Questions: Electron Affinity

Question 1

An atom has an electron affinity of −349 kJ/mol. Which statement best explains why this value is negative?

• A. The atom had excess electrons it needed to release, making the process exothermic
• B. The added electron enters a more stable configuration, and energy is released to the surroundings
• C. The nucleus repels the incoming electron, requiring energy input from the surroundings
• D. A negative electron affinity means the atom resists gaining electrons

Answer: (B)

A negative electron affinity means the process X(g) + e⁻ → X⁻(g) is exothermic — energy is released because the resulting anion is more stable than the neutral atom. The extra electron is attracted by the nuclear charge and enters an available orbital at lower potential energy, releasing that energy difference. A more negative value means a stronger tendency to gain electrons, not a weaker one.

Question 2

Which element would you expect to have a near-zero or positive electron affinity?

• A. Fluorine — small and highly electronegative
• B. Nitrogen — has a half-filled 2p subshell
• C. Neon — has a completely filled valence shell
• D. Chlorine — one electron short of a noble gas configuration

Answer: (C)

Noble gases like neon have completely filled valence shells. An incoming electron would have to enter the next higher energy shell with no gain in stability, so the process is endothermic (positive electron affinity) or essentially zero. Fluorine and chlorine have strong tendencies to gain electrons (very negative EA). Nitrogen has an anomalously low EA but still negative — the half-filled 2p shell creates repulsion with the incoming electron, but the nuclear attraction still wins slightly.

Question 3

Electron affinity generally becomes more negative (more exothermic) moving from left to right across a period.

• T. True
• F. False

Answer: True

Across a period, nuclear charge increases while the principal quantum number stays the same, so the nucleus pulls incoming electrons more strongly and atomic radius shrinks. This makes electron gain progressively more favorable — the electron affinity trend increases (becomes more negative) across a period. Exceptions exist (e.g., nitrogen's half-filled 2p, noble gases' filled shells), but the general trend holds.

Question 4

Fluorine has a more negative electron affinity than chlorine, consistent with the periodic trend that smaller atoms attract incoming electrons more strongly.

• T. True
• F. False

Answer: False

This is the key exception: chlorine (−349 kJ/mol) actually has a more negative electron affinity than fluorine (−328 kJ/mol), despite fluorine being smaller and more electronegative. Fluorine's 2p orbitals are so compact that adding an electron creates significant electron-electron repulsion, partially offsetting the nuclear attraction. Chlorine's larger 3p orbitals accommodate the extra electron with less repulsion. This exception shows that the periodic trend for electron affinity has important nuances driven by orbital size.

Question 5

Why does nitrogen have a lower (less negative) electron affinity than the elements on either side of it in the same period?

Model answer: Nitrogen's 2p subshell is exactly half-filled, with one electron in each of the three 2p orbitals (parallel spins). An incoming electron must pair with one of these existing electrons, introducing electron-electron repulsion in that orbital. This pairing energy partially offsets the stabilization from nuclear attraction, resulting in a lower electron affinity than carbon (which has a partially empty 2p orbital with room for the electron) or oxygen (where the pairing still occurs but is outweighed by greater nuclear charge).

The half-filled subshell is an unusually stable configuration because of exchange energy. Disrupting it by adding an electron is energetically penalized by the pairing repulsion, making nitrogen's electron affinity anomalously low. The same logic applies to elements with half-filled d subshells.