If the pair of electrons shared between two atoms comes exclusively from one of the atoms, the bond formed is said to be a coordinate covalent bond. One of the common examples is the formation of NH4+ from NH3 and H+. We have
where, for the sake of convenience, the electrons belonging to nitrogen are represented by dots whereas those belonging to hydrogen atoms by crosses. Three of the four bonds between N and H atoms include two electrons, one coming from nitrogen and the other from hydrogen. In the fourth bond, both the electrons have come from nitrogen. Since all electrons are indistinguishable, it will not be possible to identify the electrons the way we have done above. In fact, once the bond is formed it will not be possible to distinguish between pure covalent bond and coordinate covalent bonds, as all the four hydrogen atoms are equivalent. The molecule of sulphuric acid provides the other example in which this distinction can be made. We can write its Lewis structure as
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