Question

How would you explain why the electron affinity of nitrogen is lower (less negative) than those of both carbon and oxygen?

Answer 1

Electron affinity is the change in energy of an atom when one electron is added to the atom (in gaseous state). Negative electron affinity means that energy is released when one electron is added to the atom. This means that nitrogen releases less energy than carbon and oxygen when one electron is added to the atom. This can be understood by checking the tendency of an atom to take an electron.

Complete answer:
The electronic configurations of carbon, oxygen and nitrogen are:
Carbon: $[He]2{s^2}2{p^2}$ , Oxygen: $[He]2{s^2}2{p^4}$ , Nitrogen: $[He]2{s^2}2{p^3}$
As we know that half filled and fully filled electron configurations give more stability to atoms, so when carbon takes an electron, it will have three electrons in 2p orbitals and thus this will give extra stability and thus more energy will be released (Hence greater electron affinity). But in case of nitrogen, 2p subshell is already half filled and addition of one more electron to it makes it partially filled and hence less stabilized and thus less energy is released and thus its electron affinity is lesser than carbon. Also, oxygen’s electron affinity is greater than nitrogen because oxygen is more electronegative than nitrogen and hence it has more tendency to take electrons.

Additional information: When one electron is added to the atom (in gaseous state), the energy change is called first electron affinity and this gives a negative charge to the atom. When a second electron is added to the negatively charged atom, it is called second electron affinity and similarly, addition of third electron is called third electron affinity.
First electron affinity:
$A(g)\, + \,{e^ - }\,\xrightarrow{{}}\,{A^ - }(g)\,\,\,\,\,\,$
Second electron affinity:
${A^ - }(g)\, + \,\,{e^ - }\,\xrightarrow{{}}\,\,{A^{2 - }}(g)$

Note: First electron affinity is negative because addition of electron to neutral atom results in release of energy (and thus the process is exothermic). But we need to supply energy to add another electron to negatively charged species (after first electron addition) and thus the 2nd and upcoming (3rd, 4th……) electron affinities are positive.