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Assignment for Class 10 Science Chapter 5 Periodic Classification Of Elements
Class 10 Science students should refer to the following printable assignment in Pdf for Chapter 5 Periodic Classification Of Elements in Class 10. This test paper with questions and answers for Class 10 Science will be very useful for exams and help you to score good marks
Chapter 5 Periodic Classification Of Elements Class 10 Science Assignment
Case study questions
Around the year 1800, only 30 elements were known. Dobereiner in 1817 and Newlands in 1866 tried to arrange the then known elements and framed laws which were rejected by the scientists. Even after the rejection of the proposed laws, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses.
The main credit for classifying elements goes to Mendeleev for his most important contribution to the early development of a Periodic table of elements wherein he arranged the elements on the basis of their fundamental property, the atomic mass and also on the similarity of chemical properties. The formulae of their hydrides and oxides were treated as basic criteria for the classification of the elements.
However, Mendeleev’s classification also had some limitations as it could not assign the position to isotopes. He also left some gaps in the periodic table.
Question. If the letter ‘R’ was used to represent any of the elements in the group, then the hydride and oxide of carbon would respectively be represented as
(a) RH4, RO
(b) RH4, RO2
(c) RH2, RO2
(d) RH2, RO
Answer : B
Question. Why did Mendeleev leave some gaps in the Periodic table?
(a) For elements to be discovered
(b) For isotopes
(c) For isobars
(d) None of these
Answer : A
Question. Isotopes are:
(a) Atoms of an element with similar chemical properties but different atomic masses.
(b) Atoms of different elements with similar chemical properties but different atomic masses.
(c) Atoms of an element with different chemical properties but same atomic masses.
(d) Atoms of different elements with different chemical properties but same atomic masses.
Answer : A
Question. According to Mendeleev’s Periodic Law properties of elements are periodic function of
(a) atomic mass
(b) atomic number
(c) number of protons
(d) number of electrons
Answer : A
Question. How many groups and periods are there in Mandeleev’s periodic table?
(a) 6 group, 8 period
(b) 18 group, 7 period
(c) 7 group, 18 period
(d) 8 group, 6 period
Answer : D
Assertion-Reason Type Questions
For question numbers 1 to 4 two statements are given-one labeled as Assertion (A) and the other labeled
Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
(a) Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.
(b) Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the assertion.
(c) ‘A’ is true but ‘R’ is false.
(d) ‘A’ is false but ‘R’ is true.
Question. Assertion: The formula of oxide of nitrogen is N2O5. and H2O is oxide of hydrogen.
Reason: N2O5 is acidic oxide H2O is neutral oxide.
Answer : B
Question. Assertion: SiCl4 is covalent compound
Reason: Si can lose electrons and chlorine can gain electron.
Answer : C
Question. Assertion: Element with atomic number 118 belongs to group 18.
Reason: Element with atomic number 118 has 8 valence electrons
Answer : B
Question. Assertion: Ionic compounds have high melting point.
Reason: Ionic compounds do not conduct electricity in solid state.
Answer : B
Very Short Answer type Questions :
Question. Give reason why noble gases are placed in a separate group in the modern periodic table.
Answer : It is because they have their outermost shell completely filled and resemble with each other.
Question. Give the number of elements in 2nd and 5th period in the modern periodic table.
Answer : In 2nd period, there are 8 elements. In 5th period, there are 18 elements.
Question. Write the formula which is used to determine the maximum number of electrons that a shell can accommodate.
Answer : 2n2 , where n is the shell number.
Question. What is the valency of phosphorus with atomic number 15?
Answer : P(2, 8, 5) has 5 valence electrons, therefore its valency is equal to 3.
Question. Find the atomic number of the element whoseelectronic configuration is 2, 8, 5.
Answer : Atomic number = 2 + 8 + 5 = 15.
Question. The atomic number of three elements A, B and C are 12, 18 and 20 respectively. State giving reason, which two elements will show similar properties.
