Metallic Solids

Arrangement of metallic atoms with closest approach in a space lattice is called closed packing which can be divided in two categories :

I. 2-D close packing: Packing only in one plane which may be in two different ways : Square close packing and Hexagonal close packing as shown in the diagrams below:
 

Square close packing (2D) Co. No=4 Packing fraction = 0.524	Hexagonal close packing (2D)  Co. No. = 6 Packing fraction = 0.604

II. 3-D close packing: In 3D packing, arrangement of spheres of second layer is in depression of Ist layer that leads to appearance of two different types of depressions (gaps/voids) marked as a and b below:
 

Dark - Ist Layer Dotted sphere - IInd layer

Now, when we arrange the IIIrd layer we have two options for a regular ordered pattern.

Hexagonal Closed Packed Structure (HCP)

Arrangement of spheres of IIIrd layer in 'a' type of depression will bring Ilird layer of spheres exactly above the Ist layer i.e. Ist and IIIrd layer spheres will coincide (overlap), this 3D close packing is known as Hexagonal close packing. This is also known as AB-AB-AB... type packing because every third layer is identical to first one.
 

Top View
AB - AB type of Packing

Cubic Closed Packing (CCP)

Arrangement of spheres of IIIrd layer in b type of depression will generate a new layer which will coincide neither with I nor with ll. But fourth layer in the same order will overlap with Ist layer and it is also labelled as ABC - ABC... type packing.
 

Top View
ABC - ABC type of Packing

Features of CCP	Features of HCP
(1) Co-ordination number (C.N.) = 12 (2) Packing efficiency = 74% (3) Unit cell is FCC type (4) ABC-ABC type (5) Example: Ni, Cu, Ag, Au, etc.	Co-ordination number (C.N.) = 12 Packing efficiency = 74% Hexagonal primitive type AB - AB type Examples: Be, Mg, Zn, Cd, etc.