Metallic bonding

In a metal, the atoms contribute outer electrons to the structure. These electrons are delocalised: they are no longer tied to one atom and can move throughout the structure. The atoms become positive metal ions, arranged in a regular lattice surrounded by the delocalised electrons.

The positive ions attract the negatively charged delocalised electrons. This strong electrostatic attraction holds the lattice together and is called metallic bonding. It occurs in metals and alloys.

The electrons are shared across the structure, rather than as pairs shared between individual atoms. Metallic bonding also differs from bonding in which electrons are transferred from a metal to a non-metal.

Covalent bonding

A covalent bond forms when two atoms share a pair of electrons. The negatively charged shared electrons are attracted to the positively charged nuclei of both atoms. This strong electrostatic attraction holds the atoms together.

For example, two hydrogen atoms can each contribute one electron to a shared pair, forming a hydrogen molecule, $$\ce{H2}$$. The electrons are shared between the atoms rather than transferred from one atom to the other.

Covalent bonding is found in most non-metal elements and in compounds made only from non-metals.

Ionic bonding

Ionic bonding occurs when positive and negative ions attract each other. An ion is an atom or group of atoms with an electric charge. In ionic compounds formed from a metal and a non-metal, the ions form when electrons transfer from metal atoms to non-metal atoms.

A metal atom loses electrons and becomes a positive ion because it then has more positive charges than negative charges. A non-metal atom gains electrons and becomes a negative ion because it then has more negative charges than positive charges.

For example, a sodium atom transfers one electron to a chlorine atom. The sodium atom becomes a positive sodium ion, $$\ce{Na+}$$, and the chlorine atom becomes a negative chloride ion, $$\ce{Cl-}$$. The oppositely charged ions attract each other.

The ionic bond is the strong electrostatic attraction between oppositely charged ions. Electron transfer produces the ions; the attraction between their opposite charges holds them together.