INTER PARTICLE BONDING
There several types of bond.

1. Ionic bond, the atoms involved are ions thar are attracted together by electostatic forces.
2. Covalent bond, atoms electrons have distorted orbits so that they are spread between the two atoms that are bonded.There is a hazy line between covalent and ionic as the attraction of the electrons becomes biased to one atom.
3. Van der Waal's forces, are weaker than the above two, and result from instantous weak ions formed by the random movement of electrons.

BOND LENGTH
There is a balence between repulsion and attraction for bonds that accounts for the packing of atoms in cyrstal structures. The atoms always seek to be at the point of least potential energy. If we assume the molecules in a crystalline solid are each at the corner of a cube, so that in each direction there is a molecule at the same distance r and measure its density to be d, we can calculate the volume per molecule and hence the distance between the centres of the atoms.

Notes:

• There are other packing arrangements other than cubic, but you will only be asked to caculate ones like above.