Atomic carbon, systematically named carbon and λ⁰-methane, is a colourless gaseous inorganic chemical with the chemical formula C (also written [C]). It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.

Atomic carbon is the simplest of the allotropes of carbon, and is also the progenitor of carbon clusters. In addition, it may be considered to be the monomer of all (condensed) carbon allotropes like graphite and diamond.

The trivial name monocarbon is the most commonly used and preferred IUPAC name. The systematic name carbon, a valid IUPAC name, is constructed according to the compositional nomenclature. However, as a compositional name, it does not distinguish between different forms of pure carbon. The systematic name λ⁰-methane, also valid IUPAC name, is constructed according to the substitutive nomenclature. Along with monocarbon, this name does distinguish the titular compound as they derived using structural information about the molecule. To better reflect its structure, free atomic carbon is often written as [C]. λ²-methylium ([CH]⁺
) is the ion resulting from the gain of H⁺
by atomic carbon.

A Lewis acid can join with an electron pair of atomic carbon, and an electron pair of a Lewis base can join with atomic carbon by adduction:

Because of this donation or acceptance of an adducted electron pair, atomic carbon has Lewis amphoteric character. Atomic carbon has the capacity to donate up to two electron pairs to Lewis acids, or accept up to two pairs from Lewis bases.

A proton can join with the atomic carbon by protonation:

Because of this capture of the proton (H⁺
), atomic carbon and its adducts of Lewis bases, such as water, also have Brønsted–Lowry basic character. Atomic carbon's conjugate acid is λ²-methylium (CH⁺
).

Aqueous solutions of adducts are however, unstable due to hydration of the carbon centre and the λ²-methylium group to produce λ²-methanol (CHOH) or λ²-methane (CH
₂), or hydroxymethylium (CH
₂OH⁺
) groups, respectively.