Types of Chemical Bonding

Bonding type and structure tell us different things

Chemical bonds hold atoms or ions together. There are three types of strong chemical bonding: ionic, covalent and metallic. They differ in the particles involved and in how electrons are involved in holding those particles together.

A substance’s structure describes how its particles and bonds are arranged. Knowing the bonding type is therefore only part of the description: covalent bonds, for example, can join atoms into small molecules, long polymer molecules or giant networks.

Bonding is not restricted to compounds. A compound contains two or more different elements chemically combined, but atoms of the same element can also bond together. Hydrogen molecules contain only hydrogen atoms; a metal can also be a single element.

Ionic compounds: a giant arrangement of charged particles

Ionic bonding is the strong electrostatic attraction between oppositely charged ions. Electrostatic attraction means attraction between opposite electrical charges. An ion is an atom or group of atoms with an overall electrical charge because it has lost or gained electrons.

Ionic bonding happens when a metal combines with a non-metal. Sodium chloride is an example: it contains positive sodium ions, Na⁺, and negative chloride ions, Cl⁻.

These ions form a giant ionic lattice: a regular, repeating arrangement extending in three dimensions. Each ion is attracted to surrounding ions of the opposite charge, so the attractions act in all directions. The structure is not made of separate sodium chloride molecules.

Covalent bonding: shared pairs of electrons

A covalent bond forms when two atoms share a pair of electrons. Atoms can share one or more pairs. Covalent bonding is found in non-metal elements and in compounds made only of non-metals.

The same bonding type can produce three different arrangements.

Simple molecules are small, separate groups of covalently bonded atoms. In hydrogen, H₂, two hydrogen atoms share a pair of electrons. Methane, CH₄, contains a carbon atom covalently bonded to four hydrogen atoms. The strong covalent bonds are within each molecule, not between separate molecules. Forces between molecules are called intermolecular forces; in substances made of small molecules, these are much weaker than the covalent bonds.

Polymers have very large molecules built from repeating units. Their atoms are joined by strong covalent bonds along long chains. A polymer chain is still an individual molecule: neighbouring chains are not simply one continuous covalent network. Forces between the chains are distinct from the covalent bonds within them.

Giant covalent structures contain huge numbers of atoms linked by covalent bonds in an extended network. Diamond and silicon dioxide are examples. Unlike a simple molecular substance, there are no separate small molecules: the covalent bonding continues throughout the structure.

Metals: electrons shared throughout the structure

Metallic bonding occurs in metallic elements and alloys. Metal atoms share delocalised electrons: electrons that are not attached to one particular atom, but are shared throughout the metal and can move through its structure.

The metal contains a giant arrangement of positive metal ions surrounded by these delocalised electrons. The strong attraction between the positive ions and the negative electrons holds the structure together. In a pure metal, the ions form a regular arrangement, often represented as layers.

This differs from covalent bonding. A covalent bond involves a shared pair between particular atoms; metallic bonding involves electrons shared throughout the giant structure.

Comparing the five arrangements

The diagram separates two questions: what holds the particles together, and how far does that bonding extend?

For an ionic compound, look for opposite charges in a repeating lattice. For a simple molecular substance, look for separate small groups of bonded atoms. A polymer has separate, very long molecules, whereas a giant covalent structure has an extended bonded network. In a metal, look for positive and electrons shared throughout the structure, rather than electron pairs belonging to individual bonds.

Three strong bonding types

• Ionic: attraction between oppositely charged ions; compounds of metals with non-metals.
• Covalent: atoms share pairs of electrons; non-metallic elements and compounds of non-metals.
• Metallic: metal atoms share delocalised electrons; metallic elements and alloys.

Compare the arrangements