middle element is the mean of the other two.
2. Atomic mass of Y = (7 + 39) / 2
3. Atomic mass of Y = 46 / 2 = 23
Teacher's Note:
a) X, Y and Z here are lithium, sodium and potassium.
b) Always take the average of the two outer elements to find the middle one.
84. A and B are the two elements having similar properties which obey Newlands law of octaves. How many elements are there in between A and B?
Answer:
1. According to Newlands' law, every eighth element has similar properties.
2. A and B are 8 places apart in the arrangement.
3. So, there are 6 elements in between A and B.
Teacher's Note:
a) Count the positions between the two elements, not the elements themselves.
b) The first and eighth elements of an octave are similar.
85. Why Na is greater in atomic size than Na+ ?
Answer:
1. A sodium atom (2, 8, 1) has three shells, while the sodium ion Na+ (2, 8) has only two shells.
2. Sodium loses its outer electron to form Na+, which removes one whole shell.
3. Hence, Na is larger than Na+.
Teacher's Note:
a) Losing electrons makes a cation smaller.
b) Fewer electrons also means less repulsion between them.
86. Why does ionization energy generally decrease going down a group or family?
Answer:
1. Going down a group, the number of shells increases, so the outer electrons are farther from the nucleus.
2. Inner shells also shield the outer electrons more from the nucleus.
3. So, less energy is needed to remove an outer electron, and the ionization energy decreases.
Teacher's Note:
a) Weaker nuclear attraction makes electron removal easier.
b) Larger atoms lose electrons more easily, which explains increasing metallic character.
87. An element X (2,8,2) combines separately with NO3- and (SO4)2-, (PO4)3- radicals. Write the formulae of the three compounds so formed. To which group of the periodic table does the element 'X' belong? Will it form covalent or ionic compound? Why?
Answer:
1. Element X (2, 8, 2) is magnesium, with valency 2.
2. Formula with nitrate: Mg(NO3)2
3. Formula with sulphate: MgSO4
4. Formula with phosphate: Mg3(PO4)2
5. Element X belongs to group 2.
6. It forms an ionic compound, because magnesium is a metal that loses its 2 valence electrons to form Mg2+, which then attracts the negative radicals.
Teacher's Note:
a) The formula is written by balancing the charges of the metal ion and the radical.
b) Mg2+ combines with NO3- in a 1:2 ratio.
c) Ionic compounds generally have high melting points.
88. A metal M forms an oxide having the formula M2O3. It belongs to 3rd period in the modern periodic table. Write the atomic number and valency of the metal.
Answer:
1. The oxide M2O3 shows that the valency of M is 3.
2. The metal is in period 3 and has valency 3, so it is aluminium (Al), with group 13.
3. The atomic number of aluminium is 13.
Teacher's Note:
a) Oxygen has valency 2, so 2 oxygen atoms balance 3 metal valencies in M2O3.
b) This is the same metal as in question 76.
89. Which of the two elements A=2,8,1 B = 2,8,8,1 is more electropositive?
Answer:
1. Element A (2, 8, 1) is sodium, and element B (2, 8, 8, 1) is potassium.
2. Both have 1 valence electron, but B has more shells, so its outer electron is farther from the nucleus.
3. The outer electron of B is held more weakly, so B is more electropositive.
4. Hence, B (potassium) is more electropositive.
Teacher's Note:
a) Electropositive character increases down a group.
b) Potassium reacts more vigorously than sodium with water.
90. How does the atomic size vary in going from
A) Left to right in a period
B) Top to Bottom in a group
Answer:
A) 1. The atomic size decreases from left to right in a period.
B) 1. The atomic size increases from top to bottom in a group.
Teacher's Note:
a) Left to right, the nuclear charge increases with the same number of shells.
b) Top to bottom, each new shell adds to the atomic size.
91. An element has atomic number 13. In which group and period it should be placed?
Answer:
1. The element with atomic number 13 is aluminium, with configuration 2, 8, 3.
2. It has 3 shells, so it is in period 3.
3. It has 3 valence electrons, so it is in group 13.
Teacher's Note:
a) Atomic number equals the number of protons and electrons in a neutral atom.
b) Aluminium is the most abundant metal in the Earth's crust.
92. How many periods and groups are there in the long from of P.T?
Answer: There are 7 periods and 18 groups in the long form of the periodic table.
Teacher's Note:
a) The first period is the shortest, with only 2 elements.
b) Periods 6 and 7 are the longest, because they include the lanthanoids and actinoids.
93. Why does the size of the atoms progressively become smaller when we move from sodium (Na) to chlorine (Cl) in the third period of the period table ?
