An easy explanation on how to write equations for ion formation
let us consider two elements Na and Cl
In sodium electronic configuration is 2,8,1. If sodium loses one electron ,it will get stable configuration.
By loosing one electron ,Na become `Na^+` ion
In chlorine electronic configuration is 2,8,7 .If chlorine gets one electron ,it will get stable configuration.
By gaining one electron Cl ,changed to `Cl^-` ion.
Thus either by gaining or loosing ,a formation of ions take place.