Coordinate Covalent Bonding
Sometimes one atom supplies both of the electrons that are shared in a covalent bond. When ammonia, NH3, is placed in an acidic solution, it picks up a hydrogen ion, H+, and becomes NH4+.
The H+ ion has a vacant valence shell that can accommodate two electrons. When the H+ is bonded to the nitrogen of NH3, the nitrogen donates both of the electrons to the bond. This type of bond, in which both electrons of the shared pari come from one of the atoms, is called a coordinate covalent bond. Even though we are making a distinction about where the electrons come from, once the NH4+ forms all four of the N-H bonds are identical.
Another example of a coordinate bond occurs when a molecule having an incomplete valence shell reacts with a molecule having valence shell electrons that aren't being used in bonding.