What is the mass number of an atom?

The mass number is the total count of protons plus neutrons in an atom’s nucleus.

Think of an atom like a tiny Lego tower where protons and neutrons are the solid blocks stacked in the middle; the mass number tells you how many of those blocks are there altogether. Electrons float around the tower but are so light they do not change the mass number. Protons each have a positive charge and neutrons are neutral, but for mass number we only care about counting them, not their charge. Different atoms of the same element can have different mass numbers because they can have different numbers of neutrons.

A common mistake is thinking the mass number is the same as the atomic number, but the atomic number counts only protons and not neutrons; to fix this, remember atomic number names the element while mass number tells how heavy the nucleus is. Another error is including electrons when adding up particles, which makes the count too large; remind yourself that electrons do not add to the mass number. Some students confuse mass number with atomic mass from the periodic table, which is an average that includes naturally occurring versions; check whether you need a whole number count for mass number or a decimal average for atomic mass.

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Mass Number: Protons Plus Neutrons

1. Count protons and neutrons only
2. Ignore electrons for mass
3. Protons positive, neutrons neutral
4. Mass number = total nucleus blocks

Common Mass Number Mistakes

1. Confusing mass number with atomic number
2. Counting electrons in mass number
3. Mixing mass number with atomic mass

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