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 momentμn = 0, and 13C (1.1% ofnatural carbon) with I = ½ andμn =0.7024μN(μN is the nuclear magneton), see Radzig& Smirnov (1985). Like most of the chemical elements, it originates fromnucleosynthesis 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–protonchain reaction, which results in the synthesis of one α-particle (nucleus4He) from four protons, p. Further nuclear fusion reactions mightlead to the formation of either of the isotopes 5He and5Li (p + α collisions) or of 8Be (α +α collisions); however, all these nuclei are very unstable. As was firstrealized by F. Hoyle, the chemical evolution does not stop at helium only due toa lucky coincidence – the nucleus 12C has an energy levelclose enough to the energy of three α-particles, thus, thetriple fusion reaction 3α → 12C,being resonant, has a high enough probability. This opens up a way to overcomethe mass gap (the absence of stable isotopes with masses 5 and 8) and providesthe prerequisites for nucleosynthesis up to the most stable nucleus,56Fe; heavier elements are synthesized in supernovaexplosions.