Oddo–Harkins rule

The Oddo-Harkins rule holds that elements with an even atomic number (such as carbon) are more common than elements with an odd atomic number (such as nitrogen).

This is due to the fact that elements with odd atomic number have one unpaired electron and are more likely to capture another neutron, thus increasing their atomic number. In elements with even atomic number, protons are paired, with each member of the pair offsetting the spin of the other, enhancing stability.