The packing of identical spheres to fill space is the simplest struct ural model for a solid when the inter-atomic or molecular forces are non directional. The binding may be strong, as in the metallic state; or it may be weak, as is obviously the case for species that condense to solids only at low temperatures and/or high pressures - for example, noble gas atoms (mutually attracted only through the London dispersion force) and spherical top molecules (bound together by the various Van der Waals forces, and often still freely rotating). Sphere packing of ions will be considered later.
The sphere radius represents the contact limit; at smaller inter nuclear distances the repulsive forces increase sharply. The energy is minimised when the spheres are in contact and the number of near neighbours (coordination number) is maximised. This corresponds to fill ing the space as efficiently as possible.
In your drawings, do follow the colour scheme suggested and note that only freehand sketches are required. Do not dismantle any models you make if at all avoidable; they will be needed for comparison purposes later in the practical. You should have your answers to each section checked by the Junior Demonstrator before proceeding.