Question

The electronic configuration of four elements are given below. Arrange these elements in the correct order of magnitude (without sign) of their electron affinity.
$(i)$ $2{s^2}2{p^5}$
$(ii)$ $3{s^2}3{p^5}$
$(iii)$ $2{s^2}2{p^4}$
$(iv)$ $3{s^2}3{p^4}$

Select the correct answer using the codes given below.
A.$(i) < (ii) < (iii) < (iv)$
B. $(iii) < (iv) < (i) < (ii)$
C. $(iii) < (i)v < (ii) < (i)$
D. $(ii) < (i) < (iv) < (iii)$

Answer 1

We are given electronic configuration of the four elements .Identify the elements and find their electron affinity which means tendency to gain electrons.

Complete step by step answer:
Now from the question
(i) $2{s^2}2{p^5}$ - F
(ii) $3{s^2}3{p^5}$ -Cl
(iii) $2{s^2}2{p^4}$ -O
(iv) $3{s^2}3{p^4}$ -S

In general, electron affinity increases across the period account of increase of effective nuclear charge while decreases on moving down the group on account of increase of size. Elements $(i)$and $(iii)$ are succeeding members of the 2nd period and belong to groups 17 and 16 respectively, while elements $(ii)$ and $(iv)$are succeeding members of the 3rd period and belong to groups 17 and 16, respectively. $(i)$ is F, $(ii)$ is Cl, $(iii)$ is O, while $(iv)$ is S.
Since Electron affinity = $\dfrac{1}{{AtomicSize}}$

O and F have unexpectedly low electron affinities on account of relatively small size.
Similarly S has higher electron affinity than O due to lower electron density in O ,but it is less than both F and Cl. As the electron density of Cl is lower than F it has higher electron affinity . Hence O has the least electron affinity then followed by S then F and at last Cl.
$O < S < F < Cl$
So, the correct answer is “Option C”.

Note: Factors affecting electron affinity
1. Atomic size: If the atomic size is small, then there will be greater electron gain enthalpy because the effective nuclear forces will be greater in the smaller atoms and the electrons will be held firmly.
2. Nuclear charge: The greater the nuclear charge more will be the value for electron gain enthalpy because an increase in nuclear charge will increase the effective nuclear force on valence electrons.