# Question #55e72

Mar 29, 2016

Ionic compounds are made up of atoms that have gained or lost electrons to form ions, which are charged particles.

Opposite charges attract each other and these electrostatic forces of attraction (coulombic forces) are what hold the particles together in the crystal lattice structure.

By Coulomb's Law in Electrostatics, the magnitude of the electrostatic force $F$ between any 2 charged particles ${Q}_{1} \mathmr{and} {Q}_{2}$ is directly proportional to the product of the 2 charges and inversely proportional to the square of the distance $r$ between them, and related by Coulomb's constant $k = 9 \times {10}^{9} N . {m}^{2} / {C}^{2}$.
Thus $F = k \frac{{Q}_{1} {Q}_{2}}{r} ^ 2$.

Furthermore, the direction of this electrostatic force is determined from the theory of electrostatics - like charges repel each other and unlike charges attract each other.

Now if we take an ionic compound example sodium chloride (NaCl) in which 1 atom has been reduced and gained electrons to form negative ions ($C {l}^{-}$ in this case), and the other has been oxidized and lost electrons to form positive ions ($N {a}^{+}$ in this case), then these 2 ions are unlike charges as they are opposite in sign and so from the theory of electrostatics they will attract each other. The one side of the Nacl particle is then positively charged and the other side negative.

By the same principle, nearby NaCl particles are then also attracted abd eventually an entire crystal lattice is built up, and the intermolecular forces holding them together are precisely these coulombic forces of attraction.

Note - ionic bonds hold the individual atoms together, intermolecular forces hold the molecules together in a crystal lattice.