Ionic bonding happens when one atom transfers one or more electrons to another atom. This usually occurs between a metal and a non-metal. The metal atom loses electrons and becomes a positively charged ion (a cation); the non-metal atom gains electrons and becomes a negatively charged ion (an anion).An ionic bond is the strong electrostatic attraction between oppositely charged ions. In a solid such as sodium chloride, millions of Na+ and Cl− ions are held in a regular giant lattice.
Na → Na+ + e− Cl + e− → Cl− Atoms lose or gain electrons so that they end up with a full outer shell. The charge on the ion equals the number of electrons lost (positive) or gained (negative).Worked example: a magnesium atom loses two electrons to form Mg2+. Each chlorine atom gains one electron to form Cl−. To balance the charges, two chloride ions are needed for every magnesium ion, so the formula of magnesium chloride is MgCl2.Some ions contain more than one atom, for example phosphate (PO43−) and ammonium (NH4+). The atoms inside these ions are held together by covalent bonds, but the ion as a whole carries a charge.Why does this matter in biology? Living things contain many ionic compounds. When they dissolve in water the lattice breaks up and the ions separate, so they can be carried around organisms and take part in reactions. The same attraction between opposite charges also holds parts of large molecules together.