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Ordering the elements in the Periodic Table
There are hundreds of ways of presenting Periodic Tables. You can see some of them at The most familiar form is flat and rectangular with rows and
columns, but this is only because this fits the pages of a book easily.
All modern Periodic Tables list the elements in order of increasing atomic number, often
given the symbol Z. We know now that this is the number of protons in the nucleus of an
atom and is sometimes called the proton number. However Dmitri Mendeleev, the Russian
chemist who made the first Periodic Table in 1869, listed the elements known at that time in
order of their relative atomic masses, Ar. This was for the simple reason that, at that time,
the idea of atoms being made up of smaller sub-atomic particles, such as the proton, neutron
and electron had not been developed.
Fortunately these two arrangements – in order of atomic number and of relative atomic mass
- are almost identical. In the same way, you would probably get pretty much the same order if
you listed all your classmates first in order of the width of their hands and then in order of the
length of their feet.
Problems with the order by relative atomic mass
If you look carefully at a modern Periodic Table you will see that there are three instances
where an element is followed by another element of smaller relative atomic mass.
Activity 1
Find the three cases where this happens.
Activity 2
Use the Before function of the interactive Periodic Table to view all the elements known
in 1869. Which one of the pairs of elements you found in Activity 1 would not have
concerned Mendeleev? Why?
One of the other cases is cobalt (Z = 27) and nickel (Z = 28). They have very similar relative
atomic masses (58.93 and 58.69 respectively) so a possible explanation was that the values
had been measured incorrectly. However tellurium (Z = 52, Ar = 127.6) was listed as heavier
than iodine (Z = 53, Ar = 126.9) yet their chemical properties clearly demanded that iodine
follow tellurium in the Table. Iodine fits much better in Group 7 and Tellurium in Group 6
rather than the other way round. Mendeleev was so convinced of the relative positions of the
two elements that he believed the relative atomic mass of tellurium was wrong.
Activity 3
Find out some of the properties of iodine and of tellurium – you could use a reference
book or search on the web, at, for example. Explain why tellurium
fits into Group 6 and iodine into Group 7 rather than the other way round. You may need
to look up the properties of other elements in Groups 6 and 7 as a comparison. Some
properties you could look for include the formulae of their compounds with hydrogen and
with metals.
Mendeleev’s practical assistant, Bohuslav Brauner, re-measured the relative atomic mass of
tellurium but still got the same result. He put this down to impurities he had failed to remove.
There is a message for us all here; sometimes there are unexpected results which are
nevertheless correct.
Henry Moseley finds a link to atomic number
By 1907, when Mendeleev died, chemists were sure that iodine followed tellurium in the
Periodic Table and that there was something odd about their relative atomic masses.
However no-one was able to measure atomic number, it was just the position of an element in
the Periodic Table sequence. For example lithium was known to be the third element but this
number three was only because its properties meant that it slotted in between helium and
Henry Moseley (see box) found and measured a property linked to Periodic Table position.
Hence atomic number became more meaningful and the three pairs of elements that seemed
to be in the wrong order could be explained.
Moseley used what was then brand-new technology in his experiments. A device now called
an electron gun had just been developed. He used this to fire a stream of electrons (like
machine gun bullets) at samples of different elements. He found that the elements gave off Xrays. (This is how the X-rays used in hospitals are produced.)
Moseley measured the frequency of the X-rays given off by different elements. Each element
gave a different frequency and he found that this frequency was mathematically related to the
position of the element in the Periodic Table – he could actually measure atomic number. His
graphs are shown below. You can see that the graph relating frequency to atomic number is a
very good straight line while that relating frequency to relative atomic mass is not so good.
The relationship is actually quite complicated. In fact Moseley plotted the square root of the Xray frequency against atomic number, but do not let this detail obscure how important this
result was.
Notice that there are some gaps in the graph where X-ray data are missing – there were more
of these in Moseley’s time. Some of them were because it was not easy to fire an electron
beam at some elements. Others were because there were still several elements undiscovered
when Moseley made his measurements in 1913.
Henry Moseley
Henry (Harry) Gwyn Jeffreys Moseley came from a
scientifically brilliant family. He was a scholar at
Eton College and Oxford University before starting
his research. While still in his twenties he had
applied to be a professor at Oxford and at
Birmingham Universities. However the start of the
First World War in 1914 put the appointments on
hold. Moseley turned down the opportunity to do a
safe scientific job in Britain and became an officer
in the Royal Engineers. He was killed by a sniper
in Turkey in August 1915. Many people think that
Britain lost a future Nobel Prize winner. This is
because Nobel Prizes, the most prestigious
awards for scientific achievement. are awarded
only to living people.
Henry Moseley. Reproduced courtesy of the Library and
Information Centre, The Royal Society of Chemistry.
