cation

Meaning: A cation is an atom or group of atoms with more protons than electrons, so it carries a net positive charge. It forms when a neutral atom loses electrons, which costs energy equal to its ionization energy. Elements with low ionization energy, such as the alkali and alkaline earth metals, form cations readily (Na⁺, Ca²⁺). A cation is smaller than its parent atom because the same nuclear charge pulls on fewer electrons, with less electron repulsion.

Equation: Ion charge = (protons − electrons) × e, where e = 1.60 × 10⁻¹⁹ C and charge is in coulombs; for example Na → Na⁺ + e⁻.

On the MCAT: Expect size comparisons, such as Na⁺ versus Na, or an isoelectronic series, where more protons gives a smaller radius (Mg²⁺ < Na⁺). In electrochemistry, cations migrate toward the cathode in both galvanic and electrolytic cells. Ionization energy trends predict which element forms a cation more easily.

Common confusion: Students assume a positive ion must go to the positive electrode. Cations go to the cathode, where reduction occurs, whichever its sign: the cathode is positive in a galvanic cell and negative in an electrolytic cell. Also, cations shrink relative to their atoms while anions grow.