The formation of an ionic bond involves the

Answer is: transfer of electrons.

Ionic bond is the electrostatic attraction between oppositely charged ions.  

For example, compound magnesium chloride (MgCl₂) has ionic bond.

Magnesium (Mg) transfers two electrons, became positive cation, to chlorine (Cl), which became negative anion.

Magnesium is metal from group 2 and chlorine is nonmetal from group 17 of the Periodic table.