Download 7. A timeline of symbols and signs in chemistry

1
Symbols and signs in chemical education - Part 2: the historical evolution of
conventions
John Oversby, Institute of Education, The University of Reading, Reading, RG6 1HY,
UK email: [email protected]
Introduction
A distinguishing feature of chemistry as a scientific discipline is its use of symbols
and signs as a means of communication about chemical reactions. Although they seem
to be much in common with algebraic use of symbols and signs, there are important
differences when used in chemical contexts. An understanding of how the present use
came about may shed light on some of the problems that learners in schools may have
with chemical equations.
This article aims to explore the relationship between symbolic explanation and the
phenomena that require explanation. As a major source of evidence, it refers to
textbooks use of chemical symbols and equations. The research questions were:
1. What signs and symbols appear in textbook of chemistry at school level??
2. What changes in chemical symbolism has taken place over the last 150 years and
what has contributed to these changes?
3. What can a study of chemical symbolism contribute to thinking about learning
chemistry?
Chemists use representations as part of their explanations. We use representations to
communicate ideas even though the representations have limitations. When we draw a
chemical formula, for example, we focus on some aspects of it. These might be just
the elements in the formula, or the numbers of each component. We ignore some
aspects or take them for granted. One of these might be the order in which the
symbols are arranged. We might in one formula for water write H2O, and in another
write HOH. In the first case, we may be emphasising the presence of two hydrogen
atoms and in the second we might be emphasising that the oxygen is in the middle of
the two hydrogen components. Often we do not make these distinctions clear.
When the other person looks at the formula, they may not be aware of the meaning
that is the focus for us. In the case of H2O, the other person may focus on the three
atoms. In the second case, they may interpret it as having the three atoms in a line.
The first interpretation is legitimate but not what was intended. The second one is not
a legitimate interpretation since there is experimental evidence that the water
molecule is not linear. What is likely is that the interpretation of the creator and the
receiver are more likely to be the same if the creator explains clearly how the formula
is to be viewed. We call this clarifying the representing process. In practice, creators
often fail to make their representation clear and the receiver then has to make her or
his own sense of the drawing or formula. This is how many misinterpretations happen.
Representing faithfully therefore consists of the following steps:

identification of significant features of the target or phenomenon to be
explained;

selection of one or more representations from a different source that is
reasonably well understood;
Chemical symbols, signs© John Oversby, October 2002
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
identification of features that correspond to some significant features of the
target. In general, the more features that readily correspond, the better the
representation;

dismissal of non-corresponding features. This part of representing is more
commonly omitted or poorly done. The consequence of this is that receivers of
the representation may attempt to match inappropriate features of the
representation and so be misled;

consideration of corresponding features to elaborate explanation of the target;

consideration of corresponding features in relation to similar representations
and targets. A powerful representation will also explain similar targets or be
consistent with other representations;

in appropriate circumstances, corresponding features may prove fertile, that is
spark off explanations of the target not initially recognised;

development of the representation in the light of further data about the target.
The representation may be refined or modified in the light of novel data.
Powerful representations are those that can be refined or modified without
losing their essential characteristics.

comparison with other representations. The first comparison is in terms of
universality ie how much of the target or phenomena is explained. The second
comparison is in terms of satisfaction of explanation i.e. a subjective opinion
of how far the representation succeeds in explanation. The third comparison is
in terms of predictive power, that is, the ability to predict the outcomes of
novel experiments. Quantitative predictions are preferred.
The first + sign in a chemical equation means ‘reacts with’ and not ‘plus’, which
would be an algebraic meaning. The second + sign means ‘and also’, implying that
the chemicals are not combined. Again, use of ‘plus’ would be inappropriate. The 
arrow is another indication that these are chemical equations, not algebraic equations,
where an = sign would be expected. The equations have an implied direction, from
left to right. In algebra, 2+3 = 4+1 can be read either way. It implies complete
equality in both directions. The = sign is an operator in algebra, meaning to sum the
two values. This is clearly not the same as the chemical meaning.
Dictionary definitions of symbols
A symbol in Mathematics ‘is a character, letter or special mark taken as representing
some quantity, etc.’ (James & Lee,1995). The same reference also defines ‘chemical
symbol’ as ‘a letter or letters used to signify a chemical element’ This particular
definition of chemical symbol is deficient, particularly in regard to whether it is an
abbreviation of the name, or represents a sub-microscopic form such as the atom, or
whether it represents a macroscopic quantity, such as a mole.
Summary of symbols in equations
The ideas indicated in this example have set the scene for an examination of the
syntax of symbols and signs as parts of ways of representing macroscopic chemical
phenomena.
Chemical symbols, signs© John Oversby, October 2002
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Ideas about symbols
Chemists constructed a set of symbols for a purpose, that is, to express abstract ideas.
These symbols are not identical to those used in algebra as Malvern (2000) has
suggested. The table below provides a range of ideas about symbols against which
data from textbooks can be evaluated.
Ideas about symbols
Progress in development
A symbol represents the name
of the element.
This was an early development
and remains important today
A symbol represents a
macroscopic quantity
This came before submicroscopic representations,
beginning late in the eighteenth
century. At higher academic
levels e.g. A level, the
macroscopic representation is
still significant and present as
the mole but is not often made
explicit
This came soon after the use of
symbols for macroscopic
quantities but now is either the
dominant, or often the only,
known meaning of symbol.
A symbol represents a submicroscopic quantity, at the
atomic level.
Chemical symbols, signs© John Oversby, October 2002
Challenges
Invention of new pseudo
symbols (see text) needs to be
integrated
The link between experimental
phenomena and representation
has been weakened at school
level. There is a need for greater
emphasis on macroscopic
significance of symbols.
The use of symbols at the submicroscopic levels to represent
invisible entities should be
greater explored during the
learning process. Greater rigour
in the use of pseudo symbols
should be established by the
community of chemists.
