An allotrope is one of several different physical forms of an element in the same state of matter.Diamond and graphite are both formed from carbon but, due to the different bonding present in the two substances, they have entirely different physical characteristics--they are both allotropes of carbon. The carbon atoms in graphite are arranged in flat sheets that slide easily over each other, while the atoms in diamond are bonded in a complex, honeycombed structure that makes the solid much harder. The difference between the allotropes may be in the bonding present as in the diamond carbon example or it may be due to the crystal form produced as is common with most metals. When compounds show different characteristic forms they are termed polymorphic.

In 1830, Swedish chemist Jöns Berzelius (1779-1848) questioned whether a given element could exist in two or more forms with different chemical...