The amount of energy released when an electron is added to an isolated gaseous atom to produce a monovalent anion is called electron affinity of first electron affinity or electron gain enthalpy. The first electron affinity is given a negative sign as the addition of an electron to a neutral atom is an exoergic process. The addition of electron to `A^(-)` requires energy to overcome the force of repulsion. Thus, the second electron affinity is an endoergic process. The magnitude of electron affinity depends on a number of factors such as (i) atomic size (ii) effective nuclear charge (iii) screening effects (iv) half and fully filled orbitals and (v) shape of orbital. In general, electron affinity increase as the atomic radii decrease in a period. However, there are exceptions when the atoms have stable configuration. In a group, electron affinity decreases as the size increases. However, the members of 3rd period have somewhat higher values than the members in the 2nd period of the same subgroups.
Which of the following has least electron affinity?