Exercises 14. Which contributes more to an atom's mass: electrons or protons? Which contributes more to an atom's size?
Protons, which are nearly 2000 times more massive than electrons, contribute much more to the mass of an atom. Protons, however, are held within the atomic nucleus, which is only a tiny tiny fraction of the volume of the atom. The size of the atom is spelled out by the electrons which sweep throuh a relatively large volume of space surrounding the nucleus.