About the Creator
Philip Stewart decided at an early age that he did not want
to choose between arts and sciences. After taking degrees
at the University of Oxford in Arabic and in Forestry, he
spent seven years in Algeria working in forest and soil
conservation. In 1975 he returned to his old university,
where for 31 years he taught Economics to Biology students
and Ecology to Human Sciences students, occasionally also
taking Arabic pupils. His special interest is in the way
that people's beliefs affect the way that they interact
with natural systems. Chemistry has always fascinated him
as linking the vast world of stars and galaxies to the
utterly minute world of the atoms and molecules of our
living planet.
Philip's personal website can be found
here.
A New Vision of the Periodic Table of the
Elements
The matter in the world about us is almost entirely made
from 83 elements, which differ from each other by the
positive electric charge on their central nucleus and hence
the number of electrons they are able to attract. 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.
Chemists discovered in the 1860s that if the elements are
arranged in the order of the mass of their atoms (or as
later realized, the charge on their nucleus), chemical
characteristics recur regularly. Dmitri Mendeleyev in 1869
published his Periodic Table, in which he arranged the
elements then known in rows and columns to show this
regularity, forecasting the discovery of missing elements.
This was a convenient way to present the information, but
he recognized that it did not fully represent the overall
pattern because it broke up the sequence. He thought the
ideal would be a cylindrical helix, but that needs three
dimensions.
Many
chemists before and after Mendeleyev have proposed a spiral
image, to get the advantages of a helix in two dimensions.
Philip Stewart's Chemical Galaxy is the latest of these
versions, using a starry pathway to link the elements and
to express the astronomical reach of chemistry. The
intention is not to replace the familiar table, but to
complement it and at the same time to stimulate the
imagination and to evoke wonder at the order underlying the
universe.
Chemical Galaxy II
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 meta-stable
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’).
Beginners
Guide
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 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 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.
© P J Stewart, 2006
Advanced
Guide
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
far-fetched 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.
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.
© P J Stewart, 2006
Chemical
Galaxy Illustrator
Philip
Stewart commissioned Carl Wenczek of
Born Digital Ltd
to translate the design into electronic form.
PUBLISHED
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