We've had some messages from viewers.
They're worried why is the periodic
table changing
again? Most people can understand when
you get an element with a new name like
copenitium this year. But these numbers
which have been here for ages under the
different elements the so-called atomic
weight how can that change and suddenly
they read a new story. IUPAC,
International Union of Pure and Applied
Chemistry has changed them. It's nothing
sacred. Why can they change these
things? Well, the answer is they haven't
really changed them. But
what these numbers are are the so-called
atomic weights, which are the
weights of the average weight of the
different isotopes of the elements.
Isotopes are atoms that have different
weights. Most elements have more than
one sort of atom which are chemically
the same. We've told you about hydrogen
and dutyium. These are just the same.
Lots of elements have more than one
isotope. Carbon has two. Carbon 12,
carbon 13, and a radioactive one, carbon
14, the one that was used to date the
Turin Shroud. So, what have they
changed? What they've changed are these
weights which refer to the average of
the weights of the different isotopes of
the elements. What IUPAC has recognized
is the fact that different samples of
elements from different places around
the world, whether it's sulfur from a
volcano or sulfate from a mine somewhere
in England, have slightly different
isotopic compositions and therefore they
have slightly different atomic weights.
Up till now all scientists have been
trying to fit the same atomic weight to
sulfur from everywhere and they've gone
into all sorts of snags. Now what has
been decided is there's a range of
values. Sulfur on the earth can have an
average weight from this amount to that
one. For example, here with boron
instead of being
10.811 it can be from
10.806 to 10.821.
and they've changed the things for 10
elements. This is just a minor
adjustment and it'll make it so much
easier for chemists to work. I have no
idea why they've done it now. I think
it's probably because there's been some
careful work done by the US Geological
Survey. They published their results in
this paper who've actually worked out
what are the acceptable values for this
range for the elements and IUPAC has
adopted it. The question is should you
throw away your periodic table? The
answer is if it's on a mug like my tea
mug, throw it away because it's
disposable anyway. But if you've got a
nice one like this one here on the
office wall, keep it because the
difference is very small and it's not
going to affect your daily life as a
chemist. So why would for example sulfur
from volcano A be different from sulfur
from crystal B? Why why is sulfur not
cons a consistent blend of the isotopes?
Well, when you
have isotopes, because of their
different mass, it gives chemical bonds
a very slightly different strength. And
if you start having different strength
chemical bonds, the molecules react at
different rates. So if you have a whole
series of reactions that are producing
hydrogen sulfide
or natural elemental
sulfur, there will be different
reactions. The different isotopes will
have different rates and so they'll wind
up in different ratios. Of course, the
overall ratio in the universe will be
fixed. It's just it will be slightly
different in one place and another. It
doesn't apply to chemists like me, but
it will be important to people, for
example, who are studying geology,
looking at the elemental composition of
rocks, where over millions of years,
there has been some change in the ratio.
For most chemists, they probably won't
bother very much at all because if you
want to measure the mass of atoms or
molecules very accurately, you will use
a mass spectrometer which will mi
measure the mass of the individual
isotopes. And then the average atomic
weight doesn't really matter very much
at all. So, it's something that's
interesting. It's fun. makes you think
about chemistry again, but it's not
something that's going to change our
lives enormously.