Amorphous solids are structures that are rigid but lack a well - defined form. They have no geometric shape. Hence, they are not crystalline. This is why, like crystals, they don't have edges. An amorphous solid's most common example is Glass. Also other good examples of amorphous solids are gels, plastics, various polymers, wax and thin films.
Generally speaking, a solid is said to be a crystal if the constituent particles (atoms, ions or molecules) are arranged periodically in three dimensional ways or simply have a reticular structure. The atoms are stacked regularly in crystalline solids, forming a 3-D pattern that can be obtained through a 3-D repetition of a particular pattern unit. It has long-range orderness and thus has definite properties such as a sharp melting point. So we can say that crystal is a periodic array of atoms in three dimensions. The external geometric shape of the crystal often remains unchanged when the crystal grows under a constant environment. The shape is therefore a consequence of the internal arrangement of the component particles. An infinite 3D repetition of identical units, which can be atoms or molecules, is the ideal crystal. All ionic solids are crystalline, as are most covalent solids. Under normal circumstances, all solid metals are crystalline.
When a group of atoms or molecules is attached identically to each lattice point, a crystal structure is formed. This group is called the basis of atoms or molecules. The crystalline lattice can be reproduced in three dimensions by translating the unit cell. The unit cell is the unique part of the crystal structure that generates the entire crystal structure when translated along parallel lines.
A lattice is a regular periodic array of space points that replace objects with imaginary points. It may be considered to remove the atom, but there is still the center. The Bravais and the non - Bravais are two classes of lattices. All lattice points are equivalent in a Bravais lattice and therefore necessarily all atoms in the crystal are of the same type. On the other hand, some of the lattice points are not equivalent in a non - Bravais lattice. Non - Bravais lattices are often referred to as base-based lattices. The basis is a set of atoms which is located near each site of a Bravais lattice.
|Unit Cell||Atoms per cell||Packing Fraction||Lattice Constant||Coordination Number|