The atomic mass of an element is determined by the abundance of the element’s isotope. For example, neon-20 (with 10 protons and ten neutrons in its nucleus) has a mass of 19. There are 992 amu and a 90 percent abundance. Neon-21, neon-22, and neon-23 were all present in the neon 21.
Why Is Magnesium Atomic Number 12?
Magnesium is represented by the symbol Mg on the periodic table of elements. It has an atomic number of 12 because it has 12 protons in its nucleus.