Now you’ve probably all heard – I hope
you’ve all heard – of the periodic table.
It lists all of the elements both natural
and also man-made in increasing order of atomic
number, Z, moving from left to right.
Here we have the first two groups in the periodic
table. I’m showing you here an extract from
the first two groups where we have hydrogen
(H), lithium (Li), sodium (Na) – remember
what I said: sodium actually relates to the
Latin natrium, hence the reason why it should
be potentially ‘So’ in English but the
reality is it comes back to its Latin roots
– beryllium, which is the fourth, and magnesium,
which is in the third period.
Now, as you can see in the periodic table,
you can observe we’ve got a number of different
numbers above and beneath the chemical symbols.
If we look at the number above, it is the
smallest number and therefore it is the Z
number or atomic number. This correlates to
the number of protons and the number of electrons
that that element possesses in its atomic
And, as you’ll see, you’ll understand
why I say ‘atomic form’ because a lot
of elements in nature don’t exist as atoms.
They actually exist either as ions or they
exist as molecules. But for the purposes of
explaining basic atomic structure, we consider
them in isolation, whether they exist like
this in nature or not.
As you can see on the board, I’ve introduced
another letter there, A, which correlates
to the atomic mass, okay? Now notice the atomic
mass here is larger than the atomic number
and it correlates to the number of protons
and the number of neutrons, as I’ve said
Also take note of the fact that the numbers
are not whole. This is because, in the periodic
table, you have to take into consideration
the different types of isotopes, which have
different masses overall, when calculating
the relative atomic mass. And I’ll show
you how to do that in a second.
Right. So here we have an example of how you
would calculate the relative atomic mass of
chlorine. And here, as you can see, they are
proportional to their abundance. Now recall
what I said: 75% of all chlorine atoms exist
in their 35 atomic-mass form whereas approximately
25% exist as the 37 atomic-mass form of chlorine.
So what we need to do is we need to find a
weighted average of those two isotopes to
give us the relative atomic mass that you
would see in the periodic table.
Now I’m not going to go through this entire
equation with you. Suffice to say, conceptually,
it’s relatively easy to do.
Let’s say, for the sake of argument, you
had 75 atoms in one hand each weighing 35
grams and you had 25 in the other hand each
weighing 37. All you’d have to do is multiply
the mass by each percentage and then average
That’s effectively what we’ve done here.
This gives us an average atomic mass unit
for chlorine of 35,5 shown here as 35,45 but,
on most periodic tables, you will see it’s
rounded up. And whilst the structure of the
nucleus is important in radiochemistry and
physics, in terms of the chemical characteristics
an atom of 35Cl will behave the same chemically
as an atom of 37Cl. It makes no difference
to their reactivity. The only way you can
even detect this sometimes is via certain
analytical chemistry techniques which are
beyond the scope of this course.