As shown in Figure 10-2, the electron pair from the H-Cl bond remains on the Cl atom, converting it to a Cl- ion.
The bond formed between the N atom of NH3 and the H+ ion in structure (10.5) is a coordinate covalent bond. It is important to note, however, that once the bond has formed, it is impossible to say which of the four N-H bonds is the coordinate covalent bond. Thus, a coordinate covalent bond is indistinguishable from a regular covalent bond.
Another example of coordinate covalent bonding is found in the familiar hydronium ion.