Answer : A(12): 2, 8, 2, B(18): 2, 8,8, C(20): 2, 8, 8, 2 A and C belong to the same group because they have same number of valence electrons.
Question. The formula of magnesium oxide is MgO. State the formula of barium nitrate and barium sulphate, if barium belongs to same group as magnesium.
Answer : The valency of Mg is 2. The valency of Ba is also 2 as they belong to the same group. Formula of Barium nitrate is Ba(NO3)2, and that of Barium sulphate is BaSO4.
Question. Out of the three elements P, Q and R having atomic numbers 11, 17 and 19 respectively, which two elements will show similar properties and why?
Answer : P(11): 2, 8, 1; Q(17): 2, 8, 7; R(19): 2, 8, 8, 1 P and R will show similar chemical properties because they have the same number of valence electrons.
Question. Why are H, Li, Na and K placed in group-1?
Answer : It is because they have one valence electron.
Question. State the modern periodic law of classification of elements.
Answer : Modern Periodic Law: It states ‘properties of elements are a periodic function of their atomic number’.
Question. What is the valency of an element with atomic number 35?
Answer : Atomic number is 35, it has electronic configuration of 2, 8, 18, 7. It has 7 valence electrons, therefore its valency is equal to 1.
Question. Out of Li and K, which will have stronger metallic character and why?
Answer : K will have more metallic character because it can lose electrons easily due to its bigger atomic size and less effective nuclear charge.
Question. Write the atomic numbers of two elements X and Y
having electronic configurations 2, 8, 2 and 2, 8, 6,
respectively.
Answer : X = 2 + 8 + 2 = 12
Y = 2 + 8 + 6 = 16
Question. Name any two elements which were discovered later after the formation of Mendeleev’s periodic table.
Answer : Gallium and Germanium.
Question. What were the criteria used by Mendeleev in creating his periodic table?
Answer : Increasing order of atomic mass and same formula of oxides and hydrides.
Question. What would be the nature of oxides formed by the elements on the right side of periodic table?
Answer : On right hand side of the periodic table, most of them are non-metals, therefore they form acidic oxides.
Question. Write the number of horizontal rows in the modern periodic table. What are these rows called?
Answer : There are 7 horizontal rows in the modern periodic Table. These rows are called periods.
Question. The electronic configuration of two elements X and Y are 2, 8, 7 and 2, 8, 8, 3 respectively. Write the atomic numbers of X and Y.
Answer : X = 2 + 8 + 7 = 17
Y = 2 + 8 + 8 + 3 = 21
Question. Write one difference between Group 1 and Group 2 elements?
Answer : Group 1 elements has 1 valence electron whereas group 2 elements has 2 valence electrons.
Question. Predict the maximum number of valence electrons possible for atoms in the first period of the periodic table.
Answer : Maximum number of valence electrons in 1st period is 2.
Question. How many vertical columns are there in the modern periodic table and what are they called?
Answer : There are 18 vertical columns in Modern Periodic Table. They are called groups.
Question. An element has atomic number 17. To which group and period does it belong to?
Answer : X(17): 2,8,7 is the electronic configuration. It has 7 valence electrons. It belongs to 17th group and 3rd period.
Question. What is a metalloid? Name any two of them.
Answer : Metalloids are those elements which resembles both with metals as well as non-metals e.g., Boron and Silicon.
Question. Why was the system of classification of elements into triads not found suitable?
Answer : It is because all the elements discovered at that time could not be classified as Dobereneire’s triads.
Question. The atomic numbers of three elements X, Y and Z are 3, 11 and 17 respectively. State giving reason which two elements will show similar properties.
Answer : X= 2, 1
Y = 2,8, 1; Z = 2, 8, 7
X and Y will show similar properties due to same number of valence electrons.
Question. Write two reasons responsible for the late discovery of noble gases.
Answer : a. They are inert i.e., least reactive.
b. They are less abundant in nature except Argon.
Question. Define Newlands law of octaves.
