What is the difference between mass number and atomic mass?

1 Answer
Dec 8, 2016


The mass number is the value for an individual atom but atomic mass is the average of all the isotopes of an element.


An individual atom has a set number of protons and neutrons which each have a mass of 1 amu. This causes the mass number to be a whole number.

Not all of the individual atoms of a given element the same number of neutrons ( always the same number of protons). So the average mass of an element is rarely a whole number.

An example is Boron. Roughly 20% of Boron atoms have a mass of 10, having 5 protons and 5 neutrons. The other 80% of the Boron atoms have a mass of 11, having 5 protons and 6 neutrons. This gives an atomic mass for Boron of 10.8