Solids can be broadly classified as either crystalline or non crystalline. In a crystal the atoms are arranged in a periodic manner in all the three directions whereas in non-crystalline solid the arrangement is random. A crystalline solid can either be a single crystal, where the entire solid consists of only one crystal, or an aggregate of many crystals separated by well defined boundaries called polycrystalline. -              Non-crystalline substances known as amorphous having same physical properties in all directions are called isotropic substances. They do not posses any regular shape and they have wide range of melting point. A crystal has regular shape and when it is broken all broken pieces have same regular shape. A crystal have sharp melting point. Since the crystals may have different periodic arrangements in all three direction and therefore they are called anisotropic substances. Crystalline solid may be made up of metallic crystal or non metallic crystal. Copper, silver, aluminum, tungsten and magnesium are the examples of metallic crystals while carbon crystallized polymers and plastics are examples of non metallic crystals. The thermal, electrical, mechanical and optical properties of crystal depend mainly on their structural properties. X-ray diffraction methods and neutron diffraction methods are mainly used for study of crystal of solids. The atomic arrangement in a crystal is called crystal structure. It is very in convenient to imagine periodic arrangement of points in space about which these atoms are located. This leads to the concepts of space lattice. A space lattice is defined as an infinite array of points in three dimensions in which every point has surrounding identical to that of every other point in the array. A space lattice can be defined by referring to a unit cell. The unit cell is the smallest cell. The unit cell is the smallest unit which, when repeated in space indefinitely, generates the space lattice

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