The mass of one atom is usually expressed in atomic mass units (amu), which is referred to as the atomic mass.

Rutherford’s gold foil experiment showed that atoms have a small, dense, positively charged nucleus; the positively charged particles within the nucleus are called protons.

Millikan discovered that there is a fundamental electric charge—the charge of an electron.

Thomson’s cathode ray tube showed that atoms contain small, negatively charged particles called electrons.