What are the periodic trends in electron affinity?

Remember: electron affinity is the energy released by a gaseous atom when it gains an e- in its outermost shell. In simpler terms, it measures the atom's attraction for new electrons.

1. Down a group
   The electron affinity decreases:
   • As you go down a group, more energy levels are added to the atom. E- get farther away from the nucleus. Therefore, elements located farther down a group do not attract other e- as strongly as elements higher up in a group.
   • The nuclear shielding increases, which decreases the attraction for new e-.
   • The nuclear charge (i.e. # of p+) does increase down a group, which increases the pull of the nucleus for new e-; HOWEVER, the increased nuclear shielding compensates for this. It decreases the pull of the nucleus on other e-.

2. Across a period
   The electron affinity increases:
   • Across a period, no new energy levels are added to the atom.
   • The e- configuration from L to R gets closer to a stable octet:
   => More energy is released when atoms obtain their stable octet.
   • The nuclear shielding stays the same (i.e. there are no new inner e- shells that protect outer e- from the pull of the + nucleus)
   • From L to R, electrons are held more closely to the nucleus because of the increased nuclear charge. Therefore, they attract electrons more strongly.