How is Atomic Mass Calculated?

The atomic mass of an element is calculated by adding the number of protons and neutrons in the nucleus of an atom.

In more detail, the atomic mass is the weighted average of the masses of all the isotopes of an element, where each isotope's mass is multiplied by its relative abundance.

Mathematically, we can represent the calculation of the atomic mass of an element as follows:

Atomic mass = (isotope mass 1 x abundance 1) + (isotope mass 2 x abundance 2) + ... + (isotope mass n x abundance n)

where:

isotope mass refers to the mass number (the total number of protons and neutrons in the nucleus) of each isotope of the element

abundance refers to the proportion of each isotope in a naturally occurring sample of the element

For example, the atomic mass of carbon is calculated as follows:

Atomic mass of carbon = (mass of carbon-12 isotope x abundance of carbon-12) + (mass of carbon-13 isotope x abundance of carbon-13)

= (12 amu x 0.9893) + (13.00335 amu x 0.0107) [where amu = atomic mass unit]

= 12.011 amu

This means that on average, a carbon atom has a mass of 12.011 atomic mass units, taking into account the relative abundance of its isotopes.