ionization

Meaning: Ionization is the formation of ions from a neutral species. For an atom, a gaseous atom loses an electron to form a cation, and the energy required is the ionization energy. It is larger with greater effective nuclear charge and smaller radius, so it generally rises across a period and falls down a group. Successive ionization energies increase, with a large jump after the valence electrons are removed. In solution, a molecular compound such as a weak acid ionizes by reacting with water.

Equation: X(g) → X⁺(g) + e⁻, where X is an atom and ΔH is the first ionization energy (J/mol, usually reported in kJ/mol). For an acid, HA + H₂O ⇌ H₃O⁺ + A⁻, with Ka = [H₃O⁺][A⁻]/[HA], concentrations in mol/L.

On the MCAT: Expect a table of successive ionization energies and a question asking which group the element belongs to, or a ranking of elements by first ionization energy. Know the usual exceptions: Be is higher than B, and N is higher than O. For acids, a question may ask how percent ionization changes on dilution (it increases).

Common confusion: Ionic solids dissociate into ions that already exist in the lattice, while molecular compounds ionize. Ionization energy is also not electron affinity: ionization energy involves removing an electron and is endothermic, while electron affinity concerns adding one.