Note on the Explanatory Notes
The notes exist in two versions. The entry-level notes are written for
people who know little or no chemistry. The advanced version assumes
familiarity with the conventional Periodic Table.
If you have worked your way through the entry-level notes and feel
that you have understood them, you may wish to attempt the advanced
version. If so, note that much of what is said there is the same as, or
assumes knowledge of what was said in plainer English at the entry level.
Much of the information in the advanced version is intended to persuade
chemists of the advantages of the spiral representation.
The language of ‘shells’, ‘subshells’ and ‘orbitals’ may seem
mystifying, and indeed the behaviour of fundamental ‘particles’ like
electrons and protons is mysterious. They behave both like particles and
like waves. They are not in definite places but can be described only in
terms of equations that express the probabilities of their being in
particular places. The current status of an electron is defined by four
quantum numbers, and for each it can have only one whole-number value
or another. It cannot slide from one value to another but jumps without
passing through in-between numbers, which is not how we are used to
things behaving.
When the atom was first modelled, it was thought that the electrons
went round the nucleus in orbits, rather like planets orbiting the sun, and
the word orbitals is still used for them. The words ‘shell’ and ‘subshell’
sound as if they can be intuitively understood, but the sets of orbitals,
named s, p, d, f, g etc., are not neatly inside one another. As the number
of electrons in an atom increases, each new one occupies the orbital of
lowest energy available. The third shell has three subshells, but before
the third can be occupied the first subshell of the fourth shell has to be
added, and the same thing happens with the fourth subshell of the fourth
shell. As for the fifth subshell of the fifth shell, it is never reached in the
unexcited atom.
All the beginner needs to know is that the mysterious behaviour of
electrons accounts for the complexity of the periodic system. If each
shell were built up and completed before the next shell started, the spiral
would be as regular as a coil of rope, and the colour scheme would be a
straightforward progression from violet to red. Much of the fascination
of chemistry lies in the many subtle departures from regularity.
The new edition.
Chemical Galaxy II differs from its predecessor in a number of ways
intended to make it brighter, more colourful, more legible and more
informative. I have made the disks representing the various elements as
big as they can be without any of them overlapping, and the lettering is
now uniformly black. I have changed the background from black to blue,
darkening to nearly black away from the ‘galaxy’.
As requested by some users, I have added atomic weights (already
present in the first U.S. edition), and some information on electronic
configurations has been given at the bottom. For reasons given in the
advanced notes, I have indicated elements with naturally occurring metastable isotopes (again, as in the first U.S. edition, but more discreetly).
The colour scheme for the lanthanides and transition elements (the
left-hand side of the ‘galaxy’) was a monotonous series of blues and
greens in the first edition. I was unable to decide how far to associate
them with each other and with the typical groups on the right. A paper
published in 2005 suggested the solution I was looking for, and I have
innovated by associating two sets of lanthanides with Mendeleev’s Group
VIII elements (including the ‘coinage metals’).
© P J Stewart, September 2006
Chemical Galaxy II - Explanatory Notes (Entry Level)
Why a galaxy?
The matter in the world about us is almost entirely made from 83
elements. Another 10 elements exist in trace quantities as unstable products of the radioactive
breakdown of the two heaviest elements, and another 20 elements have been made in the
laboratory. Each element has atoms of one kind, characterized by its atomic number - the
number of protons (positively charged particles) in the atomic nucleus. This dictates the
number of electrons the nucleus is able to attract, from which comes the chemical character of
the element.
A few elements can occur in nature pure, for example carbon, sulphur, copper, silver, gold.
Or they may be mixed with other elements, for example the platinum metals or oxygen,
nitrogen and argon in the atmosphere. However most elements are usually or always
combined with others, losing their distinctive character to form the substances with which we
are familiar for example the gases oxygen and hydrogen combining to form water, or the
toxic gas chlorine and the inflammable metal sodium combing to form common salt. Even in
its pure form, an element is not just a recognizable simple substance; carbon, for example, can
take the form of diamond, graphite or charcoal. The chemical character of an element lies in
the way that it combines with other elements – how many atoms with how many, how easily,
how strongly, in what spatial arrangement and so on. Some elements, such as hydrogen and
oxygen, can combine with almost any other element. Others are more choosy. The ones
shown violet and red in the ‘galaxy’ are attracted to each other so strongly that they can
combine with explosive force. The lighter noble gases, shown in grey, are too snooty to
combine with anything else at all.
