Why is the mass of an Na atom and an Na+ ion "nearly" equal?
An Na+ ion is a sodium atom that has lost one electron as that makes the number of electrons in the atom equal to that of the nearest Nobel gas Neon which has 10 electrons. This completely fills the 1st and 2nd electron shells.
The sodium atom has 11 protons, 11 electrons and 12 neutrons. On the other hand Na+ has 11 protons, 10 electrons and 12 neutrons.
The mass of the two differ by only one electron which has a mass of 9.1*10^-31 kg. This is negligible as compared to the mass of the other particles in Na and Na+. The neutrons and protons have a mass of 1.67*10^-27 kg each which is around 2000 times heavier than that of the electron and there are 23 of these in all in the sodium atom.
This makes the mass of Na and Na+ nearly equal.