The phenomenon of the existence of an element in two or more forms having the same chemical properties but different physical properties. The different forms are called allotropic forms or allotropes.

The value of a diamond as a gem depends upon: Its weight The impurities present in it. The weight of a diamond is expressed in carats. 1 carat is equal to 0 . 2 gm .

A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a regular pattern. Common salt and sugar particles are of uniform shape and size. Hence, they are called crystals. The crystals of common salt, sugar, alum, etc. are obtained from the solutions of these substances in water by a process called crystallization.

Catenation is the ability of an atom to form bonds with other atoms of the same element. Carbon has the highest catenation power among all elements. Carbon combines with other carbon atoms to form long chains or rings, this property is called catenation.