What are the periodic trends in electron affinity?
Electron affinity decreases down a group and increases along a period.
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.
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-.
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.