Many materials like carbon can exist in various forms for example it can exist as diamond in solid form and graphite is also one form of carbon and clearly dimond and graphite have different mechanical, thermal, optical and electrical properties. These properties of different forms of carbon can be understood in terms of carbon atoms in the solid structure.
The atoms having no regular arrangement of atoms in their structure are called amorphous solids and those having regular or orderly arrangement of atoms in their structure are called crystalline solids. Graphite is an amorphous solid and diamind is a crystalline one.
Crystalline solids or crystals have periodic arrangements constructed by infinite repetion of identical structural units in space. In simple crystals structural unit is a single atom but it could be group of atoms or molecules attached to every lattice point. Structure of all crystals is described in terms of lattice.
One of simple type of crystal structures is Sodium chloride is a crystal structure having face centered cubic crystal lattice. Its basis consists of one Na and one Cl atom separated by half the body digonal of a unit cube. Each atom in sodium chloride cryatal has six nearest atoms of opposite kind. Space lattice of diamond is a face centered cubic crystal. The conventional unit cube of a diamond crystal contains 8 atoms and the basis of diamond contains only one atom that is carbon atom. Thus each point of fcc lattice have two identical atoms attached as its primitive basis.
This post is purely an informational one and is beyond your course you intend to study for your board and competetive exams.

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