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.
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.