Why can no single atom of neon have atomic mass 20.18?

Answer 1

The mass of an atom is determined by the number of protons and neutrons, which each have a mass, by definition, of 1 AMU. The mass of the electrons in the surrounding cloud doesn't add enough to change the value within a reasonable number of decimal places.

Since the nuclear particles have a mas value of 1.0000..., the mass of a single atom has no decimal places. The decimal place values on the Periodic Table come from the percentages of isotopes occurring naturally changing the collective mass of the element. This gets important when we work with elements in gram amounts. In general, atomic weights (or atomic masses) of given elements that you see on the periodic table are actually weighted averages across the naturally occurring isotopes of that element.

So if you look up on WebElements.com what the isotopic abundances of neon are, you will find that there is 90.48% of neon-20, 0.27% of neon-21, and 9.25% neon-22. The atomic weight of 20.18 grams per mole comes from (0.9048 x 20) + (0.0027 x 21) + (0.0925 * 22).

So a large sample of neon will act as though its mass were 20.18 grams per mole, but any individual atom of neon will have a mass close to 20, 21 or 22 grams per mole.

Answer 2

The Atomic Mass of an element is the average mass of all its isotopes considering their natural abundances. Neon has two stable isotopes, neon-20 and neon-22, with atomic masses of 19.99 and 21.99, respectively. Therefore, the atomic mass of neon is an average of these isotopic masses, which is 20.18.