Answer:
1. Sodium and chlorine both have three shells, so the electrons are in the same shell.
2. From sodium (11 protons) to chlorine (17 protons), the nuclear charge increases.
3. The stronger nuclear attraction pulls the electrons closer to the nucleus, so the atomic size becomes smaller.
Teacher's Note:
a) The shielding effect remains almost the same across the period.
b) More protons pull the same shell of electrons more strongly.
94. Give symbols for
A. A metal of group 2
B. A metal of group 13
C. Two non metals of group 16
D. Most reactive non- metal of group 17
Answer:
A. Mg (magnesium) is a metal of group 2.
B. Al (aluminium) is a metal of group 13.
C. O (oxygen) and S (sulphur) are non-metals of group 16.
D. F (fluorine) is the most reactive non-metal of group 17.
Teacher's Note:
a) Reactivity of halogens decreases down group 17.
b) Oxygen and sulphur are the first two members of group 16.
95. Explain Why-
(a) All the elements of a group have similar chemical properties.
(b) All the elements in a period have different chemical properties.
Answer:
(a) 1. Elements of a group have the same number of valence electrons.
2. Since chemical properties depend on valence electrons, elements of a group show similar chemical properties. For example, Li, Na and K all have 1 valence electron.
(b) 1. Across a period, the number of valence electrons changes from 1 to 8.
2. Since the valence electrons are different, the elements in a period have different chemical properties. For example, Na has 1 and Cl has 7 valence electrons.
Teacher's Note:
a) Valence electrons decide how an element reacts.
b) Metals in a period change gradually into non-metals.
96. The atomic number of an element X is 17. Predict -
A. Its valency.
B. Nature of the elements.
C. Whether it is metal or non - metal.
D. Name of the element.
E. Relative size with respect to other members of its group.
Answer:
A. Valency is 1, because chlorine has 7 valence electrons and needs one more to complete its octet.
B. Chlorine is a reactive halogen that exists as a diatomic molecule, Cl2.
C. It is a non-metal.
D. The element is chlorine (Cl).
E. Chlorine is smaller than bromine, iodine and astatine, which are below it in group 17, but larger than fluorine, which is above it.
Teacher's Note:
a) Chlorine has the electronic configuration 2, 8, 7.
b) Atomic radius increases down group 17, so chlorine is smaller than the members below it.
97. The three elements predicted by mendeleev from the gaps in his periodic table were known as eka- boron, eka- aluminum, eka- silicon. What names were given to these elements when they were discovered later on?
Answer:
1. Eka-boron was discovered as scandium (Sc).
2. Eka-aluminium was discovered as gallium (Ga).
3. Eka-silicon was discovered as germanium (Ge).
Teacher's Note:
a) All three predictions matched the properties Mendeleev had forecast.
b) Scandium, gallium and germanium were discovered within about 15 years of the predictions.
98. The atomic numbers of Nitrogen, Oxygen and fluorine are 7, 8, and 9 respectively. Write the electronic configuration of each element and answer the following:
(a) Out of N, O and F which is most electronegative and which one is least electronegative?
(b) What is the number of valence electrons of F?
(c) What is the valency of each of N, O and F?
Answer:
1. Electronic configurations: N is 2, 5; O is 2, 6; F is 2, 7.
(a) 1. Fluorine is the most electronegative element of the three.
2. Nitrogen is the least electronegative of the three, because electronegativity increases from left to right across a period.
(b) 1. Fluorine has 7 valence electrons.
(c) 1. Valency of nitrogen is 3.
2. Valency of oxygen is 2.
3. Valency of fluorine is 1.
Teacher's Note:
a) Electronegativity is the power of an atom to attract the shared electrons in a bond.
b) Fluorine has the highest electronegativity of all elements.
Question. Study the given table carefully.
| G r o u p s → P e r i o d s ↓ | 1 | 2 | 3 to 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 2 | Q | R | |||||||
| 3 | P | S | T | U |
Identify P, Q, R, T and U.
P Q R T U
(a) Mg O Ne P Ar
(b) O Mg Ar P Ne
(c) Mg O Ar P Ne
(d) O Mg Ne P Ar
Answer. A
Question. The given diagram shows the electron arrangement of two elements, A and B.
Which of the following statements is correct about the two elements?
(i) A is a Period 3 element.
(ii) B is a Group III element.
(iii) Both A and B have strong metallic characteristics.
(iv) Both A and B from positive ions.
