Question

The correct order of electron affinity is:

$A.{\text{ }}Se < S < O$

$B.{\text{ }}O < Se < S$

$C.{\text{ }}S < O < Se$

$D.{\text{ }}S < Se < O$

Answer 1

In order to find the correct order of electron affinity of the given element oxygen, Sulphur and selenium. First we will see the basic definition of electron affinity and also what are the characteristics over which the electron affinity of an element depends. Further on the basis of the characteristics we will try to find out the comparison between the electron affinities of these three elements and further we will select the correct order out of the given ones.

Complete step by step answer:

Electron affinity is characterised as when an electron is applied to the atom to form a negative ion, the shift in energy (in kJ / mol) of a neutral atom (in the gaseous phase). In other terms, the probability of the neutral atom acquiring an electron. It is also known as the amount of energy released to form a negative ion when an electron is bound to a neutral atom or molecule in a gaseous state.

Electron affinity in general terms can be considered as the acceptance for the incoming electron to the atom of that element.

The electron affinity of the elements depends on various factors such as size of the atom, electron gain enthalpy of the atom as well as the position in the periodic table.

In the periodic table Oxygen, Sulphur and selenium all are present in the same group in the order Oxygen, Sulphur, selenium. As we go down the group the size of the elements increases and the electron gain enthalpy decreases.

So, the order of electron affinity must be highest in the oxygen and least in selenium according to electron gain enthalpy.

But due to the smallest size of the oxygen atoms among the group there is a huge repulsion to the incoming electron due to the small size. So, on the basis of size, electron affinity of selenium must be highest. But the size of Sulphur and selenium is not that different.

Therefore, combining the effect of size and electron gain enthalpy the electron affinity of Sulphur is a bit higher than selenium and that of oxygen is least in the group.

Hence, The correct order of electron affinity is $O < Se < S$ .

So, the correct answer is “Option B”.

Note: In order to solve such types of problems where the order of some elements is asked on the basis of some feature, students must keep in mind not only to see the trend in the group and period but also should look for different exceptions in the group as well as some other features that try to change the trend followed. Chlorine is the element with highest electron affinity in the modern periodic table.