When the chemical elements are arranged in order of their atomic number, they form a
continuous sequence, in which certain chemical characteristics come back periodically in a
regular way. This is usually shown by chopping the sequence up into sections and arranging
them as a rectangular table. The alternative is to wind the sequence round in a spiral.
Because the periodic repeats come at longer and longer intervals, increasing numbers of
elements have to be fitted on to its coils. The artist Edgar Longman showed in his 1951 mural
that this is best done by making the spiral elliptical. In this new version, for the first time, the
size of successive turns is made to increase at a constant rate.
The resulting pattern resembles a galaxy, and the likeness is the basis of my design. It
seems appropriate, as the chemical elements are what galaxies are made of (leaving aside the
question of the mysterious ‘dark matter’). Needless to say, the elements in a real galaxy are
not arranged in this way. Real galaxies consist mostly of hydrogen and helium, and the
heavier elements are found mainly in the stars in which they have formed, or scattered by the
dying explosion of giant stars. Some of this star dust has condensed into planets like ours,
with heavier elements mostly in the middle and lighter elements mostly on the surface. The
troubled history of the earth, with its drifting continents, its mountain building and volcanoes,
has mixed the elements up quite well.
The ‘spokes’ of the ‘galaxy’ link together elements with similar chemical characteristics.
They are curved in order to keep the inner elements reasonably close together while making
room for the extra elements in the outer turns. The curvature of the ‘spokes’ is governed by
the imaginary gravitational pull of a ‘great attractor’ situated off to the upper right. The eight
main chemical groups, in the right-hand half of the spiral, lie on four curves that run through
the centre, joining them in four pairs. This is done mainly for artistic reasons.
The objective is to show the shape of the whole and to express the beauty and cosmic reach of
the periodic system. Colour is used to distinguish the different chemical groups, and a little
chemical information is given, but there is no attempt to squeeze in other detailed data, which
would distract from the overall picture and which are better sought in the pages of a book or a
web-site. Characteristics such as density, state (solid, liquid or gas), metallic or non-metallic
nature, toxicity…, are only locally valid on the surface of our planet, and this image sets out
to portray the elements as they are anywhere in the universe.
Advantages of a spiral. A spiral representation has no interruptions; each element is
between both its neighbours. If Mendeleev, the great Russian pioneer of the system, had
represented it in this form, perhaps he would not have failed to notice that there was a gap in
atomic weights where the noble gases were to be found (an oversight that is said to have cost
him the Nobel Prize). Instead, he probably saw the ends of his rows as natural breaks in the
sequence.
One of the effects of preferring tables has been to make chemists think in terms of ‘blocks’
of elements. To understand the basis of these ‘blocks’, you need first to learn about the
arrangement of the electrons, which is a complicated subject. For the moment, it is enough
to know that as the atomic number increases and successive electrons are added, they group
themselves into ‘subshells’ and ‘shells’, nested like Russian dolls or the layers of an onion.
Only the most recently added electrons can get transferred between atoms to create bonds.
The ‘blocks’ result from the way the outermost electrons behave, which theoretically depends
which subshell they belong to, but nature is not so tidy, and there are various irregularities
and overlaps.
Tables, with their rows and columns, are easy to read, but the coils and spokes of a spiral
may be easier to remember. One can easily forget whether something is in the fifth or sixth
column from the left or the fourth or fifth row from the top, but the varied shapes of a spiral
give the visual memory something to grasp. Our vision evolved for millions of years in a
world of curved forms, which have a variety and evocative power lacking in the straight lines
and right angles that our ancestors first encountered when they invented bricks and writing.
Element zero. I have placed this at the centre of the ‘Galaxy’. There is no room for such
an element in a table, but a spiral arrangement virtually requires it. My intention is that this
should be seen as neutronium, ‘atoms’ are neutrons, the electrically neutral particles which
combine with protons to form the atomic nuclei of all elements heavier than ordinary
hydrogen. When the density of the very early universe was too great for protons and
electrons to exist separately, this must have been the original form of nucleonic matter.
