Coordinate Bond – IGCSE Chemistry Definition

A coordinate bond, also called a dative covalent bond, is a type of covalent bond in which both electrons in the shared pair are provided by the same atom. In a standard covalent bond, each atom contributes one electron to the shared pair. In a coordinate bond, one atom donates a lone pair of electrons to another atom that has an empty orbital.

How it forms

A coordinate bond requires two things: a donor atom with a lone pair of electrons and an acceptor atom with an empty orbital. The donor atom shares its lone pair with the acceptor, forming a bond. Once the coordinate bond is formed, it is identical in strength and character to an ordinary covalent bond. The only difference is the origin of the two electrons.

The most common IGCSE example is the reaction between ammonia (NH₃) and boron trichloride (BCl₃). Nitrogen in ammonia has a lone pair. Boron in BCl₃ has only six electrons in its outer shell and an empty orbital. The nitrogen donates its lone pair to the boron, forming a coordinate bond and producing the compound H₃N-BCl₃.

Another example: ammonium ion

When ammonia reacts with a hydrogen ion (H⁺), the lone pair on nitrogen forms a coordinate bond with the H⁺ (which has an empty 1s orbital). The result is the ammonium ion, NH₄⁺. All four N-H bonds in the ammonium ion are identical once formed, even though one originated as a coordinate bond.

Exam context

Paper 4 questions may ask candidates to explain how a coordinate bond forms, typically using the ammonia-boron trichloride or ammonia-hydrogen ion example. Marks are awarded for identifying the lone pair on the donor, the empty orbital on the acceptor, and stating that both electrons come from the same atom.

Worked exam question

Explain how a coordinate bond forms between ammonia and a hydrogen ion. (2 marks)

The nitrogen atom in ammonia has a lone pair of electrons [1]. The hydrogen ion (H⁺) has an empty orbital. The nitrogen donates its lone pair to the hydrogen ion, forming a coordinate bond where both shared electrons come from the nitrogen [1].

Common mistakes

Candidates sometimes say both atoms share one electron each, which describes a normal covalent bond, not a coordinate bond. The defining feature is that both electrons come from one atom. Another mistake is thinking coordinate bonds are weaker than normal covalent bonds. Once formed, they are identical in strength.