Which group is excluded from electron affinity trends?

Which group is excluded from electron affinity trends?

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

Answer: (C)

Electron affinity is the energy change when an atom gains an electron to form an anion. Across a period, atoms tend to attract added electrons more firmly, so the affinity becomes more negative; down a group it generally becomes less negative as atomic size grows and shielding increases. Noble gases stand out because their outer shells are already full. Adding an extra electron would have to go into a new, higher-energy shell or disrupt a very stable configuration, so energy is not released and can even be required. That means their electron affinities are essentially zero or slightly positive, so they don’t follow the same pattern as the other groups. The other groups fit the trend: halogens readily gain electrons and release energy, alkali metals have very small or unfavorable affinities because they prefer to lose electrons, and transition metals show more nuanced behavior due to d-orbital involvement but are not excluded from the general idea of how electron affinity trends behave.

Electron affinity is the energy change when an atom gains an electron to form an anion. Across a period, atoms tend to attract added electrons more firmly, so the affinity becomes more negative; down a group it generally becomes less negative as atomic size grows and shielding increases.

Noble gases stand out because their outer shells are already full. Adding an extra electron would have to go into a new, higher-energy shell or disrupt a very stable configuration, so energy is not released and can even be required. That means their electron affinities are essentially zero or slightly positive, so they don’t follow the same pattern as the other groups.

The other groups fit the trend: halogens readily gain electrons and release energy, alkali metals have very small or unfavorable affinities because they prefer to lose electrons, and transition metals show more nuanced behavior due to d-orbital involvement but are not excluded from the general idea of how electron affinity trends behave.