What are the trends for electron affinity?
Down a group, the electron affinity decreases. Across a period, the electron affinity increases.
Remember : electron affinity is the energy released by a gaseous atom when it gains an e- in its outermost shell. Simply, it is the atom's attraction to electrons.
Down a group.
The electron affinity decreases.
• The number of energy levels increases as we go down a group. As such, e- get farther and farther away from the nucleus. Elements farther down a group, therefore, do not attract other e- as strongly than elements further up a group.
• The increased nuclear shielding (due to the increasingly number of filled, inner e- shells that protect the outer e- from the pull of the nucleus) decreases the attraction for new electrons.
• Although the nuclear charge (i.e. the # of p+) increases down a group, increasing the pull from the nucleus on outer e-, the increased nuclear shielding reduces the pull of the nucleus.
Across a period
The electron affinity increases.
• From L to R, the number of energy levels stays the same. The electron configuration gets closer to a stable octet (more energy is released once atoms obtain their stable octet).
• Nuclear shielding stays the same.
• The nuclear charge increases and e- are held more closely to the nucleus. Therefore, they attract e- more strongly.