4
Textbook ideas about symbols
Date
Author
Meaning of the term symbol
1785
Lavoisier
Lavoisier used symbols to denote both
constitution and quantity.
1810
Dalton
1833
Turner
(cited in
Nye, 1993)
‘Such symbols referred to the atom and were
therefore conceptually very different from
alchemical symbols or those of Hassenfratz and
Adet which had only a hazy or qualitative
meaning. Earlier symbols had been a shorthand;
Dalton’s circles conveyed a theoretical meaning as
well as being a convenient abbreviation.’ (p139 in
Brock (1992), referring to Dalton’s (1810) New
System
The symbols ‘are only fitted to be a convenient
abbreviation among educated chemists, but may
be made a powerful instrument of instruction by
teachers of chemistry.’ (p vii-viii)
1856
Wilson
1856
Miller
1864
Wurtz
1866
Frankland
1868
Naquet
It should never be forgotten that chemical symbols
are not like cyphers, mere substitutes for words.
But are further supplements to them. Thus the
symbols HO, NO5 and NH3, are not simple
synonymes of water, nitric acid, and ammonia, but
tell us the nature and relative amount of the
elements in these bodies, which the words do not.
[Note on p 57]
The use of the comma in his book:
P434
MgO,SO3,7H2O
P497
(KO,NO5 + (KO,HO,2SO3) = HO,NO5 +
2(KO,SO3)
Used barred formulae to indicate new atomic
weights (these were twice the old ones)
e.g.
H2O = H2O
CaO = CaO
He also used the comma for combination.
Uses typical subscripts although he is not
particular about the order of the symbols.
P9
NO3H + ONaH = NO3Na + OH2
SO4H2 + 2OKH = SO4K2 + 2OH2
Frankland (1866) also has something to say about
notation (p14): formulae symbols "denote a
certain definite proportion by weight of each
element"
Notation on p27:
… a symbol is used which represents the atom
(not the molecule) of each simple body
Footnote. Some symbols are employed barred, to
indicate they are taken with the value they have in
atomic notation, and merely to avoid the
Chemical symbols, signs© John Oversby, October 2002
Comments
Quoted in Brock (1992)
(p118) relating to a
paper on the
composition of water
published by Lavoisier
in 1785.
Dalton used a set of
iconic symbols for his
representations.
Nye reports that Turner
introduced the symbols
half way through the
text after confronting
the difficulty of using
ordinary language to
give an account of
Liebig’s and Wöhler’s
work on cyanogens.
Wilson is probably
writing for what we
would now understand
to be undergraduate
chemists.
Miller is probably
writing for what we
would now understand
to be undergraduate
chemists
Wurtz is probably
writing for what we
would now understand
to be undergraduate
chemists
Frankland is probably
writing for what we
would now understand
to be undergraduate
chemists
Naquet is probably
writing for what we
would now understand
to be undergraduate
chemist
5
1870
Odling
1886
Fittig
1887
Gautier
confusion which might arise, as several authors
still use the notation of equivalents. As, however,
the latter notation is becoming disused, I have not
thought it necessary to continue the use of the
bars.
p28 superscripts, thus SO3.
In writing symbols it has been agreed to place
first, that one of the different components which is
the most electro-positive; but this rule is only
followed exactly when there are not more than two
elements present.
In the book he uses superscripts throughout but
uses braces for some formulae, a practice that was
common at the time..
P39 H2O + COCl2 = CO2 + H} + H}
Cl}
Cl}
Wrote a chemistry textbook for a mainly medical
readership, using subscripts in a special way:
P23
HgO + Cl4 = Cl2O + Cl2Hg
For elements he does not use coefficients but
multiples of atoms in formula.
A German chemist who used superscripts towards
the end of the nineteenth century. Note the use of
names under the symbols.
P29
C6H8(OH)6 + 11HJ
=
C6H13J + 6H2O + 11J
Or
C3H5(OH)3 + 5HJ =
C3H7J + 3H2O + 4J
Glycerin
isopropyljodür
Used superscripts, for example:
P29
H2O
CO2
P418
2C2H5 + 2Na = 2NaBr + (C6H5)2
P53
2HCl
+ Fe =
Acide chlorhydrique Fer
1894
Jago
FeCl2
+ H2
Chlorure ferreuse Hydrogène
P169
6KOH + 6Cl = 5KCl + ClO3K + 3H2O
[Bold type is original.] ‘For convenience of
description each element has an abbreviation of
its full name, called its symbol. This is, where
practicable, the first letter of its Latin name.
Simply as abbreviations of the full names symbols
and formulae are of great service; this, however, is
but a small part of their significance and value in
chemistry. Their further use may best be explained
by reference to certain information gained by
experiment, to which careful attention is
requested. [ … ] It is possible to assign to every
element a number, which number or its
multiple shall represent the proportionate
quantity by weight of that element which enters
into any chemical compound. These numbers
are termed the ‘combining or atomic weights’
of the elements, and are deduced from results
obtained on actual analysis. [ … ] The symbol
represents the quantity of the element indicated
by the combining weight; where multiples of that
quantity exist in a compound the fact is expressed
by placing a small figure after the symbol and
slightly below the line. A large figure placed
before a symbol signifies that number of the
whole compound.’ (p33-34)
Chemical symbols, signs© John Oversby, October 2002
Odling is writing for
what we would now
understand to be
undergraduate medical
students
Fittig is probably
writing for what we
would now understand
to be undergraduate
chemists
Gautier is probably
writing for what we
would now understand
to be undergraduate
chemists
Jago’s book is an
inorganic textbook,
probably aimed at
undergraduate chemists.
6
1899
Newth
States (p52) that the symbol is atom and mass.
1904
Moissan
French chemist was still using superscripts in
1904 e.g. p 9 Cs2O and on P39
K2SiF6 + 4KOH = 6KF + SiO2.2H2O
1930
Hooton
1954
Holderness
& Lambert
1954
Brown
1964
Fowler &
Fowler
1980
Ramsden
1984
Hunt &
Sykes
‘For every element there has been adopted as short
symbol to represent in these [formulae] equations.
