Q1.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

Q 2. 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.

Q 3. 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.

Q 4. How can the valency of an element  be determined if its electronic
configuration is known? What will be ‘ the valency of an element of atomic number 9 (nine)?
Answer. If the element has 1, 2, 3, 4 valence electrons, its valency will be 1, 2, 3, 4
respectively. If the element has 5, 6, 7, 8 valence electrons, its valency will be 3, 2, 1, 0. Element with atomic number 9 has electronic configuration 2, 7. So, its valency will be 1.

Q 5. How does the electronic configuration of an atom of an element relate to its position in the modern periodic table? Explain with one example.
Answer. The position of element depends upon number of valence electrons which depend upon electronic configuration. Those elements which have same valence electrons, occupy same group.
Those elements which have one valence electron belong to group 1.
Elements with two valence electrons belong to group 2.
Period number is equal to number of shells.
If valence electrons are equal to 1, it belongs to group 1. If it has 2 shells, it belongs to second period, e.g. if element ‘X’ has atomic number 11, its electronic configuration is 2, 8,1. It has one valence electron, it belongs to group 1 and it has three shells therefore, it is in third period.

Q 6. The atomic number of an element is 16. Predict

1.  the number of valence electrons in its atom
2. its valency
3. its group number
4. whether it is a metal or a non- metal
5.  the nature of oxide formed by it
6. the formula of its chloride

Answer. The electronic configuration of S(16) is 2, 8, 6.

1.  6
2. 2
3. 16
4.  Non-metal
5.  Acidic oxide
6. SCl₂is a formula of its chloride.

Q 7. (a) did Mendeleev have gaps in his periodic table?
(b) any three limitations of Mendeleev’s classification.
(c) does electronic configurations of atoms change in a period with increase in atomic number?
Answer.(a)Gaps were left for undiscovered elements in the Mendeleev’s Periodic Table.
(b)(i) Position of hydrogen was not justified.
(ii)Increasing order of atomic mass could not be maintained.
(iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.
(c)Number of shells remains the same, number of valence electrons goes on increasing
from left to right in a period till octet is comilete,e.g.

Q.8 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 Q.9 The atomic numbers of three elements, X, Y and Z are 9,11 and 17 respectively. Which two of these elements will show similar chemical properties? Why?
Answer. Electronic configuration of X, Y and Z will be:
X(9) : 2, 7
Y(11) : 2, 8, 1
Z(17) : 2, 8, 7
X and Z will show similar chemical properties due to same number of valence electrons
Q 10. An element ‘X’ belongs to 3rd period and group 17 of the periodic table. State its (a) electronic configuration, (b) valency. Justify your answer with reasoning.
Answer.
(a) X(17) : 2, 8, 7
(b) Valency : 1
It has atomic number 17 and therefore, electronic configuration will be 2, 8, 7. It can gain one electron to become stable therefore, its valency is equal to 1. It belongs to third period, as it has three shells. It belongs to group 17 because it has 7 valence electrons

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