Coordinate bond is a covalent bond in which the shared pair of electrons is contributed by one of the atoms only. A coordinate bond is denoted by an arrow (→) pointing towards the acceptor atom.
Formation of ozone (O3) molecule:
The electronic configuration of oxygen atom is 2, 6. it is two electrons short of neon configuration. Thus, two atoms of oxygen share two electrons each to attain stable octet configurations, leading to the formation of double bond between them. These two atoms of oxygen do not require any more electrons because they have already attained stable configuration. Any of these two oxygen atoms can however donate a pair of electrons to the third oxygen atom without sharing any of the electrons of the third oxygen atom. As a result, a coordinate bond is formed between oxygen atoms