An atomic nucleus is built from two major kinds of particles: protons and neutrons. A proton carries one unit of positive charge, which balances the negative charge on an electron. The neutron is uncharged. The standard unit for measuring masses of atoms is the atomic mass unit (amu) defined such that the most common kind of carbon atom weighs exactly 12 amu.

On this scale, a proton has a mass of 1.00728 amu and is slightly lighter than a neutron, which has a mass of 1.00867 amu. Protons and neutrons usually are thought of as having unit masses (1 amu) unless exact calculations are called for. On this scale, an electron weighs only 0.00055 amu. The charge and mass relationships between these three fundamental particles are summarized in the table to the left.