Why Diamonds Have a Higher Density Than Graphite

Carbon Allotropes

• Pure carbon exists in different forms. Allotropes are types of pure carbon that differ in the way the atoms are arranged. Graphite and diamond are two carbon allotropes with different densities. The density of graphite ranges between 1.9 and 2.3 grams per cubic centimeter. The range of diamond density is 3.5 to 3.53 grams per cubic centimeter.

Graphite Structure

• The atoms of graphite are arranged in a crystalline pattern. Graphite molecules are composed of layered sheets of atoms. Covalent bonds hold the atoms in each sheet together in a hexagonal array. Each ring of six carbon atoms is bonded to its three nearest neighbors. Weak Van der Waals forces, rather than chemical bonds, hold the layers together.

Diamond Structure

• Diamonds are also crystalline structures. The atoms of diamond molecules are arranged in a tetrahedral shape. Each carbon atom is bonded to its four closest neighbors. The angle of these bonds is 109 degrees. When four atoms are bonded at this angle they form a tetrahedron. These tetrahedral-shaped molecules share interlocking bonds give diamonds greater density than graphite.

High Pressure

• Graphite is more stable than diamonds under normal atmospheric pressure in part because the carbon atoms are double-bonded and evenly spaced. When high pressure is applied to graphite, diamond can form because it is more stable at high pressure than graphite. The high pressure compresses the atoms in graphite and pushes them more closely together. Under high pressure the carbon atoms gain stability by rearranging into diamond's tetrahedral structure. As the atoms are pushed together, the substance increases its mass per volume ratio and becomes more dense.