# Chemistry: Solid State: Calculation of Density of Unit Cell and Close Packed Structures of Solids (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Calculation of Density of Unit Cell

Density =

Volume of unit cell = a3 (if the edge length of cubic unit cell is ‘a’ )

Mass of an atom or molecule = (where M is molar mass of substance,

Atoms or molecules of the substance present/unit cell = Z

Mass of unit cell = (Number of atoms/molecules present/unit cell x mass of one atom/molecule) =

Density =

## Close Packed Structures of Solids

• In the process of the formation of a crystal the constituent particles are closely packed.
• The crystal structures of the solids are closely packed with identical spheres as shown in figure below.
• These are held together by forces of attraction.
• A linear horizontal arrangement of identical spheres in one dimension forms a row.
• A two-dimensional close packed structure can be obtained by arranging a number of such rows to form a layer.

This can be done in two possible ways they are:

We can align these rows so that each sphere is in contact with four other spheres. This arrangement in two dimensions is called as square close packing.