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Higher and foundation tiers

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; so we have 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!

Now recall that all the mass of an atom is due to the presence of the protons and neutrons found inside the nucleus, for example the element carbon has 6 protons and 6 electrons inside its nucleus. Now the masses of the protons and neutrons are almost identical, that is 1.67 x 10-27kg. This number is obviously very small and not exactly easy to handle or use. So scientists use a unit of mass called the atomic mass unit (amu or u) to measure the masses of atoms.


Atomic structure diagram for an atom of carbon along with its chemical symbol.

The atomic mass unit is defined as 1/12thof the mass of an atom of carbon-12 (12C). It is this relative (relative just means compared to) mass scale which is used as a reference scale from which the mass of all the other atoms are measured from. Or we could simply say the masses of all other elements are measured by comparing them to an atom of carbon-12, which has a mass of exactly 12 amu or 12u. However if you look in the periodic table you will see that carbon has a relative atomic mass of 12.011! This is a similar problem we found above with chlorine which has a relative atomic mass of 35.5. The reason that these relative masses are fractions is due to the presence of isotopes.

Now do not get confused with mass numbers and relative atomic masses. The mass number for an atom is simply the sum of the number of protons and neutrons inside the nucleus while in the periodic table the relative atomic mass is used to represent the mass of a particular element. This relative atomic mass of course takes into account the presence of isotopes of the particular element, this is shown for the element chlorine in the image below:

Explanation of the difference between mass number and relative atomic mass

So what exactly are isotopes?

Definition of what an isotope is.

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 chemical tests to identify isotopes since these rely on the chemical reactions and properties of the elements and these are determined by the 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 are 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 so 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:

The isotopes of chlorine showing the numberes of protons, neutons and electrons in each isotope.

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 mass is not what is shown in the periodic table. In the periodic table the mass of chlorine is given as 35.5. You may be wondering where the 35.5 has come from?

If each isotope was present in equal amounts; that is 50% of chlorine atoms were 35Cl and 50% were 37Cl then we could simply tale an average of the two masses which would give us an average mass of 36. However analysis of the two isotopes of chlorine shows 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 each isotope. The calculation you need to carry out is shown below:

How to calculate the relative atomic mass from the percantage abundances of isotopes.

The calculation gives a mass of 35.5; this is the relative atomic 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 atoms with over 99% of all hydrogen atoms being protium (1H).

Key Points


Practice questions- Check your understanding - Isotope questions

Quick Quiz Check your understanding - Isotope quick quiz

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