Answer : Newlands Law of Octaves: When elements are arranged in increasing order of their atomic mass, every eighth element resembles with the first.
Question. An element A has atomic number 16. To which group and period does it belong to?
Answer : A(16) has electronic configuration: 2, 8, 6. It has 6 valence electrons and three shells. It belong to Group 16, 3rd period.
Question. Arrange the following metals in decreasing order of their atomic size:
Ca, Mg, Ba, Be
Answer : Ba > Ca > Mg > Be is the decreasing order of atomic size.
Question. State the modern periodic law for the classification of elements. How many (a) groups, (b) periods are there in Modern Periodic Table?
Answer : Properties of elements are a periodic function of their atomic numbers. There are 18 groups and 7 periods in modern periodic table.
Question. Elements in Periodic table show periodicity in properties. List any four such properties.
Answer : Valence electrons, Valency, Atomic size, Metallic character.
Question. Name any two pairs of elements which were adjusted by Newlands in the same slot.
Answer : (i) Co and Ni,
(ii) Ce and La
Question. Why does atomic size decreases as we move from left to right along a period in a periodic table?
Answer : It is because one proton and one electron is being added successively, therefore effective nuclear charge increases, atomic size decreases.
Question. List any two properties of elements belonging to the first group of modem periodic table.
Answer : a. They have 1 valence electron.
b. Their valency is equal to 1.
Question. State Mendeleev’s periodic law.
Answer : Mendeleev’s Periodic Law: Properties of elements are a periodic function of their atomic mass.
Question. Which has larger atomic radius, K(19) or Ca(20)?
Answer : K has larger atomic radius than Ca.
Question. What is the valency of silicon with atomic number 14?
Answer : Si(2, 8, 4), the number of valence electrons in Si is 4. Therefore, its valency is 4.
Question. The electrons in the atoms of four elements A, B, C and D are distributed in three shells havingl, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.
Answer : A: 2, 8, 1
B: 2, 8, 3
C: 2, 8, 5
D: 2, 8, 7g
They belong to the 3rd period.
AD is the molecular formula.
Question. Examine elements of the third period: Na, Mg, Al, Si, P, S, Cl and Ar Answer the following:
a. Choose (i) Metals, (ii) Non-metals out of these elements.
b. On which side of periodic table we find (i) metals (ii) non-metals.
c. Name metalloids out of the elements given above.
Where are they located in the periodic table?
Answer : a. (i) Na, Mg, A1 are metals.
(ii) P, S, Cl, Ar are non-metals.
b. (i) Metals are placed on the left hand side and middle part of the periodic table.
(ii) Non-metals are placed on the right hand side of periodic table.
c. Silicon is a metalloid. They are located between metals and non-metals at the border line in a zigzag manner.
Question. Two elements P and Q belong to the same period of the modern periodic table and are in Group-1 and Group-2 respectively. Compare their following characteristics in tabular form:
a. The number of electrons in their atoms.
b. The size of their atoms.
c. Their metallic character.
d. Their tendency to lose electrons.
e. The formula of their oxides.
f. The formula of their chlorides.
Answer : a. P has 1 valence electron, Q has 2 valence electrons.
b. P is bigger than Q.
c. P is more metallic than Q.
d. P can lose electrons more easily than Q.
e. P2O and QO
f. PCl and QCl2
Question. Explain the variation in the following properties of the elements in periodic table:
a. Atomic radius in the periodic table,
b. Metallic character in a period,
c. Valency in a group.
Answer : a. Atomic radius and Metallic character increases down the group. Valency remains the same in a group.
b. Atomic size and metallic character decrease along
a period from left to right in a period.
Question. Write the main aim of classifying elements. Name the basic property of elements which is used in the development of Modern Periodic Table. State the Modern Periodic Law. On which side (part) of the Modern Periodic Table do you find metals, metalloids and non- metals?
