Oddo-Harkins Rule
Elements having even atomic number (Z), even neutrons (N) are more stable, more abundant and richer in isotopes and never have less than three stable isotopes. Elements with odd atomic number and odd neutrons 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 protons (P) and neutrons (N) in the nucleous 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. Other examples are oxygen (atomic number 8) is more abundant than either nitrogen (atomic number 7) or fluorine (atomic number 9). Calcium (20) is more abundant than potassium(19) or scandium (21). Hydrogen and beryllium are the exceptions of this rule.
Explanation of the Oddo-Harkins Rule
Atoms form when protons and neutrons bind together and form an atomic nucleus. For most elements, this happens when the immense temperature, pressure, and gravity within a star fuses protons and neutrons together. An element's atomic number is the number of protons in its atom.
One explanation for the higher abundance of even-numbered elements is that helium(atomic number 2) is a major building block for element formation. Fusion of helium nuclei builds subsequent even atomic number elements.
Another explanation is that even atomic numbers mean protons are paired within the nucleus. Parity makes the nucleons more stable, as the spin of one proton offsets the spin of the other. Unpaired protons (odd number elements) more easily capture another proton and form an even-numbered atom.
Exceptions to the Oddo-Harkins Rule
Elements that serve as two exceptions to the Oddo-Harkins rule are hydrogen (atomic number 1) and beryllium (atomic number 4). Hydrogen is the most abundant element in the universe. It is more abundant than helium because of its creation in the Big Bang. However, stars continuously fuse hydrogen into helium. In the distant future, hydrogen will follow the even-odd rule. Beryllium is even more rare than lithium (atomic number 3) and boron (atomic number 5), even though the primary source of the all three elements is cosmic ray spallation. Scientists believe beryllium does not follow the rule because it only has one stable isotope. Lithium and boron each have two stable isotopes.