Coulomb's Law yields an expression for the energy of interaction for a pair of point charges.
V=2.31E-19 Q1 Q2 /r (nm)
V is the energy (in J) required to bring the two charges from infinite distance separation to distance r (in nm).
Q1 and Q2 are the charges in terms of electrons.
(i.e. the constant in the above expression is 2.31×10-19 J nm electrons-2)

For a group of "point" charges (e.g. ions) the total energy of interaction is the sum of the interaction energies for the individual pairs.
Calculate the energy of interaction for the linear arrangement of ions shown in the diagram below.

d = 0.400 nm.