when solids form, the particles have to arrange them selfs in a specific way. we can describe these using different types of packing . packing describes how layers of spheres most effeicnetly fit together.
if you where to arrange a layer of spheres they would exhibit hexagonal close packing which is when each sphere is surrounded by six others
depending on how the layers on top of the plane arrange themselves, different types of structures can arise
ABA close packing describes a structure in which the second layer of spheres fits on top of the first but shifted in one direction.
after the second layer the third reverts to the same position as the first, leaving holes in the structure
hexagonal close packing has only 2 positions of spheres and changes between these 2 each layer. HCP results in hexagonal geometries
in cubic close packing, the third layer shifts to cover up the voids in the AB structure. this creates a structure comprised of 3 layers, all with different positions
CCP results in primitive, body centred and face centred cubic unit cells and we will meet these later.
unit cells are sections of a solid that, when repeated indefinitely, will produce a macroscopic crystal. these unit cells contain the smallest amount of information needed to describe the pattern of the whole structure.
these can come in a number of different shapes and with a number of different arrangements.