Why are the elements carbon, oxygen, neon, and magnesium, whose masses are multiples of four, as well as the element iron, so common on Earth?

Answer:Elements of even atomic number and mass number are more stable than elements of odd atomic and mass numbers

Explanation: The fact that elements possessing even number of nucleons are more stable and abundant suggests that nucleons are paired in the nucleus. This even number of nucleons is commonly called a magic number because bit is associated with extra stability, abundance of elements and plenitude of isotopes of such elements.

These magic numbers are often numbers which are multiples of four.