… the symbol stands not only for the name of the
element, but also a definite quantity by weight of
it; thus, Hg represents 200 parts by weight of
mercury, and O 16 parts by weight of oxygen.’
(p48)
‘The symbol of an element consists of one or
more letters which denote one atom of the
element. It is important to keep clearly in mind
that the symbol of an element does stand for a
perfectly definite amount of it, and that amount is
one atom.’ (p21)
‘… it is possible to represent an atom of a
particular element by means of a symbol. … It is
very important to realise that the symbol O, for
instance, does not just refer to oxygen; it
represents a definite amount – one single atom –
of oxygen.’ (137)
‘…thing regarded by general consent as naturally
typifying or representing or recalling something
by possession of analogous qualities or by
association in fact or thought. Mark or character
taken as the conventional sign of some object or
idea or process e.g the astronomical signs for the
planets, the letters standing for chemical elements
… From the Greek  (sumbolon), mark or
token’
‘C stands for one atom of carbon. Cl stands for
one atom of chlorine, Co for an atom of cobalt, Cr
for an atom of chromium. A symbol stands for one
atom of an element.’
[the term symbol is not explained in this text.]
1993
Jones,
Jones and
Acaster
‘Each element has its own symbol. Hydrogen, for
example, has the symbol H. Helium has the
symbol He.’
1994
Ramsden
For every atom there is a symbol. For example,
the symbol for sulphur is S. the S stands for one
atom of sulphur.
‘A single capital letter, or a combination of a
capital letter and a small one, which is used to
represent either an atom or a mole of a chemical
element: eg the symbol for sodium is Na, for
1995
Larousse
Chemical symbols, signs© John Oversby, October 2002
Newth is probably
writing for what we
would now understand
to be undergraduate
chemists
Moissan is probably
writing for what we
would now understand
to be undergraduate
chemists
Hooton’s book for older
school pupils (16-18
years old) is typical of
this period.
Holderness and
Lambert’s work was
one of the dominant
textbooks in the 1930s,
1940s and 1950s in the
UK.
Brown’s book was one
of the do`minant
textbooks in the 1950s
and 1960s in the UK.
This definition is given
in a standard dictionary.
It is written for the
intelligent lay person
rather than the
specialist chemist.
Ramsden writes here
for the secondary
student (11-14 years
old)
This textbook by Hunt
and Sykes supported
the very popular
Nuffield chemistry
course widely used in
secondary schools at
this time.
These authors are
writing for secondary
students (14-16 years
old)
Ramsden is writing for
secondary students (1416 years old)
Larousse is used as a
reference of good
practice in recent times.
7
1999
Wilson
sulphur is S.’
‘Symbols: hydrogen is H and calcium is Ca while
chlorine is Cl.’
2000
McDuell
[the term symbol is not explained in this text.]
Wilson is writing for
teachers of chemistry in
secondary schools (1416 years old).
McDuells’ book is
aimed at teachers of
chemistry in the
secondary school (1116 years old).
Pseudo symbols
Chemists also use a range of chemical symbols that are similar to those used for
elements. The table below indicates the symbols, their uses, and the approximate
stages at which they are introduced.
Pseudo symbol
Meaning and comment
Age at which
introduced in the UK
e
An electron (free electron) with or without the
minus
Ligand entity
Cyanide – used by Wilson (1856)
Ethylene diamine tetra-acetic acid – used in
complex metal chemistry
Diamino ethane – used in complex metal
chemistry
Oxalic acid – used in complex metal
chemistry
General symbol for metal, used in general
formulae and equations e.g. for alkali metals
2M + Cl2  2MCl
General symbol for non-metal, used in general
formulae and equations e.g. for halogens
2Na + X2  2NaX
General symbol for anion, used most
frequently in algebraic functions relating to
weak acids e.g. HA.
CH3 – the methyl group. Used to identify this
group in condensed and display formulae
C2H5 – the ethyl group. Used to identify this
group in condensed and display formulae
C3H7 – the propyl group. Used to identify this
group in condensed and display formulae
C4H9 – the butyl group. Used to identify this
group in condensed and display formulae
(CH3)3C – trimethyl methyl or tertiary butyl
group. Used to identify the goup in condensed
and display formulae
Acetate (CH3COO) or acetyl (CH3CO). Used
to identify this group in condensed and
display formulae
Phenyl, (C6H5). Used to identify this group in
condensed and display formulae
Any alkyl group. Used to identify this group
in condensed and display formulae
Any aryl group, containing a benzene ring
Upper secondary (14-16) and
post 16
Post 16 and university
Post 16 and university
Post 16 and university
L
Cy
Edta
En
Ox
M
X
A
Me
Et
Pr
Bu
t-Bu
Ac
Ph
R
Ar
Chemical symbols, signs© John Oversby, October 2002
Post 16 and university
Post 16 and university
Upper secondary school (1416) and post 16
Upper secondary school (1416) and post 16
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
Post 16 and university
8
type structure. Used to identify this group in
condensed and display formulae
Pseudo symbols appear to be most frequently used in generalising about reactions, or
in chunking parts of a formula to aid recognition. In many respects they are used in
place of element symbols in condensed and display formulae.
A textbook discussion about good practice in the use of symbols
The original Nuffield Chemistry Handbook for teachers (Coulson, 1967) was very
clear about the issues behind the use of symbols.
‘With the idea of the atom comes the idea of the symbol, the sign to represent
the atom. In some ways it is unfortunate that the ideas of Berzelius prevailed
over Dalton’s picture symbols. … Had chemists adopted picture symbols,
beginners might have found it easier to remember that a symbol can represent
a single atom. With the less able children it is sometimes helpful to write the
symbol in a circle e.g. Mg, S, H, until the idea is well established.’ (p 64)
[The Nuffield Scheme suggested that learners should ‘use symbols and
formulae in an experimentally meaningful context’ (p111) i.e. from a
macroscopic origin.]
