Electron Affinity
Definition: Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in
the gaseous phase) when an electron is added to the atom to form a negative ion. In other words,
the neutral atom's likelihood of gaining an electron.
A(g) + e- → A-(g) + thermal energy
Unit: KJ mol-1
A chemical reaction that releases energy is called an exothermic reaction and a chemical reaction
that absorbs energy is called an endothermic reaction. Energy from an exothermic reaction is
negative, thus energy is given a negative sign; whereas, energy from an endothermic reaction is
positive and energy is given a positive sign. When an electron is added to a neutral atom (i.e.,
first electron affinity) energy is released; thus, the first electron affinities are negative. However,
more energy is required to add an electron to a negative ion (i.e., second electron affinity) which
overwhelms any the release of energy from the electron attachment process and hence, second
electron affinities are positive.
The higher an element’s electron affinity, the more thermal energy is given off when an electron
is added to an atom of the element.
First Electron Affinity (negative energy because energy released):
X(g) + e− → X−(g)
Second Electron Affinity (positive energy because energy needed is more than gained):
X−(g) + e− → X2− (g)
Factors affecting E.A
1. Atomic Size: The electron affinity decreases with increase in atomic size and increases with
decrease in atomic size.
E.A Size
2. Eff ective Nuclear Charge : (Zeff)
