Question

Choose the atom with the more exothermic electron affinity in each pair:

a) I or Br:

b) Ga or As:

Answer 1

Choose the atom with the more exothermic electron affinity in each pair:

Electron affinity is the change in energy of a neutral atom (in gaseous phase) when an electron is added to the atom to form a negative ion. ( 1st E.I.). It is usually an exothermic process. When, an electron is added to a neutral atom ( 1st E.I.) usually energy is released; thus, the first electron affinities are negative. (exothermic process).

More negative the electron affinity value, higher an atom's affinity for electrons.

a) I or Br: Br will have more exothermic electron affinity

Electron affinity is a measure of attraction between the incoming electron and the nucleus - the stronger the attraction, the more energy is released (more exothermic ). The factors which affect this attraction are exactly are - nuclear charge, distance and screening.

Br have electron added in 4p , whereas I in 5p, :  because electrons are placed in a higher energy level (n= 5 ; n =4) for I than Br , thus far from the nucleus, so lesser the attraction energy ( a decrease from its pull ) or less energy is released : less exothermic process.

and increased nuclear charge is offset by extra screening electrons.

b) Ga or As: As will have more exothermic electron affinity

Ga(31) and As(33) , both have added electron in 4p , electron added to energy levels become closer to the nucleus from Ga to As, thus a more stronger attraction between the nucleus and added electron : thus more energy is released in case of As than Ga (thus more exothermic for As than Ga). And, also more valence electrons an element has, more likely it is to gain electrons to form a stable octet.

(Note : Ge (32) have more exothermic electron affinity than As(33) due to half filled p-orbital in As).