Mass number
The total number of protons plus neutrons in the nucleus of an atom. Electrons are too light to count.
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Analogy
It is like counting the heavy bricks in a wall and ignoring the dust.
Example
Chlorine-35Mass number = 35Protons = 17Neutrons = 35 - 17 = 18
In practice
To find neutrons, take the atomic number away from the mass number.
Practice
Flashcards
- How do you work out the number of neutrons?Mass number minus atomic number.
Questions people ask
Why is the mass number always a whole number?
It counts particles, and you cannot have half a proton or neutron. So the mass number of a single atom is always a whole number. The relative atomic mass on the periodic table can be a decimal because it is an average over many isotopes.
Do electrons count towards mass number?
No. An electron has about 1/2000 the mass of a proton or neutron, so its contribution is tiny. Mass number is only the protons plus neutrons in the nucleus. That is why losing or gaining electrons never changes the mass number of an atom.