An atom of sodium has a larger mass than an atom of neon. how is this possible when neon has so many more valence electrons?

what particles make up most of the mass of an atom? what takes up most of the volume of an atom?

The electrons contribute just about zero to the mass of an atom.
It takes more than 1,800 electrons to make the mass of one
proton or neutron.

The naturally occurring element with the most complex atom is
Uranium. That's element #92 , so a neutral uranium atom has
92 electrons.  It would take almost exactly 20 times that many
electrons to add the mass of one proton or neutron to the atom!
(And no other element has that many electrons in an atom of it.)