monatomic ion

Meaning: A monatomic ion is one atom with an unequal number of protons and electrons. Metals commonly lose electrons to form cations, and nonmetals commonly gain electrons to form anions, often reaching a noble-gas configuration. Group 1 gives +1, group 2 gives +2, group 17 gives −1 and group 16 gives −2. Transition metals often form more than one cation (Fe²⁺, Fe³⁺) and lose 4s electrons before 3d electrons. A polyatomic ion (NH₄⁺, SO₄²⁻) has several bonded atoms sharing one net charge.

Equation: charge = Z − Ne, where Z is the number of protons and Ne is the number of electrons (both are counts). Multiply by e = 1.60 × 10⁻¹⁹ C to get charge in coulombs.

On the MCAT: Expect questions that ask you to predict an ion's charge from the periodic table, write its electron configuration, or name it (Fe³⁺ is iron(III), Cl⁻ is chloride). Size comparisons are common. A cation is smaller than its parent atom, and an anion is larger. In an isoelectronic series, more protons means a smaller radius: O²⁻ > F⁻ > Na⁺ > Mg²⁺. Ion charge and radius also feed into Coulomb's law and lattice energy.

Common confusion: Forming an ion changes the electron count. The proton count stays the same, so the element stays the same. Monatomic describes the particle's composition, not its charge. A neutral He atom is monatomic but is not an ion.