Answer : Classification is done so as to study the properties of 118 elements easily. Modern periodic table is based on atomic number of the atom an element. ‘Properties of elements are a periodic function of their atomic numbers.’ Metals are placed on the left and middle, non-metals are placed on the right and Metalloids are placed on the border line between metals and nonmetals in a zig-zag manner.
Question. The position of three elements A, B and C in the Periodic Table is shown below:
Giving reasons, explain the following:
a. Element ‘A’ is a non-metal.
b. Element ‘B’ has a larger atomic size than element ‘C’.
c. Element ‘C’ has a valency of 1.
Answer : a. A is a non-metal because it has 7 valence electrons, it can gain one electron to form anion.
b. B has less electrons and protons, less forces of attraction between the nucleus and valence electrons, therefore it is bigger in size.
c. C can gain 1 electron to become stable, therefore its valency is equal to 1.
Question. Atoms of seven elements A, B, C, D, E, F and G have a different number of electronic shells but have the same number of electrons in their outermost shells. The elements A and C combines with chlorine to form an acid and common salt respectively. The oxide of element A is liquid at room temperature and it is a neutral substance, while the oxides of the remaining six elements are basic in nature. Based on the above information, answer the following question:
a. What could the element A be?
b. Will elements A to G belong to the same period or same group of the periodic table?
c. Write the formula of the compound formed by the reaction of the element A with oxygen.
b. What would be the ratio of number of combining atoms in a compound formed by the combination of element A with carbon?
e. Which one of the given elements is likely to have the smallest atomic radius?
Answer : a. A is hydrogen.
b. A and G will belong to the same group.
c. H2O is the formula of the compound.
d. CA4 i.e., 1:4
e. A has smallest atomic radius.
Question. An element M with electronic configuration (2,8,2) combines separately with (NO3)-, (SO4)2- and (SO4)3- radicals. Write the formula of the three compounds so formed. To which group and period of the Modern Periodic Table does the element M belong to? Will M form covalent or ionic compounds? Give reason to justify your answer.
Answer : M(2, 8, 2). It has valency equal to 2.
M belongs to group 2.
It belongs to 3rd period.
M will form ionic compound because M can easily lose electrons.
Bond will be formed by transfer of electrons.
Question. Given below are some elements of the Modern Periodic Table. Atomic number of the element is given in the parentheses:
A(4), B(9), C(14), D(19), E(20)
a. Select the element that has only one electron in the outermost shell. Also write the electronic configuration of this element.
b. Which two elements amongst these belong to the same group? Give reason for your answer.
c. Which two elements amongst these belong to the same period? Which among the two has bigger atomic radius?
Answer : a. D(19): 2,8,8,1 has 1 valence electron.
b. A(4): 2, 2 and E(20): 2, 8, 8, 2 belong to same group because they have the same number of valence electrons.
c. A(2, 2), B(2, 7) belong to the same period. A has bigger atomic radius than B.
Question. a. Why did Mendeleev leave gaps in his periodic table?
b. State any three limitations of Mendeleev’s periodic table.
c. How do the electronic configuration of atoms change in a period with increase in atomic number.
Answer : a. He left gaps for the new elements which are to be discovered.
b. (i) Increasing order of atomic mass could not be maintained.
(ii) Position of hydrogen was not justified.
(iii) Isotopes could not be placed in different slots due to different atomic mass but same properties.
c. Number of valence electrons keep on increasing along a period from left to right in a period.
Question. a. Predict which of the following will form anions and which will form cations.
(i) Na, (ii) Al, (iii) Cl, (iv) O
b Name two elements that are inert.
Answer : a. Cl and O will form anions i.e., Cl- and O2- Na and A1 will form cations i.e., Na+ and Al3+.
b. He and Ne are inert elements.
Question. Two elements X and Y have atomic 12 and 17 respectively.
a. Write the electronic configuration of both these elements.
b. Which type of bond will they form?
Answer : a. X: 2, 8, 2; Y: 2, 8,7
b. They will form ionic bond.