‘symbols are used to represent atoms of elements, but when they are used to
give information about material in a bottle, the symbol by itself is insufficient
– the idea of giant structures and molecules is by now established and we can
write:
Cu(c) for a copper atom that is part of a crystal lattice
Hg(l) for an atom of mercury in a liquid
H2(g) for a molecule of hydrogen in the gaseous form’ (p 23-25)
S8 (orthorhombic) and S8 (monoclinic) when there is a need to distinguish the
allotropes
Zn(c) meaning ‘an atom of zinc as it is in crystalline metal’
Zn(g) meaning ‘an atom of zinc as it is in the gas’
[this comes after a section on counting atoms in a bag and invoking the idea
of proportionality of mass.] Zn represents one mole of atoms of zinc and S one
mole of atoms of sulphur. … the symbol will later be used to represent one
atom of an element (p 49)
… we can represent these atoms by symbols: we could use circles or triangles
to represent atoms taking part in chemical reactions, for example
We still use symbols to represent the atoms of elements but these are now
related to the names of elements. (p 66)
They may revise the symbols for the elements and learn that these may be used
to represent the quantity of element expressed in moles of atoms (gramatoms). This change in the use of symbols requires emphasis. (p 112)
Commentary on the Nuffield Handbook (Coulson, 1967) discussion
Chemical symbols, signs© John Oversby, October 2002
9
The Nuffield Handbook, almost uniquely, provides some advice about suitable
pedagogy about symbols. It provides:
 some of the history of the origin of symbols with an evaluation of the
effect of decisions taken then on potential teaching challenges today;
 clear identification of the meanings of symbols at all three levels
 a comparison of the use of algebraic symbols with the use of other
representations such as circles or triangles.
QCA Key Stage 3 Scheme of Work
Unit 8E Atoms and elements
In this unit:
most pupils will explain that compounds are made when atoms of different
elements join together; begin to use symbols for elements and to represent
reactions in word equations
some pupils will name some elements and represent these by symbols;
distinguish between symbols for elements and formulae for compounds
some will begin to represent compounds by formulae
Comments:
recognise the symbols for some elements; identify that elements are
made from atoms of one kind; explain the existence of compounds in
terms of atoms joining together; describe compounds e.g. water is
made of hydrogen and oxygen joined together; describe some simple
molecules, eg carbon dioxide is made of one carbon atom joined to
two oxygen atoms; {some pupils make their own representations of
some simple molecules and relate these to their chemical formulae} {it
is important to avoid talking about a molecule of sodium chloride, for
example}; establish the idea that when these materials are made the
atoms combine or join. Introduce the terms ‘molecule’ and
‘compound’, show representations or photographs of representations,
of simple and more complicated molecules, to illustrate the point. If
possible also provide samples of the compounds and their formulae.
Show pupil some reactions between elements eg
- hydrogen and oxygen
- sodium and chlorine
- carbon and oxygen
- copper and sulphur
and ask then to record their observations in terms of appearance of the
reactants and the products and to represent the reactions by word
equations, simulation software, representations or diagrams, making
clear that when the compound is formed the atoms join. Help pupils to
represent the compounds formed by formulae and representations. Ask
pupils to explain individual reactions in these terms and to question
others about their representations. Explain compound formation in
terms of atoms joining e.g. using equations, diagrams, representations.
Pupils will need time to practise using the chemical names of
compounds e.g. making the change from oxygen to oxide. It is helpful
for pupils to become familiar with formulae for common compounds
and to relate these to representations showing the numbers of atoms.
Chemical symbols, signs© John Oversby, October 2002
10
Teachers will be able to judge how far to take explanations of
differences in formulae with different pupils. For some pupils, list the
formulae of the products of the reactions seen and introduce the idea of
‘combining power’, using a simple representation e.g. card cut-outs of
elements; ball-and-spoke representations. Ask pupils to use the
representations to predict the formulae of other metallic oxides,
chlorides and sulfides. It is helpful to be explicit that the
representations are not representations of real atoms. Help pupils to
summarise the reactions of copper carbonate in a word equation,
making clear which reactants and products are elements and which are
compounds.
Language for learning: understand, use and spell correctly:
Scientific words eg element, compound, atom, molecule,
symbol, formula
Names of elements and compounds eg oxygen, carbon dioxide,
sodium, chlorine, sodium chloride
Words and phrases with different meanings in scientific and
everyday contexts, eg element, equation, state
Unit 8F compounds and mixtures
In this unit pupils:
Distinguish between elements and compounds and how they are
represented by symbols and formulae
Recognise chemical change as a process in which atoms join together
in new ways
That elements contain only one kind of atom. That compounds contain
more than one kind of atom joined together. That formulae indicate the
(relative) numbers of atoms in a compound. If ionic compounds are
included, it is sufficient to treat the formula as a ratio, eg one atom of
magnesium to one atom of oxygen in magnesium oxide. It is important
not to imply that ionic compounds consist of molecules.
Discussion of the QCA KS3 Scheme of Work
The QCA Scheme of Work is unclear about the meaning of chemical symbols,
referring only to the vague term ‘elements’. Similarly, the Scheme of Work is also
unclear about the specific ways in which the symbols represent chemical substances.
The only comment about formulae states that sodium chloride should not be
considered as being made up of molecules, with only a little indication of why, or of
how general this advice should be. The Scheme of Work is unhelpful in explaining
reasons for its advice.
Chemical symbols, signs© John Oversby, October 2002
11
The meanings of chemical signs in textbooks, including chemical arrows
Date
1856
Author
Wilson
Equations
p120
Before decomposition, KO,NO5 and HO,SO3;
After decomposition, HO,NO5 and KO,SO3.
Or more fully thus:-
Signs
See previous cell. No arrows are used in
the text.
