Nucleon number
We used to call the nucleon number the “mass number”.
The nucleon number is the mass of one mole of a specific substance.
Calculating nucleon number
The nucleon number of an atom is the total number of protons and neutrons in the atom.
\text{Nucleon number} = \text{Number of protons} + \text{Number of neutrons}
Notation
We usually represent the nucleon number with the letter
Why add the protons and neutrons?
We established that electrons have almost no mass. When calculating the mass (which is related to the nucleon number) of an atom, we can ignore the electrons and just add up the protons and neutrons, which are much heavier and so contribute almost all of the mass of the atom.
And the protons and neutrons have almost exactly the same mass, so we can just add the two.
Relative mass
It’s important to note that this calculated mass is the relative mass of the atom, compared to a single proton. It isn’t the absolute mass.