Electron Affinity in a Gaseous Atom

To study about electron affinity we must know:

Ionization potential is the energy required to remove an electron from a gaseous atom. Energy is supplied for removing an atom implies that energy will be released if an extra electron is added to the atom. The amount of energy released when a neutral atom in its gaseous state accepts an electron and gets converted into a negatively charged ion is known as electron affinity.

\(X (g) ~+~ e^-~ \rightarrow ~X^- ~(g) ~+ ~E.A.\)

Here X is any element taken in the gaseous state and E.A. is the electron affinity. The unit of electron affinity is electron volts per atom or kilo joule per mole. It is represented by a negative sign [-] for example,

\(Cl ~(g) ~+ ~e^- \rightarrow Cl^-~ (g)~ +~ 349~ KJ/mol\)

Therefore, the electron affinity of chlorine is – 349 KJ/mol.

Energy is released when first electron is added to an atom and leads to the formation of a monovalent anion and this is known as the first electron affinity. Now if we add another electron to this anion, a force of repulsion is experienced by the electron and energy is absorbed. Therefore, the second electron affinity and further affinities are positive in nature.

Electron Affinity

Electron Affinity

Electron affinity or electron gain enthalpy depends on:

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 increase in nuclear charge will increase the effective nuclear force on valence electrons.

Trends in electron gain enthalpy

In a period, as we move from left to right the atomic size decreases due to the increase in the nuclear force hence the electron gain enthalpy increases. Whereas while moving down a group in the periodic table, the atomic size increases thereby causing a decrease in the value of electron gain enthalpy.

The article briefly describes the concept of electron gain enthalpy, for any query related to this topic install Byju’s the learning app.’

Practise This Question

The correct order of ionic size of the species K+,Cl,Ca2+ and S2 is: