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?

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.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.