Higher and foundation tiers
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.
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.
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:
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.
Now recall the following:
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?
As an example consider chlorine gas. Chlorine has two isotopes; these are shown below:
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:
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.
As a final example consider the element hydrogen. Hydrogen has 3 naturally occurring isotopes. These are shown below.
The three stable isotopes of hydrogen | ||
---|---|---|
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).