Almost all of the mass of an atom (more than 99%) is in its nucleus, so "atomic mass" is pretty much a measure of the size of the nucleus of an atom. The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size.
The sand provides a rough surface on top of the ice for the cars' tires to grip onto. It provides more friction. Salt melts the ice and often provides more friction (the disadvantage is that it eats concrete!).