Mendeleev himself believed that there would be an element of ‘group zero in period zero’.
The word neutronium was coined in 1926 by Andreas von Antropoff, who placed it to the left
of hydrogen in his periodic table. I have represented it by a question mark, partly to suggest
its mystery and partly in order not to upset conservative chemists, to whom the idea is
anathema.
Neutronium is thought to exist inside neutron stars, which account for up to 1% of all nondark matter. These form when nuclear fusion no longer supplies enough energy to counteract
gravity, so that the nuclei and electrons of all the elements are crushed together to become so
dense that a thimbleful would weigh about 300 million tons, equivalent to six thousand
Titanics. Neutronium has caught the imagination of science fiction fans in the brilliant novels
of Robert Forward - Dragon’s Egg and Starquake. Unfortunately silly authors have made it
the material of armour or weapons. Here on Earth, cold neutrons, slowed to speeds
comparable with those of the molecules in ordinary matter at room temperature, can be
contained in a bottle to form an artificial and highly radioactive ‘noble gas’, which decays
with a half life of 14.64 minutes to form protons and electrons – the constituents of hydrogen.
Hydrogen and helium. I have placed these at the mid-point and the end of the first turn
of the spiral, next to carbon and the noble gases respectively. Hydrogen sits comfortably
above carbon, which it resembles in many ways, whereas it has hardly any resemblance –
apart from its valency - to lithium and the other alkali metals or to fluorine and the other
halogens, with which it is usually grouped in tables. Both hydrogen and carbon are half way
to having a full outer shell of electrons, and because of this they combine with other elements
mainly by sharing electrons instead of losing or gaining them. Carbon and hydrogen have a
great affinity for each other and are found together in a vast number of compounds, including
the carbohydrates and proteins so central to life, and the hydrocarbons which are our main
source of energy – and source of greenhouse gases. Still, the disk for hydrogen is coloured
differently to signify its uniqueness.
The colours of the disks are used to pick out the groups of elements with similar chemical
properties. The eight groups in the right-hand half of the ‘galaxy’ are the typical groups. The
noble gases are shown in grey. Their atoms have a complete outer shell of electrons and they
are not ready either to lose electrons to other atoms or to gain them, so they exist in splendid
isolation (though the heavier ones can with difficulty cede electrons to specially hungry
elements). Moving round anti-clockwise through the spectrum from violet to red, the atoms
of each group add one more electron to the ‘sealed unit’ formed by the noble gas at the
beginning of that turn of the spiral. The violet and blue groups, with one or two outer
electrons respectively, shed them easily, leaving a positively charged atom or cation. The
orange and red groups, with six and seven outer electrons, one or two short of the complete
complement of eight, readily seize electrons from other atoms, becoming negatively charged
anions. Cations and anions attract each other to form salts. In between these extremes,
atoms of elements in the cyan, green and yellow groups would need to lose or gain too many
electrons and tend to bond by sharing electrons with other atoms.
The picture is more complicated, because the further an element is from the centre, the
further its outer electrons from their nucleus, and the easier it is to remove them or the harder
it is to hold on to extra ones. Elements whose atoms easily let go of their outer electrons are
metals, when they are pure or alloyed with other metals. In them, a sea of unattached
electrons move around, gluing the atoms together, conducting electricity, reflecting light and
so on. In the coil of the spiral from atomic no. 3, lithium, to 9, fluorine, only the first two
elements are metals, in the next coil three, in the next four and so on. The molecules of life
are formed mostly from the non-metals hydrogen, carbon, nitrogen and oxygen, nos. 1, 6, 7
and 8, plus phosphorus and sulphur, nos. 15 and 16, with metallic atoms added at strategic
places.
