Energetics of Forming Ionic Bonds
Energy Costs
Mg  Mg2+ + 2e
O + 2e  O2
H = 2200 kJ
H = 900 kJ
H = 3100 kJ
Energy Savings
Mg2+ + O2  MgO
H = 3900 kJ
Bottom Line
Htotal = 3100 kJ – 3900 kJ = 800 kJ
Ionic bonds form because the end result is a lower energy
arrangement (Htotal is negative).
QQ
1 2 ,
r
where Q1 and Q2 = ion charges (this is Coulomb’s law).
Note that the energy of attraction of an ionic bond 
Question: The ionic bond between [Mg3+][ O3] would have a
larger energy of attraction due to the higher charges, but it
doesn’t form. Why?