I don't think I'd agree that you can characterize the amplitude of a wave according to the density at only one point in it. After all ... a tiny wave in steel would be much denser at a compression than a huge wave in air would be.
The amplitude of any wave is described as the difference between a peak and the resting value. Or even better ... half of the difference between a maximum and a minimum.
So if you're looking at a longitudinal wave, like sound, I'd say if you want to describe its amplitude, then you have to look at the density at two points ... either the difference between the compression and the resting densities, or the difference between the greatest compression and the greatest rarefaction.
The information given in the question tells us about the number of electrons in an atom and also the number of shells in the atom. So, we will come to know about the atomic number, size and chemical properties of the atom. But we cannot determine atomic mass. Atomic mass is a function of number of neutrons and protons.