Comments
Letterpress
p16 The sign +, …is placed between the
formulae of the molecules of the different
substances which are brought into contact
before the reaction, and of those which
result from the change. This sign must
never be used to connect together the
constituents of one and the same chemical
compound.
Na2O + H2O = 2NaHO
Sodium oxide
Sodium hydrate
Letterpress
KO
NO5 = HO,NO 5
HO
SO 3 = KO,SO 3
Here the dotted horizontal lines indicate the
bodies in combination at the beginning of the
process, the oblique unbroken lines the
exchange of acid and base that occurs during
the decomposition, and the double horizontal
lines = (the algebraic sign of equal to) the
bodies in combination at the end of the
process.
PO5 + 5C = P and 5CO
This equation (p162) shows the symbol + on the
left hand side and uses 'and' on the right hand
side.
2HCl + MnO2 = 2HO:Cl and MnCl
The coefficients can be seen from the word
equation.
1866
Frankland
1894
Jago
Na2O + H2O = 2NaHO
Sodium oxide
Sodium hydrate
Chemical symbols, signs© John Oversby, October 2002
Letterpress
No use of arrows in this book
12
p117
Chemical changes are most conveniently
expressed by what are termed 'chemical
equations'; these consist of the symbols and
formulae of the bodies participating placed just
before the sign =, while those of the resultant
bodies follow.
For example, the action of iodide of potassium and
chloride of mercury on each other is thus
expressed:-
P117
HgCl2 + 2KI = HgI2 + 2KCl
Mercury chloride Potassium iodide Mercury iodide
Potassium chloride
[The words are under the formulae in the original
text]
As no chemical change affects the weight of
matter, the weight of the quantity of a
compound represented by its formula must be
the sum of those constituent elements; so, too,
the weight of the bodies resulting from a
chemical change must be the same as that of
the bodies before the change, whatever it may
be, had occurred. Also the same number of
atoms of each element must appear on each side
of the equation. Although from a chemical equation
and table of combining weights it is possible to
state what relative weight of each element is
concerned in each reaction, it must never be
forgotten that the combining weights were first
determined by experiment, and then the table
combined therefrom.
1897
Roscoe
NH4NO2 = N2 + 2H2O
P47
Chemical symbols, signs© John Oversby, October 2002
Letterpress
Arrows are not used except as
part of the reversible reaction
13
sign
p69
1899
Newth
P54 Atomic equation, Hg + 2Cl = HgCl2
Molecular equation, Hg + Cl2 = HgCl2
1906
Armitage
No chemical equations provided
1913
Schmidt
1930
Hooton
Used the arrow  sign for formulae and = for
line equations mainly using = signs.
P65
C6H5AsCl4 + 3H2O =
C6H5AsO(OH)2 + 4HCl
But (on the same page) (C6H5)3As + Cl2 
(C6H5)3AsCl2
Elements and Symbols
Mercuric oxide = mercury + oxygen
The sign of equality indicates that the total
weight remains unchanged; mercuric oxide
breaks up into the two elements, mercury and
oxygen, and their weights are together equal
to the oxide which yields them.
HgO = Hg + O
P47-48
P24 Zn + H2SO4  ZnSO4 + H2
This equation is interpreted in terms of
1954
Holderness &
Lambert
Chemical symbols, signs© John Oversby, October 2002
P22 The sign + has a different
significance as used on the left side of the
equation to that which it bears upon the
right. On the left side it implies that
chemical action takes place between the
substances, while on the opposite side it
has the simple algebraic meaning … "and
also"
P52 symbol is atom and mass
P54 Atomic equation, Hg + 2Cl = HgCl2
Molecular equation, Hg + Cl2 = HgCl2
p109 referring to Berzelius SO3 and
Cr2O3
sulphur trioxide
Used the arrow  sign for formulae and
= for line equations mainly using = signs.
P65
C6H5AsCl4 + 3H2O =
C6H5AsO(OH)2 + 4HCl
But (on the same page) (C6H5)3As + Cl2
 (C6H5)3AsCl2
for reversible reactions
P83 Up arrows and down arrows are
mentioned on this page and occasionally
Letterpress
Letterpress
This textbook is concerned with
the history of chemistry
Arrows are not used
Offset litho
Letterpress
↑↓ are not used.
Offset litho
14
1956
Clynes &
Williams
molecules, not macroscopic.
Later on the same page:
Mg + H2SO4  MgSO4 + H2
Fe + H2SO4  FeSO4 + H2
The equations correspond to, and represent,
the observed fact that when these two metals
are brought into contact with dilute sulphuric
acid, hydrogen is liberated and the metals are
converted into their sulphates.
P 135, 136 By use of equations the materials
taking part in a reaction and the products of
reaction can be shown, and it is possible,
moreover, to calculate the weights of the
different substances concerned.
Magnesium and Oxygen give Magnesium
oxide
Or, Magnesium + Oxygen  Magnesium
oxide
used elsewhere. Arrows are used
throughout.
The + sign on the left of the equation
means “reacts with”, but on the right it
means simply “and”, while the arrow
means “producing”.
Both arrows and = signs are used
p176
Zn + 2HCl = ZnCl 2 + H2↑
Offset litho
2Mg + O2 = 2MgO
1959
Emeleus and
Anderson
1973
Spiers &
Stebbens
1978
Hall
The book then treats the matter of balancing
equations in terms of conservation of mass.
Explicit explanation of coefficients is not
provided.
Used both old and new notation:
P 322 6WO42- + 7H+  [HW6O21] 5- + 3H2O
P392
3NaBH4 + 4BF3 = 3NaBF4 + 2B2H6
Chapter 12 Investigating Chemical Equations
pp 114 ff
This is entirely based on calculations.
p 27
blue copper sulphate + heat {equil arrow}
Chemical symbols, signs© John Oversby, October 2002
Used both old and new notation:
P 322 6WO42- + 7H+  [HW6O21] 5- +
3H2O
P392
3NaBH4 + 4BF3 = 3NaBF4 +
2B2H6
Other chapters use full symbol equations
with arrows.
