Question

(a) How does ionisation energy vary along the period and down the group in s-block elements? Why does this variation occur? [ 3 Marks]

(b)How do atomic size vary in a period and in a group in p-block elements? How do you explain the variation? [2 Marks]

Answer 1

(a)
i) For s- block elements ionisation energy increases along the period from left to right and it decreases down the group.
[1 Mark]
ii) In a group there is an increase of number shells and a decrease in the force of attraction of the nucleus over the shells. So, the ionisation energy decrease down the group.
[1 Mark]
iii) Along the period, electrons are added to the same shell which increases the force of attraction of the nucleus over the shell. So, ionisation energy increases along the period from left to right.
[1 Mark]

(b)
(i) For p-block elements within the group, the atomic size increases down the group. This is because a new energy shell is added at each succeeding element while the number of electrons in the valence shell remains to be the same. [1 Mark]

(ii) Therefore, the electrons added successively lie far away from the nucleus. The force of attraction of the nucleus decreases in electrons, so the atomic size increases. In a period, the electrons are added to the same nucleus, so the nuclear charge increases with each successive element. This causes an increase in the force of attraction of the nucleus resulting in a decrease in atomic size. [1 Mark]