Oddo–Harkins rule
The Oddo–Harkins rule holds that an element with an even atomic number is more abundant than the elements with immediately adjacent atomic numbers. For example, carbon, with atomic number 6, is more abundant than boron (5) and nitrogen (7). This pattern was first reported by Giuseppe Oddo in 1914 and William Draper Harkins in 1917. Definition All atoms bigger than hydrogen are formed in stars or supernovae through nucleosynthesis, when gravity, temperature and pressure reach levels high enough to fuse protons and neutrons together. Protons and neutrons form the atomic nucleus, which accumulates electrons to form atoms. The number of protons in the nucleus, called atomic number, uniquely identifies a chemical element. The Oddo–Harkins rule argues that elements with odd atomic numbers have one unpaired proton and so are more likely to capture another, increasing their atomic number. It is possible that in elements with even atomic numbers, protons are paired, with each member of the pair balancing the spin of the other; even parity thus enhances nucleon stability. Exceptions to the rule This postulate, however, does not apply to the universe's most abundant and simplest element: hydrogen, with an atomic number of 1. This may be because, in its