The standard unit for measuring the mass of atoms and their subatomic particles is a dalton, also known as an atomic mass unit (amu). A neutron has a mass of 1.008 daltons, and a proton has a mass of 1.0007 daltons. The mass of an electron, at 0.0005 dalton, is almost 2000 times smaller than the mass of a neutron or proton. The atomic mass (also called the atomic weight) of an element is the average mass of all its naturally occurring isotopes. Typically, the atomic mass of an element is close to the mass number of its most abundant isotope.