CBSE Class 10 Science Periodic Classification Of Elements Assignment Set 08

Very Short Answer Type Questions :

Question. F, Cl and Br are the elements each having seven valence electrons. Which of these (a) has the largest atomic radius, (b) is most reactive? Justify your answer stating reason for each.
Answer: (a) Bromine has largest atomic radius because it has four shells: 2, 8, 18, 7.
(b) Fluorine is most reactive because it is smallest in size and can gain electron easily.

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. Choose from the following: 4Be, 9F, 19K, 20Ca
a. The element having one electron in the outermost shell.
b. Two elements of the same group.
Answer: a. 19K(2,8,8,1) has one valence electron,
b. 4Be(2, 2) and 20Ca(2, 8, 8, 2) belongs to the same group.

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. 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. An element A has atomic number 16. To which group mand 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. Why are H, Li, Na and K placed in group-1?
Answer: It is because they have one valence electron.

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. 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. 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. 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. 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.

Short Answer Type Questions :

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 non-metal because it can gain electron easily as it has 7 valence electrons and forms negative ion with stable electronic configuration.
(b) It is because ‘B’ has lesser atomic number, less nuclear charge, less force of attraction between valence electrons and nucleus therefore, has larger atomic size.
(c) ‘C’ has 7 valence electrons. It can gain one electron to become stable. So, its valency is equal to one.

Question. (a) How are the following related?
1. Number of valence electrons of different elements in the same group.
2. Number of shells of elements in the same period.
(b) How do the following change?
1. Number of shells of elements as we go down a group.
2. Number of valence electrons of elements on moving from , left to right in a period.
3. Atomic radius in moving from left to right along a period.
4. Atomic size down a group.
Answer: (a)
1. Different elements in same group have same number of valence electrons.
2. Number of shells of elements in same period are equal.
(b)
1. Number of shells of elements goes on increasing down the group.
2. Number of valence electrons of elements goes on increasing on moving from left to right in a period, e.g. lithium has 1, beryllium has 2, boron has 3, carbon has 4, nitrogen has 5, oxygen has 6, fluorine has 7 and neon has 8 valence electrons.
3. Atomic radius goes on decreasing in moving from left to right along a period.
4. Atomic size goes on increasing down a group.

Question. Li, Be, B, C are the elements of same period of Modem Periodic table.
a. Arrange them in increasing order of their atomic size.
b. In which shell (number) would last electron enter for all of them.
c. Calculate the valence electrons in each.
d. Which element amongst them is most electropositive?
Answer: a. C < B < Be < Li
b. Second shell 93
They have 1, 2, 3, 4 valence electrons respectively.
d. Li is the most electropositive element.

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. The elements Li, Na and K each having one valence electron, are in period 2, 3 and 4 respectively of modern periodic table.
a. In which group of the periodic table should they be placed?
b. Which one of them is least reactive?
c. Which one of them has the largest atomic radius?
d. Give reason to justify your answer in each case.
Answer: a. They belong to group 1 because they have one only valence electron.
b. Lithium (Li) is the least reactive because it is smallest in size, has least tendency to lose electrons.
c. K has largest atomic radius because it has maximum number of shells.

Question. What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev’s Periodic Law.
Answer: a. Increasing order of atomic mass was the physical property.
b. Formulae of oxides and hydrides was the chemical property.
i. Increasing order of atomic mass could not be maintained.
ii. Position of isotopes posed a challenge for the classification of elements.

Question. In the following table, six elements A, B, C, D, E and F (here letters are not the usual symbols of the elements) of the Modern Periodic Table with atomic numbers 3 to 18 are given as follows:

a. Which of these is (i) a noble gas, (ii) a halogen?
b. If B combines with F, what would be the formula of the compound formed?
c. Write the electronic configurations of ‘C’and’E’.
Answer: a. (i) G is a noble gas, (ii) F is a halogen.
b. BF is the formula of compound.
c. C: 2, 8, 2 E: 2, 8, 6

Question. Study the variation in atomic radii of first group elements given below and arrange them in an increasing order:
a. Name the element which have the smallest and the largest atoms

b. How does the atomic size vary as you go down the group?
Answer: Li < Na < K < Rb < Cs
a. Li is the smallest, Cs is the largest atom.
b. Atomic size increases down the group.

Question. In the following table, six elements A, B, C, D, E and F (here letters are not the usual symbols of the elements) of the Modern Periodic Table with atomic numbers 3 to 18 are given:

(a) Which of these halogen ?is (i) a noble gas, (ii) a halogen
(b) If B combines with F, what would be the formula of the compound formed?
(c) Write the electronic configurations of C and E.
Answer: 

Question. In the following table, are given eight elements A, B, C, D, E, F, G and H (here letters are not the usual symbols of the elements) of the Modern Periodic Table with the atomic numbers of the elements in parenthesis.

