The document discusses different arrangements for closely packing spheres and their applications to crystal structures. There are three main arrangements discussed: hexagonal close packing (HCP), cubic close packing (CCP), and body centered cubic (BCC). HCP and CCP both efficiently fill 74% of space, with HCP stacking spheres in an A-B-A-B pattern and CCP stacking in an A-B-C-A-B-C pattern over four layers. BCC has a coordination number of 8 and two atoms per unit cell. These arrangements can be used to model the bonding and structure of metallic crystals.