Definition · Plain-language
Ionic bond
An ionic bond is the electrostatic attraction between oppositely charged ions, formed when electrons transfer from one atom to another.
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Forming ions by transferring electrons
An ionic bond begins when a metal atom, which holds its outer electrons loosely, gives one or more of them to a non-metal atom that readily accepts them. Losing electrons leaves the metal as a positively charged cation; gaining them makes the non-metal a negatively charged anion. In sodium chloride, a sodium atom transfers its single outer electron to a chlorine atom, forming Na⁺ and Cl⁻. Both ions now have full, stable outer shells, and their opposite charges attract — that attraction is the ionic bond.
Lattices, not molecules
Ionic bonding does not produce discrete molecules. Instead, each ion attracts several oppositely charged neighbours in all directions, building a regular, repeating three-dimensional arrangement called an ionic, or crystal, lattice. In salt, every sodium ion is surrounded by chloride ions and vice versa. The chemical formula, NaCl, gives the simplest whole-number ratio of ions, not the size of a molecule. This lattice structure, with countless strong attractions throughout, explains why ionic compounds are hard, brittle solids with high melting points.
Properties of ionic compounds
The strong, multidirectional attractions in the lattice give ionic compounds characteristic properties. They have high melting and boiling points because a great deal of energy is needed to overcome the attractions. They are hard but brittle, shattering when layers are forced to shift and like charges meet. They do not conduct electricity as solids, because the ions are locked in place, but they conduct when molten or dissolved in water, since the ions are then free to move and carry charge. Many ionic compounds also dissolve readily in water.
Key facts
At a glance
- Definition: the electrostatic attraction between oppositely charged ions
- Formed by: transfer of electrons from one atom to another
- Ions: positive cations and negative anions
- Forms between: usually a metal and a non-metal
- Structure: a repeating crystal lattice, not molecules
- Conducts: when molten or dissolved, not as a solid
Common misconceptions
What people often get wrong
Often heard: Ionic compounds are made of molecules, like NaCl molecules.
Actually: Ionic compounds form extended lattices of ions, not discrete molecules. The formula NaCl gives the ratio of sodium to chloride ions, not the contents of a single molecule.
Often heard: Ionic compounds never conduct electricity.
Actually: They do not conduct as solids, because the ions are fixed in the lattice. But when molten or dissolved in water, the ions become free to move and the compound conducts electricity.
Often heard: In an ionic bond, electrons are shared between the atoms.
Actually: Sharing electrons is covalent bonding. In an ionic bond, electrons are transferred from one atom to another, creating the oppositely charged ions that then attract.
