Which group is excluded from electron affinity trends?

Which group is excluded from electron affinity trends?

• A. Alkali metals
• B. Noble gases
• C. Halogens
• D. Transition metals

Answer: (B)

Electron affinity is the energy change when a neutral gaseous atom gains an extra electron. As you move across a period, atoms become more eager to complete their valence shells, so the energy released (the affinity) becomes more exothermic. Down a group, added electrons enter a larger, less strongly attracting environment, so the energy release is smaller and the tendency to gain an electron weakens. Noble gases don't fit this pattern because their valence shells are already full. Adding an electron would have to enter a new, higher-energy shell, which is not energetically favorable. As a result, their electron affinities are essentially negligible or even endothermic, so they are excluded from the usual trend. The other groups—alkali metals, halogens, and transition metals—follow the general trend more closely, with halogens showing the strongest tendency to gain electrons.

Electron affinity is the energy change when a neutral gaseous atom gains an extra electron. As you move across a period, atoms become more eager to complete their valence shells, so the energy released (the affinity) becomes more exothermic. Down a group, added electrons enter a larger, less strongly attracting environment, so the energy release is smaller and the tendency to gain an electron weakens.

Noble gases don't fit this pattern because their valence shells are already full. Adding an electron would have to enter a new, higher-energy shell, which is not energetically favorable. As a result, their electron affinities are essentially negligible or even endothermic, so they are excluded from the usual trend. The other groups—alkali metals, halogens, and transition metals—follow the general trend more closely, with halogens showing the strongest tendency to gain electrons.