Question. Out of the elements H(1), Be(4), Na(11) and Mg(12).
a. Write the pair of elements having similar chemical properties.
b. State the group number of each pair,
c. Name one another element belonging to each of these groups.
Answer : a. Be(4) and Mg(12) have similar chemical properties.
H( 1) and Na(ll) have similar chemical properties.
b. Be and Mg belong to group 2, H and Na belong to group 1.
c. K belongs to group 1 and Ca belongs to group 2.
Question. Calcium is an element with atomic number 20. Stating the reason, answer each of the following questions:
a. Is calcium a metal or a non-metal?
b. Will its atomic radius be larger or smaller than that of potassium with atomic number 19?
c. Write the formula of its oxide.
Answer : a. Calcium is a metal because it can lose electrons to form cations.
b. Its atomic radius will be smaller due to more number of protons and electrons, more forces of attraction.
c. CaO
Question. a. How does electropositivity of elements gets affected as we move (i) down the group, (ii) across the period?
b. Which atomic property increases both ways: as we move across the period or down the group?
Answer : a. (i) Electropositivity increases down the group.
(ii) Electropositivity decreases across the period from left to right.
b. Atomic number increases across the period as well as down the group.
Question. Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the modern periodic table:
On the basis of the above table, answer the following questions:
a. Name the element which forms only covalent compounds.
b. Which element is a metal with a valency of one?
c. Which element is a non-metal with a valency of two?
d. Out of D and E, which has a bigger atomic radius and why?
Answer : a. E forms only covalent compounds.
b. D is a metal with valency of 1.
c. B is a non-metal with valency of 2.
d. D has bigger atomic size because it has less effective nuclear charge.
Question. The electronic configuration of two elements A and B are 2, 8, 3 and 2, 8, 7 respectively. Find the atomic number of these elements. State the nature and formula of the compound formed by union of these elements.
Answer : A has 2 + 8 + 3 = 13 as its atomic number B has 2 + 8 + 7 = 17 as its atomic number
AB3 is the formula of compound. It is an ionic compound.
Question. As we move across a period in the periodic table, what is gradation in the following properties: (a) Atomic size, (b) Atomic number, (c) Electronegativity?
Answer : a. Atomic size goes on decreasing across the period from left to right.
b. Atomic number goes on increasing along the period from left to right.
c. Electronegativity goes on increasing along a period from left to right.
Question. An element X belongs to 3rd period and group 16 of the Modern Periodic table.
a. Determine the number of valence electrons and the valency of X.
b. Molecular formula of the compound of X when it reacts with hydrogen and write its electron dot diagram.
c. Name the element X and state whether it is metallic or non-metallic.
Answer : a. X has 6 valence electrons and its valency is equal to 2.
b. H2X is its formula,
c. X is sulphur. It is a non-metallic element.
Question. Why do all elements of the
a. same group have similar properties?
b. same period have different properties?
Answer : a. It is due to same number of valence electrons which will decide the chemical properties.
b. They differ in number of valence electrons, therefore they differ in chemical properties. They have the same number of shells.
Gradation in Periodic Properties
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. 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. In the Modern Periodic Table, calcium (Z = 20) is surrounded by the elements with atomic numbers 12, 19, 21 and 38. Which of the following will have physical and chemical properties resembling calcium?
(a) 12, 38
(b) 12, 19, 38
(c) 19, 38
(d) 12, 19
Answer : A
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
Question. Element X has a proton number equal to 17. Which of the following statements about X is not correct?
(a) X is a member of Group VII.
(b) X has 7 electrons in the outermost electron shell.
(c) X has three occupied electron shells.
(d) X gains one electron to form X +.
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. Which of the following is the most nonmetallic element?
(a) Br
(b) Cl
(c) P
(d) S
Answer : B
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. 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. The position of elements A and B are shown in the periodic table below:
Which of the following statements is correct about the two elements?
(a) A and B have the same electron structure.
(b) A and B have the same number of filled electron shells.
(c) A is a metal and B is a non-metal.
