An ion is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention, and this charge is equal and opposite to the charge of a proton. Ions can be positively charged, called cations, or negatively charged, called anions. Ions are formed by the addition or removal of electrons from neutral atoms or molecules or other ions, by the combination of ions with other particles, or by the rupture of a covalent bond between two atoms in such a way that both of the electrons of the bond are left in association with one of the formerly bonded atoms.

For example, when an atom gains or loses electrons, it develops a charge, and these charged particles are called ions. When an atom loses electrons, it becomes positively charged, and positively charged ions are called cations. When an atom gains electrons, it becomes negatively charged, and negatively charged ions are called anions.

Ions are important in chemistry and can be found in many substances in the body, such as sodium, potassium, calcium, chloride, and bicarbonate. They can also be created using radiation, such as x-rays, and ionizing radiation is often used to diagnose or treat medical conditions.

In their gas-like state, ions can be highly reactive and will rapidly interact with ions of opposite charge to give neutral molecules or ionic salts. Ions are also produced in the liquid or solid state when salts interact with solvents, such as water, to produce solvated ions, which are more stable for reasons involving a combination of energy and entropy changes as the ions move away from each other to interact with the liquid.