Arrows used where equations are given.
Offset litho
15
white copper sulphate + water
1980
Palmer &
Shaw
1981
Shayer &
Adey
p31 ‘Chemical equations are shorthand ways
of describing chemical reactions.’
Chapter 5: elements, compounds, and atoms
explains how to write equations.
i. The formulas of the reactants are
written on the left.
ii. The formulas of the products are
written on the right.
iii. An arrow, , is used instead of the
‘equals’ sign, =.
iv. Like equations in maths, there must
be the same number of each type of
particle on the left as on the right.
P94
2B late concrete Chemical combinations
remembered without general rules being
appreciated. Composition of compounds as a
kind of mnemonic, e.g. water is made of
hydrogen and oxygen, and will produce
hydrogen and oxygen. Word equations could
be used to indicate a reversible reaction, such
as heat on hydrated copper sulphate. But
‘copper’ in that name is used only as a label,
so word equation is only a statement of fact.
3A early formal. Can handle the conservation
of elements in an exchange reaction, so for
the first time has a representation of a
chemical reaction. Use of balanced chemical
equations is possible, providing that plenty of
drill is given in learning the rules of the game.
The relation of chemical equations to
reactions will be perceived, but do not expect
pupils to know how to use them to estimate
Chemical symbols, signs© John Oversby, October 2002
Chapter 4: chemical change: elements and
compounds explains word equations. The
arrow means react to give.
No discussion of signs
Offset litho
16
1986
1989
Allen,
CrockerMichell,
Hannon, PageJones,
Thornley
Partington
1989
Gallagher &
Ingram
1989
Hill, Holman,
Lazonby,
quantities except by practice in specified
situations. May use atomic theory and simple
representation of atomic structure to account
for chemical change.
3B late formal. The functional use of
chemical symbols. The Nuffield approach to
equations – from the experimental situation,
through the nearest idealisation of the facts
that fits the measurement, to a balanced
equation
The term ‘equations’ is not in the index.
Equation syntax is not explained in this text.
P197 ‘Symbols were little used in textbooks
early in the century {19th}. Turner’s Elements
of chemistry (4th edition, 1833) explains
symbols and uses them, together with
chemical equations, but with an apology in
the preface. Liebig (Traité de chimie
organique, 3 vols, Paris, 1840-44) uses
equations but not liberally. Symbols and
equations are freely used in Gmelin’s
Handbook (English translation, 1847-1872)’
Chapter 5 pp66-67
You could show the reaction by a diagram,
like this:
{particle representations of carbon and
oxygen giving carbon dioxide and words
stating molecules underneath}
symbols C + O2  CO2
Equations are not mentioned in the index.
P9: calcium + water  calcium hydroxide +
Chemical symbols, signs© John Oversby, October 2002
No explanations of signs is given in tis
text. Arrows are used.
Offset litho
Partington does not give any detail about
the signs.
Offset litho
A history of chemistry textbook
p 67
Question 1: What do + and  mean, in
an equation?
Offset litho
no discussion of signs is apparent
Offset litho
17
Raffan &
Waddington
1993
Jones, Jones &
Acaster
1993
Hill
1995
Larousse
1995
Lister &
Renshaw
1996
Earl &
hydrogen
P12 questions:
magnesium + copper(II) oxide  copper +
magnesium oxide
p25:
methane + oxygen  carbon dioxide + water
CH4(g) + O2(g)  CO2(g) + 2H2O(l).
Pp 58-61 (Equations) balancing equations –
implicit reference to Law of Conservation of
mass only referenced to atoms not being able
to be added or removed..
Equations show what happens during a
chemical reaction.
Signs are not discussed.
Offset litho
heat
bread  carbon + carbon dioxide + water +
toast
food + oxygen  carbon dioxide + water
fuel + oxygen  carbon dioxide + water
iron + oxygen + water  hydrated iron oxide
Signs are not discussed
Offset litho
A quantitative symbolic representation of
the changes occurring in a chemical
reaction, based on the requirement that
matter is neither added nor removed
during the reaction.
Chapter 6 Equations
Word equations (p27) Equations are used as a
simple way to represent what is happening in
a chemical reaction, and the simplest is the
word equation. … equations should always be
based on experimental evidence.
Equations do not say how fast a reaction
happens. Conditions (heat, pressure, etc.) may
be shown on arrows, for example –heat 
{heat over arrow}
Signs are not discussed
Offset litho
Signs are not discussed
Offset litho
This is aimed at the 16-18 age
range
Word equation:
No discussion of signs is given
Offset litho
Chemical symbols, signs© John Oversby, October 2002
18
Wilford
copper
carbon
heat
+ carbon(s
copper(s) +

oxide(s)
dioxide(g) (the reactants)
(the products)
1996
McLean
2000
McDuell
If we replace the chemical names of the
reactants by their symbol or formulae we
can produce a full chemical equation. The
word equation for the thermal
decomposition of copper carbonate can
be replaced by:
heat
CuCO3(s)

CuO(s) + CO2(g)
Only reference to equations in the index is to
balancing equations. This is treated as writing
the word equation, then the formulae and then
checking the atom numbers. Finally, adding
state symbols. On this page the explanation is
algorithmic.
p41-45 (Chapter written by Pritchard)
‘Word equations should be seen by pupils as a
way of summarising a chemical reaction, with
the reacting substances (reactants) on the lefthand side and the substances produced
(products0 on the right-hand side. An arrow is
used rather than an equals sign because the
two sides are not strictly equal and an arrow
shows the direction of reaction. Pupils should
be encouraged to write word equations
whenever they come across a chemical
reaction. For example:
Chemical symbols, signs© John Oversby, October 2002
Signs are not discussed
Offset litho
p 41-45 An arrow is used rather than an
equals sign because the two sides are not
strictly equal and an arrow shows the
direction of reaction.