To complicate matters still further, the shells of electrons do not fill up in orderly
succession. The third shell behaves as complete in the noble gas argon, no. 18, and the fourth
shell starts filling up in the next two elements, but then a new subshell of the third shell starts
being added, producing elements 21 to 30 in the bottom left quarter of the spiral. These are
the ‘transition elements’, all of them metals. The earlier ones use the electrons in the new
subshell in rather the same way as the elements in the typical groups, and for this reason paler
version of the same colours are used, from cyan to red. From no. 21, iron, onwards, only two
or three of the electrons in this subshell are available, and this is indicated by a series of
browns. In the tenth element, zinc, none of the ten newly added electrons is available, so its
behaviour is like that of the blue group. The elements in the middle of this set have very
complicated behaviour, much of it manifested in strong colours such as chrome yellow, cobalt
blue, the purple of potassium permanganate, and the reds and browns that iron gives to so
many soils and rocks.
After this first set of transition elements, the next eight resume the filling of the fourth
shell and start to fill the fifth, but then there is another phase of catching up, with ten more
transition elements as another subshell is added to the fourth shell. Then the pattern repeats
again, with the next eight elements completing the fifth shell and starting on the sixth. This
takes us up to element no. 56, barium, but then there is a new surprise, with a jump back to
add fourteen new electrons to the fourth shell, producing the elements in the top left quarter of
the spiral, known as the lanthanides. These are so similar to each other that it took more than
a century to separate them. They mostly prefer to give up one electron from the latest
subshell, but no. 63 europium and no. 70, ytterbium, are comfortable not giving up any, while
nos. 58 and 65, cerium and terbium, easily give up two, and the elements in between are
specially prone to give up one. In this they resemble the transition elements in the blue, cyan
and green groups, and this is symbolized by paler versions of the same colours, with pale
browns for the rest echoing those of the later transition elements. Triplets of cyan, blue and
green cross over the boundaries between these divisions of the spiral, emphasizing the
continuity of the system.
Radioactivity. The pattern repeats again in the next turn of the spiral, with a series of
elements, the actinides, in which 14 electrons are added to the fifth shell, but these are all
radioactive, with nuclei so unstable that eventually they break up, emitting radiation. This is
indicated by printing their symbols in italics. Two radioactive elements, no. 43, technetium,
and no. 61, promethium, occur in the second set of transition elements and in the lanthanides
respectively. From no. 84, polonium, onwards, all elements are radioactive, and most of them
have short half lives (the period during which half the nuclei decay), indicated by a black ring
surrounding the disk. Two of the actinides, no. 90, thorium, and no. 92, uranium, have such
long half lives that they are quite common in nature, indicated by a largely white ring. The
lighter radioactive elements are found only in trace quantities, continually renewed by the
decay of uranium and thorium. Of these fleeting elements, radium has a certain value in
medicine, and its decay product, radon, is a locally dangerous to health, being a gas that can
escape from certain rocks. Elements heavier than uranium have only existed since physicists
started producing them artificially in the 1940s. Since then some twenty new elements have
been created, and more than half a turn added to the spiral, but the end is undoubtedly in
sight.
Many elements contain a mixture of isotopes (atoms of different weights, with different
numbers of neutrons but the same number of protons). Some naturally occurring isotopes are
radioactive, with very long half lives - so feebly radioactive as to present no practical danger.
Elements that have such isotopes are indicated by three black nicks in the white rings that
surround them. Radioactive isotopes are also produced artificially, and some of these are
valuable in biology and medicine, in engineering and in materials science. Nuclear
explosions release dangerous isotopes like strontium 90, iodine 129 and caesium 137 into the
atmosphere. Nuclear power generation would pose no danger if there were no accidents, no
risks of terrorist attack, and no problem in disposing of radioactive waste. If thorium and
uranium had not been so long-lived, we might never have discovered radioactivity.
Chemistry and physics would have been less interesting, but the world might have been a
safer place.
Chemical Galaxy II - Explanatory Notes (advanced version)
Why a galaxy? Of the six men who pioneered the periodic system, four – Mendeleev,
Meyer Odling and Newland, represented it in tabular form. It was perhaps the prestige of
Mendeleev that led to tables becoming dominant, though he himself wrote in his 1871 paper
‘In reality the series of elements is uninterrupted, and represents in a certain degree a spiral
function.’ The other two pioneers, de Chancourtois and Hinrichs, followed by many later
chemists, gave this function visual form, drawing the sequence as a spiral. Because the
periodic repeats come at longer and longer intervals, increasing numbers of elements have to
be fitted on to successive ‘coils’. The artist Edgar Longman showed in his 1951 mural that
this is best done by making the spiral elliptical. In the present version, for the first time, the
size of successive coils increases at a constant rate. The resulting pattern resembles a galaxy,
and the likeness is the basis of my design. It seems appropriate, as the chemical elements are
what galaxies are made of (leaving aside the question of ‘dark matter’).
