The Physics of a Crystal
Most simple chemical compounds consist of crystals.If you were to examine a lump of crystalline material very closely,you would be able to see tiny individual crystals.All crystals have a definite geometric shape,determined by the way the atoms are linked together.
Mineralogists recognize 32 different classes of crystal,which are grouped into seven crystal systems.Crystal systems are described by their axis,which are imaginary lines that join the center of opposing faces of a crystal.For example,if a cubic crystal has three sets of opposing faces,it has three axes.They are all of equal length and are all at right angles to each other.
Cubic crystals are described as being isometric,but not all isometric crystals are plain cubes.If the corners of a cube are cut off,the result is a polyhedron with six octagonal faced and eight triangular ones.
Aside from the isometric system,there is also; a tetragonal,orthorhombic,monoclinic,triclinic and hexagonal system.
Crystals may form when a solution of a substance evaporates.Crystals may also form when a vapor or molten substance solidifies.
Allotropes are different forms of the same element.For Example,oxygen has two allotropes,normal oxygen and ozone.Pure carbon also has two allotropes,diamond and graphite.A crystal of diamond is in fact a single giant molecule in which every carbon atom is linked to four other,by four equal,strong bonds.These bonds are arranged tetrahedrally round each atom and there are no planes along which the giant molecule can easily be split.This quality is what gives diamond its tremendous hardness.
Graphite on the other hand,consists of flat sheets of carbon atoms arranged in adjoining rings.Each carbon atom is linked to three others by fairly strong bond that all lie in the same plane.These can easily be overcome,allowing the sheets to slide over each other.As a result,graphite is a soft slippery substance that can be used as a lubricant.
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