metal cation

Meaning: Metals have low ionization energies and low electronegativities, so they lose valence electrons readily. Main-group metals typically reach a noble-gas configuration: group 1 gives +1, group 2 gives +2, and aluminum gives +3. Transition metals lose their outer s electrons first and often form more than one charge, such as Fe²⁺ and Fe³⁺. The cation is smaller than the atom because the outer shell is usually lost and the same nuclear charge holds fewer electrons. In complexes, cations act as Lewis acids by accepting electron pairs from ligands.

Equation: M → Mⁿ⁺ + n e⁻, where M is the neutral metal atom, n is the number of electrons lost (dimensionless), and the energy cost is the sum of the first n ionization energies (J/mol).

On the MCAT: Expect ranking of ionic radii, especially isoelectronic series (Al³⁺ < Mg²⁺ < Na⁺, because more protons pull the same electrons closer). Also expect formulas of ionic compounds, electrochemistry (the metal is oxidized at the anode, and cations are reduced at the cathode), and Lewis acid strength.

Common confusion: Students think a cation is larger than its atom. It is smaller, while an anion is larger than its atom. Also, transition metals lose 4s electrons before 3d electrons.