(d) A is in period 2 and B is in group III.
Answer : C
Question. Element X is in period 3 and group III of the periodic table. The electronic configuration of X is
(a) 2, 1
(b) 2, 3
(c) 2, 8, 3
(d) 2, 8, 5
Answer : C
Question. Which of the following combination of elements belong to the same group?
(a) N, P, As
(b) Li, Be, Al
(c) Na, Mg, Al
(d) O, S, Cl
Answer : A
Question. From top to bottom in a group of the periodic table the electropositive character of the element
(a) increases
(b) decreases
(c) remains unchanged
(d) changes irregularly.
Answer : A
Question. Which of the following statements is correct about elements 19 9A and 3717B?
(a) A is more electronegative than B.
(b) A forms a positive ion and B forms a negative ion.
(c) A and B have the same number of neutrons.
(d) A and B have the same number of electrons.
Answer : A
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. Which of the following elements is a metalloid?
(a) Pb
(b) Sb
(c) Bi
(d) Zn
Answer : B
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. 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
(a) Both assertion and reason are true, and reason is correct explanation of the assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false, but reason is true.
Question. Assertion : Decreasing order of atomic radii is : Cl > F > O > S.
Reason : Atomic radius increases as the number of energy level increases and decreases as nuclear charge increases.
Answer : D
Question. Assertion : Down the group, atomic radius increases.
Reason : Electrons are added in new shell.
Answer : A
Question. Assertion : Increasing order of metallic character is : P < Si < Be < Mg < Na
Reason : Metallic character increases along a period and decreases down a group.
Answer : C
Question. Assertion : Alkali metals do not form dipositive ions.
Reason : After loss of one electron alkali metals achieve stable configuration of noble gas.
Answer : A
Question. Assertion : Atomic radius in general decreases along a period.
Reason : In a period, effective nuclear charge decreases.
Answer : C
Question. Assertion : In Dobereiner’s triad, the three elements present have same gaps of atomic numbers.
Reason : Elements in a triad have similar properties.
Answer : D
Very Short Answers
Answer : It states that “the properties of the elements are periodic functions of their atomic numbers.”
Question : State one reason for placing Mg and Ca in the same group of the periodic table.
Answer : They have same number of valence electrons.
Question : What was the drawback of these two laws?
Answer : All the elements discovered at that time could not be classified.
Question : Lithium, sodium and potassium form a Dobereiner’s triad. The atomic masses of lithium and potassium are 7 and 39 respectively. Predict the atomic mass of sodium.
Answer : Atomic mass of Na 7 + 39 / 2 = 46/2 = 23
Question : Chlorine, bromine and iodine form a Dobereiner’s triad. The atomic masses of chlorine and iodine are 35.5 and 126.9 respectively. Predict the atomic mass of bromine.
Answer : Atomic mass of Br 35.5 + 126.9 / 2 = 162.4 / 2 = 81.2
Question : What was the basis of classification of elements by Newland?
Answer : Increasing order of atomic mass
Question : What were the limitations of Dobereiner’s classification ?
Answer : Dobereiner could identify only three triads from the elements known at that time. Hence, this system of classification into triads was not found useful.
Question : Name any three metalloids
Answer : Boron, Silicon and Germanium.
Question : Why was the system of classification of elements into triads not found suitable?
Answer : It is because all the elements discovered at that time could not be classified into triads.
Question : An element ‘B’ belongs to the second period and Group 13. Give the formula of its oxide.
Answer : B2O3
Question : Where would you locate the element with electronic configuration: 2, 8 in the modern periodic table?
Answer : It belongs to Group 18 and second period of the periodic table.
Question : 1735Cl and 1737Cl are isotopes of chlorine, would you place them in different slots because their atomic masses are different? Or would you place them in the same position because their chemical properties are the same?
Answer : They will be placed in the same slot.
Question : Explain, why the number of elements in the third period are 8.
Answer : It is because 3rd shell could accommodate a maximum of 18 electrons, but if it is the outermost shell it could not have more than 8 electrons. Therefore, this period has 8 elements.