The ‘spokes’, representing the groups, are curved in order to keep the inner elements
reasonably close together while making room for the extra elements in the outer coils, but the
gap begins to open out even in the second turn of the spiral, between beryllium and boron.
This is balanced by a corresponding though smaller gap between oxygen and fluorine, which
closes up as you move outwards. The curvature of the ‘spokes’ is governed by the imaginary
gravitational pull of a ‘great attractor’ situated off to the upper right. The eight typical groups
lie on four curves that run through the centre, joining them in four pairs. This is done for
artistic reasons, but it does not seem entirely fanciful to see hydrogen and helium, lithium and
nitrogen, beryllium and oxygen, boron and fluorine as in a sense complementary pairs or
opposites.
The objective is to show the shape of the whole and to express the beauty and cosmic reach of
the periodic system. Colour is used to distinguish the different chemical groups, and
information is given on relative atomic mass, radioactivity and exceptional electronic
configurations, but there is no attempt to squeeze in other detailed data, which would distract
from the overall picture and which are better sought in the pages of a book or a web-site.
Characteristics such as density, state (solid, liquid or gas), metallic or non-metallic nature,
toxicity…, are in any case only locally valid on the surface of our planet, and this image sets
out to portray the elements as they are anywhere in the universe.
Advantages of a spiral. The information conveyed by the spiral is exactly the same as
that in a table, the coils corresponding to the rows and the spokes corresponding to the
columns. Extra information is made easily visible, in that a spiral has no interruptions; each
element is between both its neighbours. If Mendeleev had represented the system in this
form, perhaps he would not have failed to notice that there was a gap in atomic weights where
the noble gases were to be found (an oversight that is said to have cost him the Nobel Prize).
Instead, he probably saw the ends of his rows as natural breaks in the sequence.
One of the effects of preferring tables has been to make chemists think in terms of ‘blocks’
of elements, conceived as mirroring the subshells of electrons – s, p, d and f. Because there
are 10 electrons in the d subshells and 14 in the f subshells, the corresponding ‘blocks’ should
ideally comprise 10 and 14 elements. In fact, nature is not so tidy; the tenth electron is added
to the d block in the ninth element of the scandium and the lutetium rows (copper and gold)
and in the eighth element of the yttrium row (palladium). Lanthanum, which should be the
first element of the f block has a d electron instead of an f one. The actinides behave even
worse, with no f electrons in the first two elements and irregularities continuing as far as
curium.
Thinking in blocks would be justified if the members of a block behaved chemically more
like each other than like members of other blocks, but the first and last groups of the d block
(the scandium and zinc groups) behave more like the boron and beryllium groups respectively
– indeed, Mendeleev predicted the discovery of scandium on the basis of its resemblance to
boron, and zinc and cadmium resemble beryllium and magnesium in many respects more than
do calcium, strontium and barium (William B Jensen: ‘The Place of zinc, cadmium and
mercury in the periodic table’, Journal of Chemical Education, 80 (8), pp. 952-61, 2003). The
characteristic ‘transition element’ behaviour, with use of several d orbitals, begins with the
titanium group, becomes most marked around the chromium group and ceases after the
copper group. As for the ‘f block’, although the differentiating electron of both lanthanum
and lutetium is in a d orbital, they behave so like the thirteen elements in between that it took
a century to separate them all.
Tables, with their rows and columns, are easy to read, but the coils and spokes of a spiral
may be easier to remember. One can easily forget whether something is in the fifth or sixth
column from the left or the fourth or fifth row from the top, but the varied shapes of a spiral
give the visual memory something to grasp. Our vision evolved for millions of years in a
world of curved forms, which have a variety and evocative power lacking in the straight lines
and right angles that our ancestors first encountered when they invented bricks and writing.
