ion

An ion is a charged particle formed when an atom loses or gains electrons. Losing electrons leaves more protons than electrons, so the ion is positive (a cation); gaining electrons gives more electrons than protons, so it is negative (an anion). For example, a sodium atom that loses one electron becomes Na⁺, and a chlorine atom that gains one becomes Cl⁻. The number of protons never changes in ordinary chemistry — only the electron count moves — so an ion of an element has the same atomic number as its neutral atom but a different charge. Opposite charges attract, so ions of a metal and a nonmetal pull together to form an ionic bond, and their charges must balance: one Mg²⁺ pairs with two Cl⁻ to give MgCl₂.