The atomic mass represents the total mass of an atom, primarily coming from the combined mass of its protons and neutrons in the nucleus, with a very small contribution from electrons. It is essentially the quantity of matter contained in an atom
. Atomic mass is usually expressed in atomic mass units (amu) or daltons (Da), where 1 amu is defined as one-twelfth the mass of a carbon-12 atom
. Because of the nuclear binding energy, the actual atomic mass is slightly less than the sum of the masses of protons, neutrons, and electrons
. In practical terms, atomic mass is often approximated by adding the number of protons and neutrons in the atom, since electrons have negligible mass
. This sum is also known as the mass number, but atomic mass can be a weighted average of all isotopes of an element, reflecting their relative abundances
. In summary:
- Atomic mass is the measure of the total mass of an atom.
- It mainly includes protons and neutrons, with electrons contributing very little.
- Expressed in atomic mass units (amu) or daltons (Da).
- Slightly less than the sum of individual particle masses due to mass defect from nuclear binding energy.
- Often approximated by the sum of protons and neutrons (mass number).
- For elements, atomic mass can represent the average mass of all isotopes weighted by abundance
