Re: An amto of sodium has a larger mass than an atom of neon.

A sodium atom has only 1 valence electron, and 10 core electrons -- 11 electrons total.
A neon atom has 8 valence electrons, but only 2 core electrons -- 10 electrons total.

The number of protons in any atomic nucleus is equal to the total number of electrons (for a 
neutral atom).

Sodium has 11 protons and 12 neutrons, so its atomic mass is approximately 23.
Neon has 10 protons and 10,11, or 12 neutrons (3 stable isotopes), so it has different types of 
atoms with masses 20,21, and 22. In ordinary neon there is about 91% mass 20, 9% mass 22, and 
only a little bit of mass 21, so its atomic mass comes in at 20-and-a-bit.