Activity 4
Suggest what sort of elements it might not be easy to fire a beam of electrons at.
These would be the elements for which X-ray data was missing.
Activity 5
Use the After function of the interactive Periodic Table to find out how many elements
that are now known were undiscovered in 1913.
Moseley’s graphs
Atomic number explained from atomic structure
Within ten years of Moseley’s work, the structure of the atom was further unravelled and
atomic number seen to be the number of protons in the nucleus of an atom. Some people call
Z the proton number, but it could have been called the Moseley number.
The X-rays given out by atoms bombarded with electrons are formed as follows.
Imagine an atom of magnesium. It has the electron arrangement 2, 8, 2 as shown in the
upper part of the diagram below
If one of the electrons from the electron gun hits one of the electrons in the inner shell it may
knock the inner electron out of the atom completely leaving a gap in the inner shell as in the
middle part of the diagram below
Now, one of the electrons in shell 2 may be pulled by the attraction of the nucleus into shell 1
to fill the gap as shown in the lower part of the diagram below
As it does so, it gives out energy in the form of X-rays.
Production of X-rays
If the same process happened for another atom with more protons in its nucleus, more energy
would be given out because the electron would be attracted more strongly to the nucleus. In
general, the bigger the charge on the nucleus (ie the bigger the atomic number), the more
energy the X-rays will have. Like all electromagnetic radiation, the energy of an X-ray is
related to its frequency.
The X-rays Moseley used were caused by electrons falling from shell 2 into shell 1. The
reason that the energy (and therefore wavelength) is linked to the atomic number (the positive
charge on the nucleus) is that the greater the charge on the nucleus, the more the electrons
are attracted to it and the more energy they give out when dropping from one shell to another.
Q 1. Select the best word from the boxes to complete the sentences.
All atoms, except hydrogen, are composed of three particles: electrons, neutrons
and protons.
The nucleus contains electrons / neutrons / protons as well as neutrons. These are
both much heavier than electrons / neutrons / protons Thus nearly all the mass of
an atom is in the nucleus. Protons are almost equal in mass to electrons / neutrons
/ protons. The atomic number of an atom equals the number of electrons /
neutrons / protons in the nucleus, and is also the number of electrons / neutrons /
protons in a neutral atom.
All atoms of an element have the same number of electrons / neutrons / protons in
the nucleus but the number of electrons / neutrons / protons can vary slightly.
These different varieties of the same element are called isotopes. The relative
atomic mass is an average of the mass of the different isotopes, taking account of
the different proportions of each isotope. Most hydrogen atoms have one proton
and one electron / neutron / proton but no electron / neutron / proton.
Q 2.
(a) The atomic number of iodine is 53. Use the value of 127 for its relative atomic
mass to work out how many electrons, protons and neutrons are in each atom. 53
electrons, 64 neutrons, 53 protons / 127 electrons, 64 neutrons, 127 protons / 64
electrons, 64 protons, 64 neutrons
(b) The element before iodine in the Periodic Table is tellurium. It has seven
different isotopes of which the commonest have 74, 76 and 78 neutrons per atom.
Work out the total number of particles in the nucleus of each of these atoms.
Tellurium with 74 neutrons 74 / 127 / 126
Tellurium with 76 neutrons 76 / 127 / 128
Tellurium with 78 neutrons 78 / 127 / 130
Q 3.
Why does the number of protons in the nucleus govern the chemical properties of
an atom such as the formulae of compounds that it forms and the type of bonding
that it takes part in? It is the same as the number of electrons / It is the same as
the number of neutrons / It is the same as the relative atomic mass
Answers to activities and questions
Activity 1
Argon and potassium, cobalt and nickel and tellurium and iodine.
Activity 2
Argon and potassium – the element argon was not known in Mendeleev’s day.
Activity 3
Tellurium’s compound with hydrogen has the formula H2Te, while the other Group 6 elements
also form compounds of the formula H2X.
Iodine’s compound with hydrogen has the formula HI, while the other Group 7 elements also
form compounds with the formula HX.
Tellurium’s compound with sodium has the formula Na2Te, while the other Group 6 elements
also form compounds of the formula Na2X.
Iodine’s compound with sodium has the formula NaI, while the other Group 7 elements also
form compounds with the formula NaX.
There are many other properties that could be compared.
Activity 4
It would be difficult to fire an electron beam at gases, for example.
Activity 5
26 There were 85 elements discovered in 1913, compared with 111 in 2003.
Q 1. In order a) protons b) electrons c) neutrons d) protons e) electrons f) protons g) neutrons
h) electron i) neutron.
Q 2. (a) 53 electrons, 64 neutrons, 53 protons b) (i) 126; 128; 130
Q 3. It is the same as the number of electrons