Use coulomb\'s law to calculate the energy of a magnesium ion and an oxide ion at their equilibrium ion-pair separation distance.

The energy required to separate the ion pair = 4.297×10^(−19)J .

We know that

F = k * q1 * q2 / d^2

Force of attraction F

K = 8.99 x 10^9

q1 = charge on Mg = +2  = 2 X 1.9 x 10^-19 C

q2 = charge on O = -2 = 2 X 1.9 x 10^-19 C

d = distance between two ions = 72 + 140 pm = 212 pm

Force = 8.99 x 10^9 X 2 X 1.9 x 10^-19 C X 2 X 1.9 x 10^-19 C / (212 X 10^-12 )^2

Force = 0.002889 X 10^-5 = 2.889 X 10^-8 N

a electronegativity decreases beacuse the size of nucleus is bigger.