Question : An element ‘A’ has atomic number 16. To which group and period does it belong?
Answer : It belongs to group 16 and third (3rd) period.
Question : If an element ‘X’ is placed in group 14, what will be the formula and nature of bonding of its chloride?
Answer : XCl4, it has covalent bonding.
Question : Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.
Answer : Henry Moseley
Question : Write the formula which determines the maximum number of electrons that the shell of an atom can accommodate.
Answer : 2n2, where ‘n’ represents the number of electronic shell.
Question : How many metals are present in second period of periodic table?
Answer : There are two metals Lithium (Li) and Beryllium (Be) in second period of periodic tabl
Question : Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer : No, atomic number cannot be in fractions
Question : Write the name, symbol and electronic configuration of an element X whose atomic number is 11.
Answer : Sodium, Na: 2,8,1
Question : To which group and period should hydrogen be assigned?
Answer : It is placed in Group 1 and first period.
Question : An element ‘X’ belongs to the second group of periodic table. What is the formula of its chloride?
Answer : XCl2
Question : Find the atomic number of the element whose electronic configuration is 2, 8, 5.
Answer : Its atomic number is equal to 2 + 8 + 5 = 15.
Short Answers
Question : What is the limitation of Döbereiner triads?
Answer : He failed to arrange all the elements in triads having same chemical properties.
Question : Name three elements discovered later, which filled gaps left by Mendeleev for them.
Answer : Scandium, gallium and germanium.
Question : Give two limitations of Newlands’ law of Octaves.
Answer : Two limitations of Newlands’ law of octaves are:
(i) The law was applicable only upto calcium.
(ii) In order to fit elements into his table, Newlands adjusted two elements in wrong slot, and grouped unlike elements in same groups.
Question : State Modern Periodic Law.
Answer : Properties of elements are a periodic function of their atomic number’.
Question : What is the formula of oxide and hydride of Group I elements?
Answer : Oxide formula → R2O
Hydride formula → RH.
‘R’ represents element.
Question : How many triads could Döbereiner identify from the existing elements then?
Answer : Döbernier could identify only three triads.
Question : Which two chemical properties were considered by Mendeleev for grouping of elements?
Answer : The two chemical properties are:
(a) The nature of compounds formed by elements with oxygen.
(b) The nature of compounds formed by elements with hydrogen.
Question : State Mendeleev’s Periodic Law.
Answer : The properties of elements are the Periodic Function of their atomic masses.
Question : Give one example of Döbereiner’s Triad.
Answer : Li, Na, K
7 23 39
Question : Define ‘groups and periods’.
Answer : The vertical columns in a Periodic Table are called groups and the horizontal rows are called periods.
Question : What was the basis of classification of elements made by Newlands?
Answer : Newlands arranged the elements in the order of increasing atomic masses.
Question : How many groups and periods are present in the Modern Periodic Table?
Answer : Modern Periodic Table has 18 groups and 7 periods.
Question : In Modern Periodic Table what is common among all the elements in a group.
Answer : All elements in same group shows same valence electrons and same chemical properties.
Question : In the modern periodic table, the element Calcium (atomic number = 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these elements has physical and chemical properties resembling those of Calcium and why?
Answer : Elements with atomic number 12, 38 resemble calcium in physical and chemical properties because they have same number of valence electrons and belong to same group 2.
Mg(12) : 2, 8, 2
Ca(20) : 2, 8, 8, 2
Sr(38) : 2, 8, 18, 8, 2
Question : How does the valency of elements vary
(a) in going down a group, and
(b) in going from left to right in a period of the periodic table?
Answer : (a) Valency remains the same in a group.
(b) Valency first goes on increasing from left to right in a period till middle of period, then decreases.
Question : An element ‘M’ has atomic number 11.
(a) Write its electronic configuration.
(b) State the group to which ‘M’ belongs.
(c) Is ‘M’ a metal or a non-metal?
