Q.Explain the formation of Coordinate Covalent bond with one example.

In a coordinate covalent bond, one atom (the donor, with a lone pair) supplies both bonding electrons to another atom (the acceptor, with a vacant orbital) — shown as $NH_3 \to BF_3$.

Definition: an ordinary covalent bond forms when each of the two bonded atoms contributes one electron to the shared pair. A coordinate (or dative) covalent bond is a special case of a covalent bond in which both electrons of the shared pair come from just one of the two atoms (the donor, which must have a lone pair of electrons available), while the other atom (the acceptor) contributes no electrons but must have a vacant orbital to accept the pair into. Once formed, a coordinate bond behaves exactly like a normal covalent bond (same bond length/strength category), and is denoted by an arrow ($\rightarrow$) pointing from the donor atom to the acceptor atom.

Example — Ammonia-boron trifluoride adduct:

In $NH_3$, nitrogen has one lone pair of electrons not used in the 3 N–H bonds. In $BF_3$, boron (with only 6 electrons around it after forming 3 B–F bonds) has an empty (vacant) $p$ orbital.

When $NH_3$ and $BF_3$ react, nitrogen's lone pair is donated entirely into boron's empty orbital, forming a coordinate covalent bond between N and B: …