Question

Within a period the elements having the lowest electron affinity are:
A. Halogens
B. Actinides
C. Transition metals
D. Group 1A alkali metals

Answer 1

Hint: We know that electron affinity is the energy released when an electron is added to a neutral gaseous atom. This energy will be more if forming an ion is more stable. So, try to find the species which forms the least stable anion species.

Complete step by step answer: We know that electron affinity is also called electron gain enthalpy. Which means energy released when a neutral atom in gaseous state gains an electron. Energy is released because there will be attractions developed between the added electron and nucleus of the atom. This release is the release of energy. It is the negative of the change of enthalpy of the reaction:

Enthalpy of reaction is $\triangle H$.
Then Electron gain enthalpy will be $-\triangle H$.
Electron affinity will be more if the enthalpy of the reaction will be more negative. It means negative ions should be more stable.
Halogens gain one electron and attain the nearest noble gas configuration which is very stable. And they have very high electron gain enthalpy.
On the other hand alkali metals which are most electropositive with electronic configuration $n{ s }^{ 1 }$ try to lose electrons and form a cation. So, they are reluctant to add electrons. Therefore, group 1A metals have the least electron affinity. Option D is correct.
Note: Though alkali metals have least electron affinity this is positive because they attain configuration $n{ s }^{ 2 }$ which is somewhat stable. Transition elements and f-block elements have empty orbitals so they are stable in the negatively charged form.