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Carbon's covalent radii are normally taken as 77.2 pm (C–C), 66.7 pm (C=C) and 60.3 pm (C≡C), although these may vary depending on coordination number and what the carbon is bonded to.
In general, covalent radius decreases with lower coordination number and higher bond order.
Thus, irrespective of its allotropic form, carbon remains solid at higher temperatures than the highest melting point metals such as tungsten or rhenium.
All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form.
They are chemically resistant and require high temperature to react even with oxygen.
Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds.
Graphite is much more reactive than diamond at standard conditions, despite being more thermodynamically stable, as its delocalised pi system is much more vulnerable to attack.