Note that you can work out the number of neutrons by subtracting the atomic number from the mass number. In this example, it is 35 – 17 = 18 neutrons.

For example, the mass number of argon atoms and calcium atoms can both be 40. The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.

For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...