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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 now chlorine has an atomic number of 17; this means it contains 17 protons in the nucleus and since it is an atom it will be neutral with no overall charge and so it will also have 17 electrons. To calculate the number of neutrons in a chlorine atom you simply subtract 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 in the periodic table 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 so the mass number for an atom of carbon will be 12 a.m.u. or simply 12u, where a.m.u or u is an atomic mass unit.

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

Now you are probably used to measuring masses in kilograms, but when we are dealing with something as small as atoms it is simply just too big a unit of measurement to use, after all you would not measure your mass in tonnes would you? Its just not an appropriate unit. You might measure the mass of a lorry in tonnes but not in grams, again grams is just too small a unit of mass to use.

Relative atomic mass and mass numbers

The masses 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 instead of dealing with these very small awkward numbers scientists use a unit of mass called the atomic mass unit (amu or u) to measure the masses of atoms. Using this scale the masses of a proton and a neutron are simply 1 amu or just 1u. So what exactly is an atomic mass unit, well its simply defined as 1/12thof the mass of an atom of carbon-12 (12C). Now the diagram above is an atomic structure diagram for a carbon atom containing 6 protons and 6 neutrons in its nucleus with 6 electrons in the electron shells, so imagine if you could weigh this atom on a mass balance and get its mass in grams, then dividing by this mass by 12, then this mass would be the mass in grams of an atomic mass unit (amu or just u), the mass is 1.660538921 × 10-24. So if you divide the mass of a proton or a neutron in grams by the mass of 1 amu then you will find that the masses of a proton and a neutron are 1 amu.

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 detailed periodic table or Google the mass of carbon then will find 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, so at this point you might be wondering what is happening here! Well the reason that these relative masses are fractions is due to the presence of isotopes.

So before we look at isotopes let's look at an area that often causes confusion, 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 the key point here is that the masses given for a particular element in the periodic table are relative atomic masses and NOT mass numbers. The relative atomic mass is the weighted average mass for an element taking into account the presence of isotopes.

So what exactly are isotopes?

Now recall the following:

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 obviously 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 different isotopes present 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 then 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