Element zero. I have placed this at the centre of the ‘Galaxy’. There is no room for such
an element in a conventional table, but a spiral arrangement virtually requires it. My intention
is that this should be seen as neutronium, whose ‘atoms’ are neutrons, but I have represented
it by a question mark, partly to suggest its mystery and partly to avoid upsetting conservative
chemists, to whom the idea is anathema. Mendeleev himself believed that there would be an
element of ‘group zero’ in ‘period zero’, which he mistakenly expected to be the ether (D.I.
Mendeleev. An attempt towards a chemical conception of the ether. London: Longman,
Green. 1904). Andreas von Antropoff coined the word neutronium in 1926, six years before
the neutron was discovered, eight years before Baade and Zwicky introduced the concept of a
neutron star and 41 years before the first neutron star was observed (A von Antropoff, ‘Eine
neue Form des periodischen Systems der Elementen’, Zeitschrift für angewandte Chemie 39,
pp. 722-725, 1926). He placed it to the left of hydrogen in his ‘helical’ periodic table. It was
first put in the middle of a periodic spiral by Charles Janet (‘The helicoidal classification of
the elements’. Chemical News 138, pp. 372-374; 388-393, 1929) and subsequently by E I
Emerson (‘A New Spiral Form of the Periodic Table’, J. Chem. Education 22, pp. 111-115,
1944) and John D Clark (‘A Modern Periodic Chart of the Chemical Elements’, Science 111,
pp. 661-63, 1950).
Neutronium may have been the first form of nucleonic matter to exist in the early universe,
when it was too dense for protons and electrons to exist separately. It is thought to form
much of the mass of neutron stars, which account for up to 1% of all non-dark matter. These
form when nuclear fusion no longer supplies enough energy to counteract gravity, so that the
nuclei and electrons of all the elements are crushed together to reach a density of about 3x1014
grams per cubic centimeter. A thimbleful would weigh about 300 million tons, equivalent to
six thousand Titanics. Neutronium has caught the imagination of science fiction fans in the
brilliant novels of Robert Forward - Dragon’s Egg and Starquake. Unfortunately silly
authors have made it the material of armour or weapons. Here on Earth, thermal neutrons,
slowed to speeds comparable with those of the molecules in ordinary matter at room
temperature, can be contained in a bottle to form an artificial and highly radioactive ‘noble
gas’, which decays with a half life of 14.64 minutes to form protons and electrons – the
constituents of hydrogen.
Hydrogen and helium. I have placed these at the mid-point and the end of the first turn
of the spiral, next to carbon and the noble gases respectively. In this respect I reject the
arrangement introduced by Charles Janet, who placed them next to lithium and beryllium.
The notion that they in any way resemble the metals of the first two groups seems very farfetched and makes sense only in the context of a table, which gains in regularity if the rows
end with the alkali metals. There is nothing final about the completion of an s subshell,
except in the case of helium, which is in every respect like the other noble gases. The s
electrons of later rows join with subsequent p, d or f electrons in a way that those in a
completed ns+np combination do not.
Hydrogen sits comfortably above carbon, which it resembles in many ways, whereas it has
hardly any resemblance – apart from its valency - to lithium and the other alkali metals or to
fluorine and the other halogens. Both hydrogen and carbon are half way to having a full outer
shell of electrons, and because of this they combine with other elements mainly in covalent
compounds. Carbon and hydrogen have a great affinity for each other and are found together
in a vast number of compounds. Detailed arguments for this placing are given by Marshall
Cronyn ‘The Proper Place for Hydrogen in the Periodic Table’ Journal of Chemical
Education, Vol. 80, no. 8, pp. 947-951 (August 2003). Still, the disk for hydrogen is coloured
differently to signify its uniqueness.
