An ionic bond is a type of a chemical bond formed by the complete transfer of electrons from one atom to another atom i.e one of the atoms loses its electrons and the other gains it. This results in the formation of 2 oppositely charged ions.
In sodium chloride, sodium loses one electron from its outermost shell (valence shell) whereas chloride gains it. Due to this sodium gains, a net positive charge and chloride gain a net negative charge.
So, due to the complete transfer of electrons that takes place from sodium to chloride, the compound generated (Sodium chloride) has an ionic bond.