Offset litho
This is aimed at teachers of
chemistry at the 11-16 age range
19
magnesium oxide + sulphuric acid 
magnesium sulphate + hydrogen
For many pupils, writing word equations will
be the limit of their equation writing but for
pupils who go on to write symbol equations,
the word equation is still an important
precursor.
{The book then goes on to write about
balanced symbol equations. Explanation in
terms of the law of Conservation of Mass is
not provided but is simply algorithmic. A full
page of balanced symbol equations with
arrows and states is given on p 43. State
symbols are described on p 44. Ionic
equations are explained on p44 but it says: ‘if
an ionic substance dissolves in water it
becomes split up into its component ions, i.e.
it becomes ionised’. It also says that ‘solids,
liquids and gases remain un-ionised in
equations’ but ‘acids, alkalis and all soluble
salts can be written as component ions’.
No description of the meanings of other signs
is given.
Chemical symbols, signs© John Oversby, October 2002
20
The + sign
This section is deliberately not headed ‘the plus sign’. The + sign in chemical
equations has a meaning that is distinct and significantly different from the algebraic
meaning. In historical terms, Berzelius, in 1813, had a great influence on chemical
notation. Compounds were denoted at first (the early part of the eighteenth century)
by a + sign between the constituents element symbols e.g. copper oxide was Cu+O.
The exact meaning of the + sign is not made explicit, for example in Wilson (1856),
but it would seem reasonable to suggest that it meant ‘combined together’. The + sign
was dispensed with later but it may have been kept as a symbol for 'reacts with' or
joins with’. The + sign on the right of the equation has a different meaning. It simply
means ‘and also’. Wilson (1856) often writes symbolic equations with the term ‘and’
on the right hand side where we would put + today. The + sign then refers, at the
macroscopic level, to the existence of a chemical reaction in which one or more new
substances are formed. The use of the + sign here is peculiar to chemistry and
contrary to the meaning in algebra..
The equation arrow 
This section deals with the meaning of the main equation arrow although other
equation arrows will be treated later. It seems to mean the process of a chemical
change, or ‘yields’, ‘to give’. When chemistry teachers from a variety of countries
were questioned, they all responded with this meaning e.g. ça donne, by a French
colleague. The arrow corresponds to chemical change and is thus fundamental to the
nature of chemistry.
The position about the development of equation signs is not entirely clear. There is a
tendency for chemistry textbooks in the mid- and late nineteenth century to use the
more descriptive form and restrict the use of equations. This is understandable in
terms of printing technology for such equations would have to be specially set up by
the printer.
The equation arrow, , has not always been used to denote the process of chemical
change. From the beginning of writing chemical equations (1830s) until about the
1960s, textbooks used the = sign for the process of chemical change. This is in contradistinction to other opportunities chemists chose to use symbols and signs in a
different way to that used in algebra. Brock (1992) suggests that this may be the
influence of the printers who did not have the  in their printers’ box of type. I have
identified that books that used the letterpress method of printing, where the type
leaves an indentation in the page, always used the = sign for the equation arrow. This
method of printing was in common use until the early part of this century. A survey of
PGCE student science teachers demonstrated their unanimous preference for  over
=, many justifying this in terms of the chemical equation being distinct from algebraic
equations.
The symbol = is used in equations from the mid part of the nineteenth century. It was
available as a standard printer symbol and was so easy to use. The arrow symbol 
was occasionally used from the middle of the nineteenth century, especially in organic
chemical reactions using displayed formulae. In the early books, it was clearly a
special symbol devised by the printers and so may have been something of a nuisance.
Chemical symbols, signs© John Oversby, October 2002
21
Its use seems to be largely restricted to the line drawings used for displayed formula
equations used in organic chemistry. Even until the 1950s, the = sign was commonly
used. This may be the influence of the printing technology.
Other equation arrows
The two reversed arrows, as in the diagram below, became more popular in the early
part of the twentieth century and are in common use today. It may be that these
arrows were, by then commonly available to printers in their standard set of
characters.



The  sign (note that, in this font, the arrows are the wrong way round with the upper
arrow pointing to the left) appeared in the early twentieth century and gained ground,
as printing technology became more versatile. Books printed by letterpress, as
evidenced by the impression left on the printed page, use the = sign. Books printed by
the litho process are more likely to use the modern  form. This form is now
prevalent with the advent of modern word-processing, even though it is the wrong
way round. 
It has proved impossible to find explicit explanations of the meanings of these
symbols in textbooks used for teaching chemistry. The few books that explain the
form of chemical equations fail to explain the meanings of the signs +,  and. They
focus on the chemicals as reactants and products.
The history of the development of chemical symbols
Dalton (described in Brock, 1992) created a system of symbols for different elements
based on circles with distinctive marks inside. These symbols referred to the atom and
were therefore theoretical and not just some abbreviation for the names of the
elements. Brock suggests that Dalton's symbols may not have survived because they
were an additional expense for the printer; they required special type to be produced.
Their main contribution to chemistry was to focus on the sub-microscopic atomic
level of representation. In 1837, at the meeting of the British Association for the
Advancement of Science, British chemists agreed to use the symbols of Berzelius.
These were based on the alphabet and we can suspect the influence of the printing
industry in this decision.
Berzelius (1779 - 1848) had obtained a copy of Richter's writings on stoichiometry
and these contained explanations of the law of reciprocal proportions and chemical
equivalents. Richter first published in 1792 Anfangsgründe der Stöchyometrie
(Outlines of Stoichiometry, or the Art of Measuring Chemical Elements) (cited in
Partington, 1951). This work was based on the amounts of different acids that could
neutralise given quantities of different bases. Richter coined the term stoichiometry
from the Greek meaning to measure something that can not be divided. Richter had
expressed the law of constant proportions that characterises a fundamental property of
compounds. He also established the concept of chemical equivalents that was later
embodied in elemental symbols. These ideas influenced Berzelius' textbook, Larbok i
kemien, which was first published in 1808. Berzelius read about Dalton's work on
Chemical symbols, signs© John Oversby, October 2002
22
multiple proportions at second hand, because of the European wars at the time, but
obtained his own copy of Dalton's New System in 1812. Berzelius, in 1813, introduced
a system of symbols based on the Latin names of the elements. Compounds were
denoted at first by a + sign between the constituents element symbols eg copper oxide
was Cu + O. The plus sign was dispensed with later but it may have been kept as a
symbol for 'reacts with' as will be seen later in the Wilson (1856) text. Berzelius used
superscripts for the number of atoms in elements and these were placed to the right of
the symbol, eg S2O3 for a molecule of hyposulphuric acid. Brock states that these
'were criticised initially for being potentially confusing with algebraic symbolism'.