(a) (ii) only
(b) (i) and (ii) only
(c) (iii) and (iv) only
(d) (i), (ii), (iii) and (iv)
Answer. B
Question. The ‘law of octaves’ was enunciated by
(a) Lother Meyer
(b) Mendeleev
(c) Dobereiner
(d) Newlands.
Answer. D
Question. Döberiener’s system of classification into triads was not found to be useful as he could identify only
(a) two triads
(b) three triads
(c) four triads
(d) five triads.
Answer. B
Question. Identify the correct statements.
(i) In Mendeleev’s periodic table, elements were arranged in the order of their increasing atomic masses and it was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties.
(ii) In 1913, Dmitri Ivnovich Mendeleev showed that the atomic number of an element is more fundamental property than its atomic mass.
(iii) He was the most important contributor to the early development of a Periodic Table of elements wherein the elements were arranged on the basis of their fundamental property, the atomic mass and also on the similarity of chemical properties.
(iv) Among chemical properties, Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen.
(v) Mendeléev formulated a Periodic Law,which states that ‘the properties of elements are the periodic function of their atomic numbers’.
(vi) Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called periods.
(a) (ii) and (v)
(b) (i), (iii), (iv), (v) and (vi)
(c) (i), (iii), (iv) and (vi)
(d) All the statements are correct.
Answer. C
Question. Two elements A and B belong to group 1 and 2 respectively. Identify the correct statements.
(i) Valency of A is one while that of B is two.
(ii) Oxide of A has formula AO and that of B is B2O
(iii) Element A is more metallic as compared to element B.
(iv) Element A is smaller than B in size.
(a) (ii) and (iv)
(b) (i) and (iv)
(c) (i) and (iii)
(d) (ii) and (iv)
Answer. C
Question. Considering the elements B, Al, Mg and K, the correct order of their metallic character is
(a) B > Al > Mg > K
(b) Al > Mg > B > K
(c) Mg > Al > K > B
(d) K > Mg > Al > B
Answer. D
Question. State True (T) or False (F) for the given statements.
(i) John Newlands, arranged the known elements in the order of increasing atomic numbers.
(ii) He started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element.
(iii) He found that every ninth element had properties similar to that of the first.
(iv) The properties of lithium and sodium were found to be the same.
(v) Beryllium and magnesium resemble each other.
(vi) Newlands’ law of octaves worked well with higher elements only.
(i) (ii) (iii) (iv) (v) (vi)
(a) F T F T T T
(b) T F T F T T
(c) T F F F T T
(d) F T F T T F
Answer. D
Question. An element X belongs to group 14th and 2nd period of the periodic table. Its atomic number will be
(a) 6
(b) 14
(c) 7
(d) 15
Answer. A
Question. Which of the following pairs of atomic numbers represents elements belonging to the same group?
(a) 11 and 20
(b) 12 and 30
(c) 13 and 31
(d) 14 and 33
Answer. C
Question. The number of elements in the 5th period of periodic table are
(a) 8
(b) 18
(c) 10
(d) 32
Answer. B
Question. The basic character of MgO, SrO, K2O and NiO increases in the order
(a) K2O < SrO < MgO < NiO
(b) NiO < MgO < SrO < K2O
(c) MgO < NiO < SrO < K2O
(d) K2O < MgO < NiO < SrO
Answer. B
Question. X, Y and Z are the three elements, each one belongs to any one of the groups IA, IIIA and VA. The oxide of X is amphoteric, the oxide of Y is highly acidic, and the oxide of Z is highly basic. Identify the groups to which these elements X, Y and Z belong?
X Y Z
(a) VA IA IIIA
(b) IA VA IIIA
(c) IIIA IA VA
(d) IIIA VA IA
Answer. D
Question. Moving across a period of the Periodic Table,
(a) the elements become more electronegative
(b) the metallic character of the elements increases.
(c) the ability of the elements to lose electrons increases.
(d) the elements form ions with increasing negative charge.
Answer. A
Question. The proton number of four elements are given below :
| Element | W | X | Y | Z |
| Proton number | 8 | 13 | 15 | 19 |
Question. Which element has the largest size in the second period?
(a) N
(b) F
(c) Li
(d) Be
Answer. C
Question. From the given set of metals and non-metals identify the non-metals. S, Mg, Al, P, N, Na, K.
(a) S, P, K
(b) Mg, Al, Na
(c) S, P, N
(d) S, Al, K
Answer. C
Question. Which is true about electronegativity order?
(a) P > Si
(b) C > N
(c) Br > Cl
(d) Sr > Ca
Answer. A
Question. In the given metals one with the smallest size is
(a) Rb
(b) Cs
(c) K
(d) Na
Answer. D
Question. Which of the following is the most nonmetallic element?
