Chemguide: Support for CIE A level Chemistry


Learning outcome 2.1(d)

This statement expects you to be able to work out how many protons, neutrons and electrons there are in an atom or ion from basic information.

Before you go on, you should find and read the statement in your copy of the syllabus.


First read the page a simple view of atomic structure, paying particular attention to the terms atomic number, proton number, mass number and nucleon number. It is important that you can recognise all of these, even though, in the CIE syllabus, only proton number and nucleon number are used.

The important thing to notice is that the proton number (atomic number) counts the number of protons in the nucleus. In a neutral atom that also tells you the number of electrons. That isn't as simple in an ion, though. We will look at a couple of examples of ions later on.

The nucleon number (mass number) counts the total number of protons and neutrons. You can work out the number of neutrons present by subtracting the proton number from the nucleon number.


Some examples:

Fe: proton number 26; nucleon number 56

There are 26 protons (from the proton number).

This is a neutral atom (not an ion) and so there are 26 electrons.

There are a total of 56 protons and neutrons (from the nucleon number), and so there must be 30 neutrons (56 - 26).


S2-: proton number 16; nucleon number 32

This is an ion - take care!

There are 16 protons (from the proton number).

If it was a neutral atom, there would be 16 electrons. But this is an ion with 2 negative charges. That's because it has 2 extra electrons. There are 18 electrons.

There are a total of 32 protons and neutrons (from the nucleon number), and so there must be 16 neutrons (32 - 16).

Notice that the fact that you have an ion rather than a neutral atom makes no difference whatsoever to the number of protons or neutrons.


Al3+: proton number 13; nucleon number 27

Another ion - take care with the electrons.

There are 13 protons (from the proton number).

If it was a neutral atom, there would be 13 electrons. However, this is a 3+ ion. That's because it has lost 3 electrons. There are 10 electrons.

And you can work out the number of neutrons from the nucleon number. In this case there are 14 neutrons (27 - 13).


Note:  This all assumes that you know from earlier work what an ion is. If you are starting A level without any chemistry background, an ion is an electrically charged atom or group of atoms. Negative ions are negative because they have one or more extra electrons. Positive ions are positive because they have lost one or more electrons. The ion is negative or positive because there are no longer equal numbers of protons and electrons.



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© Jim Clark 2010 (last modified March 2014)