How Many of Them are There?

Well it's different for every atom and some atoms come in different versions called isotopes. Before reading this page you'll find it helpful to have a Periodic Table handy.

By the end of this page you should be able to:

  • Determine the number of protons, neutrons and electrons within an atom
  • Define an isotope
  • Describe the difference between an isotope and the most common form of an element


Look at the symbol for fluorine to the right - it has two numbers, 9 and 19. The smaller of the two, 9, is the proton number (or atomic number - they're the same thing), so fluorine has 9 protons. In an atom the number of protons is the same as the number of electrons, so a fluorine atom also has 9 electrons!!!! Well, that was easy.

What about neutrons? Here you need to use both numbers, the larger number is called the mass number and it's the number of protons AND neutrons. So you have to subtract the number of protons to get the neutrons on their own. 19-9=10 neutrons. Ta Da!

Explanation of Periodic Table Symbol for Fluorine

Check Your Understanding

The table below contains a few examples - use a periodic table to check that you can do this - it's guaranteed that you need this skill for several questions in the exam. For the missing values - click on the question mark to reveal the answer.

Element Number of Protons Number of Electrons Number of Neutrons

Isotopes, He's My Brother By Another Mother

The number of neutrons in an atom can vary a little bit. So Oxygen for example comes in two forms, called isotopes, one has ten neutrons and the other has 8. Their symbols are shown in the diagram to the right, notice that they're named by writing the element name plus the mass no.. Isotopes have the same number of protons and electrons but a different number of neutrons.

A common exam question is to describe the difference between two isotopes. So for Oxygen-18 and Oxygen-16 you should write that they both have 8 protons and 8 electrons but Oxygen-18 has 10 neutrons while Oxygen-16 had 8 neutrons.

Isotopes of Oxygen

Why Does Chlorine Have a Relative Atomic Mass of 35.5?

Chlorine has a relative atomic mass of 35.5 according to the Periodic Table which would give it 17 protons, 17 electrons and 18.5 neutrons. You can't have half a neutron so what's going on? The atomic mass of chlorine is an average. 75% of chlorine atoms have a mass of 35 while 25% of chlorine atoms have a mass of 37 - it's (stratified) average of the two. This calculation will be explained in detail in the calculations unit.

Mass Number or Relative Atomic Mass? (HT)

Now's a good time to talk about the difference between the two. The mass number is the number of protons and neutrons within an atom, i.e. Cl-35 has a mass number of 35. The relative atomic mass is an average of the different naturally occuring isotopes, so chlorine has a relative atomic mass of 35.5. This will be covered again in the calculations unit.

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