Ordering the elements in the Periodic Table
Introduction
There are hundreds of ways of presenting Periodic Tables. You can see some of them at
www.chemlab.pc.maricopa.edu. 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, Z. This is
the number of protons in the nucleus of an atom and is sometimes called the proton
number. However Mendeleev, who made the first Periodic Table in 1869, listed the thenknown elements in order of their relative atomic masses, Ar (then called atomic weights).
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, had not been developed – many scientists were still
having trouble coming to terms with the idea of atoms.
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
among stable elements where the atomic number and relative atomic mass order differ –
argon and potassium, cobalt and nickel and iodine and tellurium. Argon and potassium did not
originally cause a problem to Mendeleev simply because argon had not been discovered.
Cobalt (Z = 27) and nickel (Z = 28) have very similar relative atomic masses (58.93 and 58.69
respectively) so a possible explanation was that the values were incorrect. However tellurium
(Z = 52, Ar = 127.6) was listed as heavier than iodine (Z = 53, A r = 126.9) yet their chemical
properties clearly demanded that iodine follow tellurium. Mendeleev was so convinced of the
relative positions of the two elements that he believed the relative atomic mass of tellurium
was wrong.
Activity 1
Find out some of the properties of iodine and of tellurium – you could use a reference
book or search on the web, at www.webelements.com, 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.
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 property to justify the
term atomic number
By 1907, when Mendeleev died, chemists were in little doubt that iodine followed tellurium
and that their relative atomic masses were unusual. However there was no measurable
property that represented the position of an element in the overall 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 beryllium. Henry Moseley (see box)
found a property linked to Periodic Table position. It was the wavelength of X-rays given off by
solid elements when they were bombarded by a beam of electrons from the then newlydiscovered ‘electron gun’. Hence atomic number became more meaningful and the three
anomalies mentioned earlier could be explained.
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.
Moseley measured the wavelengths, λ, of particular X-rays (technically called the Kα1 lines)
given out by a range of elements. He then calculated the frequency, , using the formula  =
c/, where c is the speed of light, 3 x 108 m s-1) and plotted graphs of the square root of the
frequency (√) of these lines against atomic number (the position of the element in the
Periodic Table) and also against relative atomic mass. Table 1 gives the Kα1 X-ray
wavelengths for some elements using modern data.
Element
Na
Mg
Al
Si
P
Atomic
number
11
12
13
14
15
Relative
Wavelength
atomic
of Kα1 line /
mass
x 10-10 m
22.99
11.88500
24.31
9.86900
26.98
8.32000
28.09
7.11100
30.97
6.14200
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
32.06
35.45
39.95
39.10
40.08
44.96
47.88
50.94
52.00
54.94
55.85
58.93
58.69
63.55
65.39
69.72
72.59
74.92
78.96
79.91
83.80
85.47
87.62
88.91
91.22
92.91
95.94
98.91
101.10
102.90
106.40
107.90
112.40
114.80
118.70
121.80
127.60
126.90
131.30
132.90
137.30
138.90
140.10
140.90
144.20
146.90
150.70
152.00
157.30
158.90
162.50
164.90
5.36130
4.71820
3.73370
3.35170
3.02500
2.74320
2.49840
2.08060
1.90620
1.75300
1.61740
1.65450
1.53740
1.43220
1.33720
1.25130
1.17340
1.10250
1.03760
0.92360
0.87340
0.82710
0.78430
0.74460
0.70783
0.67780
0.64174
0.61202
0.58422
0.55824
0.53388
0.51104
0.48961
0.46937
0.45035
0.43242
0.39946
0.38431
0.36996
0.31515
0.30363
0.33115
0.31902
0.30844
0.29795
0.28778
0.27820
0.26895
0.26030
Er
Tm
Yb
Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Fr
Ra
Ac
Th
Pa
U
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
167.30
168.90
173.00
175.00
178.50
181.00
183.90
186.20
190.20
192.20
195.10
197.00
200.60
204.40
207.20
209.00
209.00
210.00
222.00
223.00
226.10
227.00
232.00
231.00
238.00
0.25198
0.24387
0.23625
0.22882
0.22173
0.21505
0.20857
0.20236
0.19639
0.19065
0.18513
0.17982
0.16979
0.16503
0.16045
0.13254
0.12569
Table 1 Kα1 X-ray wavelengths for some elements
Activity 2
You can plot the graphs yourself either on graph paper or using a spreadsheet. You will
need to use the equation above to calculate the frequency of each X-ray line and then
take the square root of this. The final graphs are shown in Figure 1.
Square root of X-ray frequency versus relative atomic mass
600000000
Square root of X-ray frequency / s-1
500000000
400000000
300000000
200000000
100000000
0
0.00
50.00
100.00
150.00
200.00
250.00
Relative atomic mass
Square root of X-ray frequency versus atomic number
600000000
Square root of X-ray frequency / s-1
500000000
400000000
300000000
200000000
100000000
0
0
10
20
30
40
50
60
Atomic number
Figure 1 Moseley’s graphs
70
80
90
100
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.
The bombarding electrons can remove one of the electrons from the inner shell of the target
atom, see Figure 2. One of the electrons from an outer shell then falls into the original shell to
fill the gap. When this happens, a packet (or 'quantum’) of electromagnetic energy (E) is
given out. If the electron in question falls back into an inner shell, this energy is in the X-ray
part of the spectrum. As is true for all electromagnetic radiation (such as light, ultra-violet and
infra-red) the energy of X-rays is linked to its frequency (and therefore wavelength) by the
expression E = h, where E is the energy,  the frequency and h a constant called Planck’s
constant, 6.626 x 10-34 J s.
Figure 2 X-rays are produced by bombardment of atoms by electrons
A Kα1 line is caused by an electron 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.
Questions
Q 1. Select the best word from the highlighted boxes to complete the questions.
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
Q4.
(a) Potassium is almost entirely composed of two isotopes potassium-39 (93% and
potassium 41 (7%). What is the relative atomic mass of potassium? 39 / 39.14 / 40
(b) What instrumental technique could be used to separate these isotopes and
measure their relative abundances? X-ray diffraction / infra-red spectrometry / mass
spectrometry
Answers to activities and questions
Activity 1
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.
Questions
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
Q 4. (a) 39.14 (b) mass spectrometry