Page "Stable isotope" Paragraph 24

Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons ( both protons and neutrons ) exhibit a lower energy state when their number is even, rather than odd.

This stability tends to prevent beta decay ( in two steps ) of many even-even nuclides into another even-even nuclide of the same mass number but lower energy ( and of course with two more protons and two fewer neutrons ), because decay proceeding one step at a time would have to pass through an odd-odd nuclide of higher energy.

This makes for a larger number of stable even-even nuclides, up to three for some mass numbers, and up to seven for some atomic ( proton ) numbers.

Conversely, of the 253 known stable nuclides, only five have both an odd number of protons and odd number of neutrons: hydrogen-2 ( deuterium ), lithium-6, boron-10, nitrogen-14, and tantalum-180m.

Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40, vanadium-50, lanthanum-138, and lutetium-176.

Odd-odd primordial nuclides are rare because most odd-odd nuclei are highly unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.