electron affinity
Summary
Electron affinity is the energy released when a neutral gaseous atom gains one electron to form an anion. It generally increases across a period and decreases down a group, with exceptions. Halogens have the largest values, so they readily form X⁻ ions.
Detail
Meaning: Electron affinity (EA) measures how strongly a neutral gaseous atom attracts one extra electron. A large EA means the process releases a lot of energy and the anion is stable. The added electron feels the effective nuclear charge, so EA generally rises from left to right across a period and tends to fall down a group as the incoming electron sits farther from the nucleus. Exceptions exist: Cl has a larger EA than F because the small 2p subshell of F crowds the new electron, and noble gases, group 2 elements and N have low or unfavorable values because of filled or half-filled subshells or a new higher subshell.
Equation: X(g) + e⁻ → X⁻(g), with EA = −ΔE, where ΔE is the energy change for adding the electron (J/mol, often given in kJ/mol). A positive EA means energy is released.
On the MCAT: Passages give periodic-table data and ask you to rank elements by EA, explain why halogens form X⁻ while alkali metals form M⁺, or compare EA with ionization energy and electronegativity. Expect to justify a trend with effective nuclear charge and atomic radius.
Common confusion: EA is not ionization energy, which is the energy needed to remove an electron, and it is not electronegativity, which describes an atom's pull on shared electrons in a bond. Also, sign conventions differ, so read whether a value is ΔE (negative if favorable) or EA (positive if favorable). Adding a second electron, as in O⁻ → O²⁻, requires energy.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.