Calculating the number of neutrons The mass number tells us how much mass is in the atom. Because electrons are very tiny (practically without mass), the mass number is used to tell us the combined total of protons and neutrons in the nucleus of the atom. Helium has a mass number of 4. This means that there are 4 particles in the nucleus. Because helium has an atomic number of 2, two of these particles must be protons. This leaves another 2 particles (4 – 2 = 2). These are the neutrons. Therefore:
Number of neutrons = Mass number – Atomic number
Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus.
Isotopes Most periodic tables do not give the mass number as a simple whole number. This is because of isotopes. Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus. The table below shows two isotopes of carbon. Carbon 12 has six neutrons and Carbon 14 has eight neutrons. Both are isotopes of the element carbon.
Isotope Atomic number
Number of protons Mass number
Number of neutrons Carbon 12
6 6
12 12 – 6 = 6 Carbon 14
6 6
14 14 – 6 = 8
The atomic mass of an element on the Periodic Table is the average mass of the atoms of that element. Because there are a small number of the heavier carbon-14 isotopes, the atomic mass of carbon is 12.01. For most calculations, however, scientists usually round the atomic mass to the nearest whole number.
Activity How many protons, electrons and neutrons?
Write down how many protons, electrons and neutrons are contained in a neutral atom of each of the elements lithium, potassium and nitrogen, where the upper number is the atomic number and the lower number is the mass number.