(a) Br
(b) Cl
(c) P
(d) S
Answer. B
Question. Which of the following elements is a metalloid?
(a) Pb
(b) Sb
(c) Bi
(d) Zn
Answer. B
Question. Identify the pair of atomic numbers representing s-block elements.
(a) 7, 15
(b) 9, 17
(c) 2, 10
(d) 11, 12
Answer. D
Question. Which of the given pairs of atomic numbers represents elements in the same group?
(a) 11, 19
(b) 6, 12
(c) 4, 16
(d) 8, 17
Answer. A
Question. The effective nuclear charge acting on the valence shell electrons
(a) increases across a period
(b) decreases down the group
(c) both (a) and (b)
(d) none of the above.
Answer. C
Question. Which of the following will have equal number of electrons?
(a) Cl– and Br–
(b) Na+ and Mg2+
(c) Ar and Ne
(d) Mg2+ and Ca2+
Answer. B
CBSE Class 10 Science Chapter 5 Periodic classification of elements Case Based MCQs
Case I : Read the passage given below and answer the following questions.
Maximum number of electrons that can be accommodated in a shell is given by the formula : 2n2, where n is the number of the outermost from the nucleus.
For example, K shell – 2 × (1)2 ⇒ 2, hence, K-shell can accommodate maximum 2 electrons.L shell – 2 × (2)2 ⇒ 8, hence, L-shell can accommodate maximum 8 electrons. In the modern periodic table, elements are placed according to their electronic configuration. The elements present in any group have the same number of valence electrons. The elements present in any period contain the same number of shells. The first period of the modern periodic table corresponds to the filling of electrons in the first energy shell, i.e., K-shell, first period has two elements. The second period of the periodic table corresponds to the filling of electrons in the second energy shell, i.e., L-shell, second period contains eight elements. The third, fourth, fifth, sixth and seventh periods have 8, 18, 18, 32 and 32 elements respectively.
Question. Electronic configuration of an element ‘X ’ is 2, 1. The number of elements present in the period to which ‘X ’ belongs is
(a) 8
(b) 32
(c) 18
(d) 2
Answer. A
Question. Which of the following have the same number of electrons in outermost shell?
(a) Elements with atomic numbers 3, 11, 19
(b) Elements with atomic numbers 14, 15, 16
(c) Elements with atomic numbers 12, 20, 28
(d) Elements with atomic numbers 10, 18, 26
Answer. A
Question. Which of the following elements has two shells and both are completely filled?
(a) Helium
(b) Neon
(c) Calcium
(d) Fluorine
Answer. B
Case II : Read the passage given below and answer the following questions.
Mendeleev arranged the 63 elements known at that time in order of their ascending atomic masses and prepared a periodic table. Mendeleev’s periodic table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’. Elements with similar properties were placed in same groups.
The basis of Mendeleev’s classification is his periodic law which states that :
(I) Atomic mass is the fundamental property of elements.
(II) The physical and chemical properties of elements are periodic function of their atomic masses.
Question. Which of the following metals is not placed in eighth group of Mendeleev’s periodic table?
(a) Fe
(b) Na
(c) Co
(d) Ni
Answer. B
Question. In Mendeleev’s periodic table, silver belongs to IB group. The group to which silver belongs in the modern periodic table is
(a) first
(b) eleventh
(c) tenth
(d) sixteenth.
Answer. B
Question. The properties of eka-aluminium predicted by Mendeleev were the same as properties of which element that was discovered later?
(a) Scandium
(b) Germanium
(c) Gallium
(d) Aluminium
Answer. C
Question. Which of the following statements is not correct about Mendeleev’s periodic table?
(a) In the Mendeleev’s periodic table, some places were left vacant for new elements which were not discovered at that time.
(b) Group VIII like groups I-VII has been divided into two sub-groups A and B.
(c) The group of an element in the periodic table represents its valency.
(d) Li and C belong to same period in Mendeleev’s periodic table.
Answer. B
Case III : Read the passage given below and answer the following questions.
| Group → Period ↓ | 1 | 2 | 3-12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 2 | A | B | C | ||||||
| 3 | D | E | F |
Question. Which element in the given table has same number of electrons as in K+ and Cl–?
(a) C
(b) F
(c) E
(d) D
Answer. B
Question. The formula of the oxide of element D will be
(a) DO
(b) D2O
(c) D2O3
(d) D2O5
Answer. C
Question. Element E forms a chloride with formula
(a) E Cl2
(b) E Cl3
(c) E Cl4
(d) ECl
Answer. C
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