How does electron affinity vary in the periodic table?

Electron affinity definition

Electron affinity is a measure of the energy released when an electron is added to an atom to form a negative ion. A positive electron affinity denotes that an atom accepts electrons easily, and thus is a nonmetallic/electronegative atom. A negative electron affinity denotes that the atom does not accept electrons (furthermore it loses them), and it is a metallic/electropositive element.

Factors on which electron affinity is dependent:

• Atomic size : Electron affinity decreases with increase in atomic size.
  Reason: The lesser is the atomic size, the lesser is the distance between the nucleus and the outermost electron shell. This means that the positive charge in the nucleus holds the outermost electrons more firmly and thus makes the atom an electron acceptor. Thus, the electron affinity is increased.
• Nuclear charge (atomic number) : Electron affinity increases with increase in nuclear charge/atomic number.
  Reason: The greater is the nuclear charge/atomic number, the greater is the attractive force of positive protons on negative electrons. Thus, the atom tends to gain electrons when its atomic size is greater.

Variation of electron affinity:

• Across the rows of the periodic table: On moving from left to right in the periodic table, the electron affinity of the elements increases.
  Reason: The atomic size gets smaller as we move across the rows of the periodic table from left to right, and the nuclear charge/atomic number also increases simultaneously. This causes the electron affinity of atoms to be increased tremendously. Thus, we find that, on moving from left to right in the periodic table, the elements become more non metallic and lesser metallic.

• Down the groups of the periodic table, the electron affinity decreases.
  Reason: the atomic size of elements decreases down the groups of the periodic table, but the atomic number increases. However, the effect of the decrease in atomic size outshines the effect of the decrease in atomic number. Thus, the attractive force of the nucleus on the outermost orbit electrons decreases, and hence the electron affinity of the elements decreases.