Electron affinity

Definition of electron affinity

Electron affinity is the affinity of an element to an electron. This is measured by the energy released when an element in its gaseous state accepts an electron to form an anion. So this is an exothermic reaction. They are expressed in KJ mol^-1.

M_(g) + e^– → M^–_(g)       EA

Since one, two or more electrons can be added to form an anion, many types of electron affinity is possible.

• The first electron affinity (EA₁) is the energy released when first electron is added to an element in gaseous state.
• The second electron affinity (EA₂) is the energy released when second electron is added to the negatively charged (M^–) element in its gaseous state.
• The successive electron affinity are also possible for the negatively charged element in its gaseous state.

M_(g) + e^– → M^–_(g)     EA₁

M^–_(g) + e^– → M^2-_(g)    EA₂

M^2-_(g) + e^– → M^3–_(g)    EA3

Order of successive electron affinity

The second electron affinity (EA₂) is always larger than the first electron affinity (EA₁) as it is hard to add an electron into a negative ion than a neutral atom. As for example the electron affinity of oxygen to add two electrons are:

O_(g) + e^– → O^–_(g)     EA₁ = -142 Kj mol^-1

O^–_(g) + e^– → O^2-_(g)    EA₂ = + 844 Kj mol^-1

Here we can see that the second electron affinity is much higher than the first one. Thus in general successive electron affinity increase in magnitude EA₁<EA₂<EA₃ and so on.

First electron affinity in the periodic table

The first electron affinity of elements in the periodic table increases across the period and decreases down the group.

First electron affinity increases across the period

Moving from left to right across a period, the number of protons and electrons increases while the number of energy shells stay same. Thus the affinity of an element (or more positive nucleus) to an electron increases across the period.

First electron affinity decreases down the group

Moving down a group, the number of energy shells also increases with the increase of protons and electrons. So because of shielding effects of electrons in the increased inner shells, the affinity to an electron of the nucleus is reduced down the group.

Fluorine is an exception

According to the above discussion, fluorine should have more electron affinity than chlorine. But the electron affinity of fluorine is less than chlorine. As fluorine is the smallest atom in this group, entering an electron to it results a greater repulsion because of the existence of electron cloud in the small shell. As a result, electron affinity decreases in fluorine.

F	1s22s22px22py22pz1	EA1 = -328 kJ mol-1
Cl	1s22s22p63s23px23py23pz1	EA1 = -349 kJ mol-1

Nitrogen is an exception

Nitrogen has very low electron affinity because of its half filled orbitals. The nitrogen is fairly stable atom than adjacent atom carbon in the periodic table. Thus the electron affinity falls from carbon to nitrogen.

C	1s22s22p2	EA1 = -122 kJ mol-1
N	1s22s22px12py12pz1	EA1 = 0.07 kJ mol-1

 Summary

• The electron affinity (EA) is the energy released to add an electron to an elements in the gaseous state.
• In general successive electron affinity increase in magnitude EA₁<EA₂<EA₃ and so on.
• First electron affinity increases across the period
• First electron affinity decreases down the group
• Exceptions are nitrogen, fluorine, berylium etc.

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