A carbon atom has four outer electrons that can each be paired with an outer electron belonging to another atom to make a strong covalent chemical bond. As well as bonding to other elements, a carbon atom can bond with another carbon atom. Different arrangements of carbon–carbon bonds produce allotropes (forms of the same element) with markedly different properties.
In the simplest arrangement, each carbon atom is surrounded by four others in a regular tetrahedron shape (1). This is diamond (2). Carbon–carbon covalent bonds are very strong and stiff, making diamond the hardest known mineral. It is rated 10.0 on the Mohs scale of hardness (defined by the ability of a harder mineral to scratch a softer one). Diamond is dense (about 3.5 g cm–3). Its electrons are locked within the covalent bonds, making diamond a good electrical insulator with resistivity of about 1018Ωm.
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