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.

and the sum of the protons and neutrons gives the atomic mass, the number of neutrons defines the isotope of that element. For example, most carbon (≈ 99 %) has 6 protons and 6 neutrons and is written ...

This means the isotopes will have the same atomic number but a different mass number. There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium (hydrogen-3): Carbon has ...