Why is elemental carbon relatively unreactive at ordinary temperatures?

Elemental carbon is relatively unreactive at ordinary temperatures primarily due to its stable crystal structure. In its most common forms, diamond and graphite, carbon atoms are held together by strong covalent bonds within an extensive, rigid lattice.1

For a chemical reaction to occur, the activation energy—the minimum energy required to initiate a transformation—must be overcome. In the case of diamond, the structure consists of an infinite three-dimensional array where every atom is covalently bonded to four others in a tetrahedral arrangement; destroying this structure requires the rupture of many strong bonds. Similarly, the layers of fused hexagonal rings in graphite are highly stable. Because these structures are so robust, they remain chemically inert under ordinary conditions and only combine with oxygen at high temperatures.213