The carbon atom

Carbon is the sixth element in the Periodic Table. It has two stable isotopes, 12C (98.9% of natural carbon) with nuclear spin I = 0 and, thus, nuclear magnetic moment μn = 0, and 13C (1.1% of natural carbon) with I = ½ and μn = 0.7024μN (μN is the nuclear magneton), see Radzig & Smirnov (1985). Like most of the chemical elements, it originates from nucleosynthesis in stars (for a review, see the Nobel lecture by Fowler (1984)). Actually, it plays a crucial role in the chemical evolution of the Universe.

The stars of the first generation produced energy only by proton–proton chain reaction, which results in the synthesis of one α-particle (nucleus 4He) from four protons, p. Further nuclear fusion reactions might lead to the formation of either of the isotopes 5He and 5Li (p + α collisions) or of 8Be (α + α collisions); however, all these nuclei are very unstable. As was first realized by F. Hoyle, the chemical evolution does not stop at helium only due to a lucky coincidence – the nucleus 12C has an energy level close enough to the energy of three α-particles, thus, the triple fusion reaction 3α → 12C, being resonant, has a high enough probability. This opens up a way to overcome the mass gap (the absence of stable isotopes with masses 5 and 8) and provides the prerequisites for nucleosynthesis up to the most stable nucleus, 56Fe; heavier elements are synthesized in supernova explosions.