Lutetium and Lawrencium pose a problem in conventional versions of the Periodic
Table, in which blocks are separated: should they be regarded as the last in the f block or the
first in the d block? The problem disappears with the spiral arrangement: they can be seen as
belonging with both. This avoids the messy solution adopted in those tables that treat
lanthanum and actinium as the first of an interrupted sequence of d block elements, in which
case they are paradoxically not in the lanthanide and actinide series to which they give their
names. Lutetium is placed next to yttrium, which, because of the lanthanide contraction, it
resembles in atomic radius and consequent properties, in the same way that hafnium and
tantalum resemble zirconium and niobium. The problem is treated in detail by W B Jensen
(The Positions of Lanthanum (Actinium) and Lutetium (Lawrecium) in the Periodic
Table. J. Chem. Education 59, pp. 634-6, 1982).
Colours. Going from the alkali metals to the halogens I have run through the spectrum
from violet to red, with grey for the noble gases. For those transition elements with highest
oxidation states like those of the main groups, I have used paler versions of the same colours,
very much along the lines suggested by Mendeleev’s a and b sub-groups. For Mendeleev’s
‘group VIII’ (the iron, cobalt, nickel and copper groups, ignoring his later opinion that the
latter formed group Ib), I have used a series of graded browns to suggest their common
feature of not being able to use all their d electrons in bonding (apart from OsVIII and possibly
RuVIII).
For the lanthanides, I have used a similar colour scheme, inspired by the periodic table
recently introduced by Michael Laing (‘A revised periodic table: with the lanthanides
repositioned.’ Foundations of Chemistry 7, pp. 203-233, 2005). Ytterbium, with a complete f
subshell, and europium, with all seven f orbitals occupied, can adopt a II oxidation state and
are therefore coloured with a pale version of the blue of the beryllium and zinc groups.
Lanthanum and gadolinium, with a d electron, merit a pale version of the cyan of the boron
group. Cerium and terbium, which easily attain an oxidation state of IV, are appropriately
given a paler version of the green of the carbon and titanium groups.
For the remaining lanthanides, I have used paler versions of the graded browns used for
the later transition elements. The Eu-Gd-Tb triplet, occurring just about where yttria were
separated from ceria or lanthana, divides them into two sets of four, which should make it
easier to remember the sequence of these confusingly similar elements. The blue-cyan-green
triplets of Ba-La-Ce and Yb-Lu-Hf bridge the gaps between the s, f and d ‘blocks’, and the
Zn-Ga-Ge triplet provides a similar link between the d and p ‘blocks’, reinforcing the
impression of continuity given by the spiral arrangement.
The actinides have the same colour scheme as the lanthanides, though it would have been
possible to justify differences to take account of the higher oxidation states displayed by some
of them such as UVI. However, oxidation states in general are too complex a matter to be
fully represented by a few colours, and priority is given to making it easy for the eye to
distinguish the different groups.
Radioactive elements are indicated by symbols in italics. A black ring surrounds the disk
for elements in which all isotopes have short half lives. Thorium and uranium are
distinguished by mainly white rings to signify the fact that their half lives are respectively 14
billion years (about equal to the age of the universe) and, for uranium-238, 4½ billion years
(about equal to the age of the earth). Transuranian elements up to 111, roentgenium, are
given the names assigned by IUPAC. Ununbium and ununquadium are given their rather
preposterous pseudo-Latin provisional names. Reports of elements 113 and 115 to 118 have
not been ratified, and they are referred to only by their atomic numbers.
I have indicated stable elements that have one or more meta-stable isotopes by black
‘nicks’ in the white rings that surround them. Most of these are so feebly radioactive as to
present no practical danger. My reason for marking them is to suggest that the periodic
system of chemistry is echoed in the physics of the nucleus. The gaps between the ‘magic
numbers’ of neutrons and protons, which make for nuclear stability, sometimes coincide with
the gaps between numbers of electrons in the periodic build-up of shells. Thus tin and lead,
with 50 and 82 protons, are in the same group (with lead 208 also having a magic number of
neutrons). Similarly, magic numbers of neutrons are found in and around calcium (also with
a magic number of protons), strontium and barium. Meta-stable nuclei are common near to
the magic numbers, suggesting that it is difficult for nuclei to ‘hold on to’ too many neutrons
or ‘make do with’ too few near these critical points. It is noticeable that technetium and
promethium occur five places after strontium and barium respectively, something that does
not show up in tables other than Michael Laing’s.
© P J Stewart, 2006