1. What is the electronic configuration of F?
2. What is the number of valence electrons in the atom of F?
3. What is the number of shells in the atom of F?
4. Write the size of the atoms of E, F, G and H in decreasing order,
5. State whether F is a metal or a non-metal.
6. Out of the three elements B, E and F, which one has the biggest atomic size?
Answer: 1. F has electronic configuration 2, 8, 2.
2. F has 2 valence electrons.
3. There are three shells in ‘F’.
4. H > G > F > E is decreasing order of size of atbms.
5. ‘F’ is a metal.
6. ‘B’ is having biggest atomic size among B, E and F.

Question. The atomic number of Na and Mg are 11 and 12 respectively and they belong to the same period a. Which one should have smaller atomic size?
b. Which would be more electropositive?
c. To which group would each one belong?
Answer: Na(11): 2, 8, 1; Mg(12): 2, 8, 2
a. Mg will have smaller size.
b. Na is more electropositive.
c. Na(11) belongs to group 1 whereas Mg(12) belongs to group 2.

Question. In the following table, the position of six elements A, B, C, D, E and F are given 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. Name the element which is a metal with valency of three.
c. Name the element which is a non-metal with valency of three.
d. Out of B and C, whose atomic radius is bigger and why?
e. Write the common name of the family to which the elements D and F belongs to?
Answer: a. E is an element which form covalent compounds.
b. B is a metal with a valency of 3.
c. C is a non-metal with a valency of 3.
d. B has bigger size.
e. D and F belong to Noble gases.

Question. (a) Name metals among the first five elements of the Modern Periodic Table.
(b) Write their symbols.
(c) Write the formula of their oxides.
Answer: a. Lithium and Beryllium are metals among first five elements.
b. Lithium (Li), Beryllium (Be).
c. Li₂O and BeO are the formulae of their oxides.

Questiona. Which atom is bigger, Na or Mg? Why?
b. Identify the most (i) metallic and (ii) non-metallic element in Period 3.
Answer: a. Na is bigger because it has 11p and 11e i.e., less forces of attraction than in Mg which has 12 protons and 12 electrons and has more forces of attraction, due to more effective nuclear charge.
b. (i) Na is the most metallic element, (ii) Cl is the most non-metallic element.

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 combine with chlorine to form an acid and common salt respectively. The oxide of element A is liquid at room temperature and is a neutral substance, while the oxides of the remaining six elements are basic in nature. Based on the above information, answer the following questions: What could the element A be?
(i) Will elements A to G belong to the same period or same group of the periodic table?
(iii) Write the formula of the compound formed by the reaction of the element A with oxygen,
(iv) Show the formation of the compound by a combination of element C with chlorine with the help of electronic structure.
(v) What would be the ratio of number of combining atoms in a compound formed by the combination of element A with carbon?
(vi)Which one of the given elements is likely to have the smallest atomic radius?
Answer: (i)’A’ is hydrogen because its oxide H2O is liquid at room temperature .
(ii)A to G belong to same group of the periodic table as these have same number of valence electrons.

Question. Two elements X and Y belong to group 1 and 2 respectively in the same period of periodic table. Compare them with respect to: periodic table from the left to the right and why?
(a) the number of valence electrons in their atoms
(b) their valencies
(c) metallic character
(d) the sizes of their atom
(e) formulae of their oxides formulae of their chlorides
Answer: (a) ‘X’ has 1 valence electron, Y has 2 valence electrons.
(b) ‘X’ has valency equal to 1, V has valency equal to 2.
(c) ‘X’ is more metallic than Y.
(d) ‘X’ is bigger than Y.
(e) X₂0 and YO are formulae of their oxides.
(f) and YCI₂ are formulae of their chlorides.

Question. What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev’s Periodic Law.
Answer: Atomic mass as a physical property and nature and formulae of oxide and hydride formed, and chemical property was used by Mendeleev.
Following are the two observations which posed a challenge to Mendeleev’s Periodic Law.
(i) Increasing order of atomic weights could not be maintained while matching chemical properties. Chemical properties do not depend upon atomic mass.
(ii) Isotopes have different atomic mass but same chemical properties.

Question. What were the limitations of Newlands’ Law of Octaves ?
Answer: Limitations:
(i) It was found that law of octaves was applicable only upto calcium because after Ca, every eighth element did not possess properties similar to first.
(ii) New elements discovered could not fit into the law of octaves on the basis of their properties.
(iii) Newland adjusted two elements in same slot e.g., Co and Ni which differ entirely from halogens. Iron resembles with Co and Ni, has been placed far away from these elements.