A special covalent bond
A dative bond (also called a coordinate bond) is a type of covalent bond. In a normal covalent bond each atom contributes one electron to the shared pair. In a dative bond, both electrons of the shared pair come from the same atom — the donor — while the other atom (the acceptor) contributes none but has an empty orbital to receive them.
The donor must have a lone pair of electrons. Once formed, a dative bond is identical in strength and length to an ordinary covalent bond; it is often drawn as an arrow (→) pointing from donor to acceptor.
Key idea / 要点
Dative bond = a shared pair where one atom supplies both electrons. It needs a donor with a lone pair and an acceptor with an empty orbital (often an H+ ion).
The ammonium ion
Ammonia, NH3, has a lone pair on the nitrogen atom. A hydrogen ion, H+, has no electrons and an empty orbital. The nitrogen lone pair is donated to the H+, forming the ammonium ion, NH4+. All four N–H bonds are now identical even though one began as a dative bond.
Worked example / 例题
Explain the bonding in the hydroxonium ion H3O+. Water, H2O, has lone pairs on oxygen. One lone pair is donated to a H+ ion by a dative bond, forming H3O+. The oxygen is the donor and H+ is the acceptor.
Remember / 记住
- Both bonding electrons come from one atom (the donor).
- Common examples: NH4+ and H3O+.
- Once formed, it behaves exactly like a normal covalent bond.