(d) Write the formula of its chloride.
Answer :
Question : Give reasons:
(a) Elements in a group have similar chemical properties.
(b) Elements of Group I form ions with a charge of +1.
Answer : (a) Elements in a group have same number of valence electrons and same valency therefore have similar chemical properties.
(b) It is because elements of group 1 lose one electron to acquire +1 charge and become stable.
Question : How does the metallic character of elements change along a period of the periodic table from the left to the right and why?
Answer : The metallic character goes on decreasing along a period from left to right because atomic size goes on decreasing therefore, tendency to lose electrons decreases.
Question : Choose from the following: 6C, 8O, 10Ne, 11Na,14Si
(a) Elements that should be in the same period.
(b) Elements that should be in the same group.
State reason for your selection in each case.
Answer : (a) 6C, 8O, 10Ne are in same period, Le. 2nd period as they have two shells. nNa and 14Si are in same period, i.e. 3rd period as they have three shells.
(b) 6C,14Si are in same group because they have same number of valence electrons, i.e. four.
Question : An element ‘X’ has atomic number 13.
(a) Write its electronic configuration.
(b) State the group to which ‘X’ belongs.
(c) Is ‘X’ a metal or a non-metal?
(d) Write the formula of its bromide.
Answer :
.
Question : In the periodic table, how does the tendency of atoms to lose electrons change on going from
(a) left to right across a period?
(b) top to bottom in a group?
Answer : (a) Tendency to lose electrons decreases from left to right across a period.
(b) Tendency to lose electrons increases from top to bottom in a group.
Question : State the Modern Periodic Law for classification of elements. How many
(a) groups and (b) periods are there in the Modern Periodic Table?
Answer : ‘Properties of elements are the periodic function of their atomic number.’
(a) There are 18 groups and
(b) 7 periods in the Modern Periodic Table.
Question : The formula of its oxide An element ‘M’ has atomic number 12.
(a) Write its electronic configuration.
(b) State the group to which ‘M’ belongs.
(c) Is ‘M’ a metal or a non-metal?
(d) Write the formula.
Answer :
Long Answers
Question : Point out the major defects in the Mendeleev's Periodic Table.
Answer : Defects in Mendeleev's Periodic Table
The Mendeleev's periodic table was quite helpful in the classification of the elements.
But it had certain defects also. These are discussed as follows:
i. Position of hydrogen: Hydrogen was placed at the top of group IA. It is a nonmetal where all other elements included in the group are metals.
ii. Position of isotopes: The periodic table is based on the basis of the atomic masses of the elements. This means that the elements with different atomic masses must be given separate place in the table. If this is correct, all the isotopes of an element must be allotted separate positions. For example, there are three isotopes for hydrogen and they must be given three separate places in the table.
But only one position for hydrogen has been given.
iii. Wrong order of atomic masses of some elements: In the table, the elements are arranged in order of increasing atomic masses. This means that the element with higher atomic mass must be placed after the element with the lower atomic mass. But in the table, there are some anomalies. For example, Co (Cobalt) with atomic mass 58.9 should be placed after Ni (Nickel) with atomic mass 58.7. But it has been placed before nickel.
iv. Elements with similar properties placed in different groups: In the periodic table, it has been found that the elements with similar properties are placed in different groups. For example, copper and mercury have many common properties. But copper has been placed in group I B and mercury in group II B.
v. No similarity in the elements placed in sub-groups: The elements present in different sub-groups of the same group are expected to have common properties. But these are quite different. For example, elements in group 1 A are very soft and reactive metals but element in group I B are hard and less reactive in nature.
vi. No explanation for the cause of periodicity: Mendeleev was not in a position to explain why the elements included in a group show similar properties.
vii. Position of lanthanides and actinides: Both these families have fourteen elements each, but they could not find proper positions in the main set up of the Mendeleef’s periodic table. Instead, these elements have been placed in two separate rows at the bottom of periodic table without assigning a proper reason.
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