Mass number is an integer equal to the sum of the number of protons and neutrons of an atomic nucleus.
The mass number allows enable us to tell the difference between elements with a different number of neutrons, called isotopes.
The mass number is a count and is unitless.
Formula to calculate mass number.
The hydrogen atom doesn’t have a neutron but it has 1 proton, so the mass number of hydrogen is 1.
Oxygen atom has 8 protons and 8 neutrons. Determine the mass number.
Thus, oxygen atom mass number is 16.