How do you calculate the atomic mass and what is its unit of measurement?

Answer 1

Usually you don't "calculate" the Atomic Mass, you measure it (okay, there are some calculations involved). You can approximate the atomic mass by adding the number of protons and neutrons; at this level of precision you can disregard the electrons since they're so light compared to the neutrons and protons, and the answer won't be "right" anyway because you're also ignoring binding energy.

The unit is formally "u" or "Da" (daltons). 1 u = 1 Da = 1/12 the mass of an isolated carbon-12 atom in its nuclear and electronic ground state. For some reason, biologists and biochemists seem to have embraced the "dalton" more rapidly than chemists, who generally still use the archaic abbreviation "amu" even when what they really mean is u (1 amu is approximately 1 u, but there was some ambiguity in the term because it was defined differently in chemistry and physics).

Answer 2

The atomic mass of an element is calculated by taking the weighted average of the masses of its isotopes, based on their relative abundance. The unit of measurement for atomic mass is the atomic mass unit (amu) or unified atomic mass unit (u). 1 amu is defined as 1/12th the mass of a carbon-12 atom, which is approximately equal to the mass of a proton or a neutron.