Question

What is the arrangement of spheres present in hexagonal close packing?

Answer 1

As we have learned in solids that they exist as amorphous or crystalline solids based on the packing of structure in the lattice space. These two differ in the arrangement of the ions and molecules of the element.

Complete answer:
In hexagonal close packing there are alternating layers of spheres or atoms arranged in a hexagon, with one additional atom at the center. A middle layer between these two atoms is in triangular form and fills the tetrahedral voids which are created due to the layer at the top and the bottom.
In a hexagonal close packing the arrangement is ABAB, where A and B represent different layers of the packing. Each sphere is in contact with the six other spheres. The top and the bottom layers contain six atoms at the corners of the hexagon and one atom is at the center of each hexagon. This is the layer ‘A’. The layer in between them, ‘B’ contains three atoms sandwiched between the atoms of top and bottom layers.
Hexagonal close packing gives the same density as that of cubic close packing since the addition of one sheet of spheres cannot affect the volume they occupy. The hexagonal close packing occupies the highest packing density of about $75\%$

Note:
Unit cell of the crystal is the smallest group of the constituent particles which has overall symmetry of the crystal and by repetition of it in three dimensions, the entire lattice is built. One unit of hexagonal close packing comprises six atoms.