Atomic mass

As the mass number reflects the sum of protons and neutrons in the nucleus of an atom, the Atomic mass, reflects the weight of these particles in the nucleus. As the weight of a proton and a neutron is almost equal to 1 amu in every atom, the atomic mass and mass number of an atom are almost the same.

The unit amu is used when defining the weight of an atom. One amu is exactly 1/12 of the mass of one carbon-12. Because each proton and each neutron contribute approximately one amu to the mass of an atom, and each electron contributes far less, the Atomic Mass of a single atom is approximately equal to its mass number (a whole number). In more details, a proton has the mass of 1.0073 amu, a neutron 1.0087 amu, and an electron 0.00055 amu.

However, the average masses of atoms of most elements are not whole numbers because most elements exist naturally as mixtures of two or more isotopes. The mass of an element shown in a periodic table or listed in a table of atomic masses is a weighted, average mass of all the isotopes present in a naturally occurring sample of that element. This is equal to the sum of each individual isotope’s mass multiplied by its fractional abundance.

Equation of average mass states that average mass equals sum of symbol with small i below, open bracket fractional abundance multiplied by isotopic mass close bracket with subscript small i