Electron affinity is defined as the amount of energy emitted when an electron is added to a neutral atom in the gaseous state to form a negative ion. The energy of an atom is determined when the atom gains or loses energy through a chemical reaction that causes the gain or loss of electrons.
A chemical reaction that releases energy is called an exothermic reaction. Examples of exothermic reactions include the burning of a substance, combustion reactions of fuels, and respiration. A chemical reaction that absorbs energy is called an endothermic reaction. Examples include photosynthesis, thermal decomposition reactions and melting ice.
The energy released from an exothermic reaction is negative, whereas energy released from an endothermic reaction is positive. In the first reaction, the electron affinities are negative, and in the second reaction, the electron affinities are positive.
The electron affinities of elements vary. For instance, metals have lower electron affinities compared to non-metals. This is because the probability of metals gaining electrons is low because it’s easier for metals to lose valence electrons. A valence electron is an electron associated with an atom that can participate in the formation of a chemical bond. Non-metals have greater electron affinities because of their atomic structures. Secondly, they have more valence electrons compared to metals; therefore, it’s easier for non-metals to gain electrons.