Oddo–Harkins Rule

Oddo–Harkins Rule

Elements having even atomic number (Z) and even N are more stable, more abundant and richer in isotopes and never have less than three stable isotopes. Elements with odd Z and odd N are less stable, less abundant and poorer in isotopes and never have more than two isotopes, generally only one isotope. This shows that there is a tendency for N and P in the nucleus to pair up i.e. to be even.
For example, carbon, with atomic number six, is more abundant than boron atomic number five and nitrogen atomic number seven. This pattern was first reported by Giuseppe Oddo in 1914 and William Draper Harkins in 1917.
This rule does not apply to the universe's most abundant and simplest element: hydrogen, with an atomic number one, may be because, in its ionized form, a hydrogen atom becomes a single proton.