ionization energy

Meaning: Ionization energy is the energy needed to remove an electron from a gaseous atom or ion. The first ionization energy (IE₁) removes the outermost electron from the neutral atom. The second removes one from the +1 ion, and so on. Removal is endothermic because the nucleus attracts the electron. Across a period IE₁ generally rises, because effective nuclear charge increases and atomic radius shrinks. Down a group it generally falls, because the valence electron is farther from the nucleus and more shielded.

Equation: X(g) → X⁺(g) + e⁻, ΔH = IE₁ > 0, where X is the gaseous atom, e⁻ is the removed electron, and IE₁ is the energy in J/mol (usually quoted in kJ/mol).

On the MCAT: Questions ask you to rank elements by IE₁ or to explain the exceptions. IE₁ of B is below Be, because the 2p electron is higher in energy than a 2s electron. IE₁ of O is below N, because of repulsion between the paired electrons in one p orbital. Successive ionization energies also appear. A very large jump between two consecutive values shows that the valence electrons are used up. For example, a big jump from IE₂ to IE₃ points to two valence electrons, a group 2 element.

Common confusion: Ionization energy is not electron affinity, which describes adding an electron. Each successive IE is larger than the one before, because the same nuclear charge holds fewer electrons.