Back to Material Science
Atoms are the building blocks of all materials. They are put together in a great variety of ways and bonded or "held together" by cohesive forces in a manner characteristic of a particular material.
A crystal is a repeating array. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell). The most fundamental property of a crystal lattice is its symmetry. In three-dimensions, unit cells stack like boxes, filling the space, making the crystal. The different colors are just to show the separate boxes - each unit cell is identical.
Unit Cell: When a solid has a crystalline structure, the atoms are arranged in repeating structures called unit cells, which are the smallest units that show the full symmetry of a crystal.
Lattice: The three dimensional array formed by the unit cells of a crystal is called lattice.
When a crystalline solid starts to form from the molten or gaseous state, these unit cells will tend to stack in a three-dimensional array, with each cell perfectly aligned, and they will form a crystal. If crystals are growing in a melt at the same time, the crystals will eventually meet and form grains. The junction of the grains is called grain boundaries.
The majority of metals have one of three well-packed crystal structures:
Face-centered cubic (F.C.C.)
Body-centered cubic (B.C.C.