isotope heading

Higher and foundation tier

The same but different!

chlorine symbol Element number 17 on the periodic table is chlorine. Its chemical symbol is shown opposite. Chlorine has an atomic number of 17, this means it contains 17 protons in the nucleus and since it is an atom, it will also have 17 electrons . To calculate the number of neutrons we take the atomic number from the mass number, 35.5-17 = 18.5 neutrons !! Obviously you cannot have half a neutron so does this mean that the mass given in the periodic table is wrong? Well if you look online at an A-level periodic table you will see that almost all the elements have masses that are decimals, this would mean then that they all contain a fraction of a neutron , obviously this cannot be so! These decimal numbers for the masses confused scientists for a long time, it was only in 1913 with the discovery of isotopes that scientists were able to solve this problem.

So what exactly are isotopes?

isotope definition

Isotope examples

chlorine gas in a flask All the elements in the periodic table have isotopes, some like chlorine have only two isotopes while others have many isotopes, Caesium, an alkali metal in group 1, has 40 isotopes. It might seem odd to think that the chlorine gas in the flask opposite contains different types of chlorine atoms. All of the chlorine atoms in the flask have 17 protons and 17 electrons but some of them have more neutrons than the others.

You cannot use any chemical test to identify the isotopes, as chemical properties depend on electron arrangements, however as some of the atoms have extra neutrons they will have different masses. Boiling point is a physical property that depends on mass, so in theory you could separate the isotopes using the fact that they will have slightly different boiling points. Rates of diffusion is another physical property that could be used to separate isotopes.

The fact that all elements have isotopes causes a big problem - since each isotope has a different mass, what mass do we record in the periodic table for an element?



Isotopes of chlorine

As an example consider chlorine gas. Chlorine has two isotopes, these are shown below:

isotopes of chlorine

The two isotopes of chlorine have masses of 35 and 37, so which mass do we use for chlorine? We could take an average of the two masses, (35 + 37)/2 = 36, however this average is not what is used in the periodic table. In the periodic table the mass of chlorine is given as 35.5. You maybe wondering where the 35.5 has come from!
If each isotope was present in equal amounts i.e. 50% of chlorine atoms were 35Cl and 50% were37Cl then we could simply tale an average of the two masses, which would give us a mass of 36. However analysis of the two isotopes of chlorine show that they are not present in equal amounts. 75% of all chlorine atoms are 35Cl while 25% are 37Cl, so when working out the average mass we need to take into account the abundance of equal isotope. The calculation you need to carry out is shown below:

isotope mass calculation

The calculation gives a mass of 35.5, this is the mass which is displayed in the periodic table for chlorine. The periodic table displays the relative atomic masses taking into account the abundance of each isotope.

Isotopes of hydrogen

As a final example consider the element hydrogen. Hydrogen has 3 naturally occurring isotopes. These are shown below.

The three stable isotopes of hydrogen
protium deuterium tritium
This isotope called protium contains 1 proton in its nucleus. Its relative atomic mass is 1. It has 1 electron in the 1st electron shell. This isotope called deuterium contains 1 proton and 1 neutron in its nucleus Its relative atomic mass is 2. It has 1 electron in the 1st electron shell This isotope called tritium contains 1 proton and 2 neutrons in its nucleus. Its relative atomic mass is 3. It has 1 electron in the 1st electron shell

As with all isotopes their chemical properties are identical. These 3 isotopes of hydrogen are no exception and all have identical chemical properties. The heavy isotopes deuterium (2H) and tritium (3H) are rare atom, with over 99% of all hydrogen atoms being protium(1H).

Key Points


Practice questions

Check your understanding - Isotope questions

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