The of an isotope of an element is equal to the number of protons plus the number of neutrons present in the nucleus. This is typically shown after a dash in the name of the isotope. For example, the mass number of hydrogen-1 is “1” and the mass number of oxygen-17 is “17”.
To find the number of protons and neutrons in an isotope from the mass number, you’ll need to do the following:
The number of protons can’t be obtained by the mass number itself, but from the name of the element. As you may already know, the number of protons of an element is the same as the . For example, oxygen has an atomic number of 8, which means it has 8 protons.
The number of neutrons in an isotope is equal to the mass number minus the number of protons. As a result, oxygen-17 has (17-8=) 9 neutrons.