We can only surmise that the argument about potential confusion was lost because of
the influence of the printers in terms of ease of use. While most chemists used
subscripts soon after it was introduced by Berzelius and adopted by the British
Association for the Advancement of Science in 1837, many French chemists used the
superscript form until the twentieth century. It may be possible that the use of the
subscript form did make some distinction between algebraic notation and chemical
notation. In 1827 Berzelius in the organic section of his textbook, used the barred
form to indicate two atoms of the elements. Thus, Ħ represents two hydrogen atoms
in the Berzelius symbolic form. H, the underlined symbol, was often used by printers
for ease in place of the barred symbol, but the barred symbolism was virtually unused
by Berzelius, except in part of his book, and it was never widely used.
1. What signs and symbols appear in textbook of chemistry at school level?
Recent textbooks show a range of signs in chemical equations. Signs for the
chemical process range from , to  and . The reaction sign is + although this
is rarely sated explicitly in modern textbooks.
2. What changes in chemical symbolism has taken place over the last 150 years and
what has contributed to these changes?
The use of the = sign was predominant throughout the nineteenth and early
twentieth centuries, and was then replaced by a variety of arrows. Chemical
symbols began originally as pictorial (e.g. Dalton) and developed through
variations on alphabetic characters, the present format being adopted at a meeting
of the British Association in 1837. This decision may have been influenced by the
expense of producing special characters rather than the standard alphabetical
symbols. The choice of superscripts or subscripts to denote the number of
elementary components seems to have been dictated by the influence of
significant journal editors. Eventually, consistency was achieved by the
international nature of publishing research. The use of pseudo symbols was
present in the mid nineteenth century and has developed to provide the power of
generalisation as well as simplifying some complex formulae. The prevalent use
of e for electron is in line with the widespread use of half equations as an
explanatory tool.
3. What can a study of chemical symbolism contribute to thinking about learning
chemistry?
In the first place, choice of symbolism often arises through external factors such
as the influence of technology. This makes learning symbols a rote learning
venture with consequent demand on memory space. The origin of chemical
symbols in Latin nomenclature is also a challenge to the learning process.
Multipliers in chemistry are placed in the opposite position to that in mathematics,
Chemical symbols, signs© John Oversby, October 2002
23
after the symbol. This can lead to a conflict that can diminish understanding. In
addition, the use of sub-scripts is yet another irrational rule to learn.
The meanings of signs such as + and  needs to made explicit in texts, as it was
in many texts from the nineteenth century.
.
The work reported here indicates that chemical notation is a neglected part of learning
about chemistry in secondary schools now compared with the past. Reference to
empirical origins is rare. Chemical word equations mean different things to different
authors and the lack of consistency is obvious. Balancing chemical equations has little
chemical purpose and is rule oriented rather than based on understanding of
fundamental chemical laws.
Lost beginnings
Larousse makes it clear that there has been very little change in the meanings of
symbols since they were first established at the beginning of the nineteenth century.
However, recent textbooks are at odds with these very principled meanings. How may
this have come about? With regard to the symbols, the loss is the quantitative
macroscopic meaning. Chemistry has moved on from equivalents to moles and the
discussion here recognises this change. Similarly, chemistry now recognises a range
of entities and representations, such as the bull's eye diagrams for electronic
structures. The language used in the textbooks appears to be one of finality. The
authors leave no room for a development of the explanation they give. The message
they leave us with is not one that accredits a progressive understanding of the
meaning of symbols as the reader's knowledge improves. Rather, it is a cut and dried
view, this is the meaning of symbol that will stand the test of your improving
knowledge. There is no later reflection on these meanings at the end of these books to
suggest there is any need for a rethink. Perhaps authors believe that their nitial
explanation of the meaning of symbol is the end of the story. If chemistry is to come
alive as a developing subject then this should be reflected in the structure of the
textbook. A section at the end relating an improved meaning of the term and relating
to the history of how this term was established would be a helpful contribution to
understanding the structure of chemistry.
Chemical formulae are poorly discussed in every single one of the recent textbooks.
There is no indication of a quantitative macroscopic meaning and this original
meaning has been lost. At the start of the nineteenth century the sub-microscopic view
was a modern contribution. The use of symbols was an accreditation of the need to
include this sub-microscopic approach. Now the macroscopic position has been left
out and the result is a detachment from empirical evidence. This may go some way to
explain why some students fail to maintain chemical formulae when balancing
equations. They are simply not aware of the strength of the law of constant
composition so well established by Richter and others at the turn of the nineteenth
century. It is little wonder that they have such little regard for the constancy of the
formulae. In a similar light, the focus on rules for balancing equations has led to a
detachment of stoichiometry from the law of conservation of matter. This ought to be
trumpeted in the texts as the basis for balancing equations. The place of the different
notations of coefficients and numbers of atoms, crafted from the numerous
discussions two hundred years ago, needs to be emphasised. The lack of explicit
Chemical symbols, signs© John Oversby, October 2002
24
teaching of this part of the chemistry curriculum suggests that it may not be
appreciated by the authors.
Word equations are a more modern phenomenon, while retaining their roots in the
Bergman-Berzelius forms. These are more used as a teaching tool. The teaching
community, rather than the community of academic chemists, may better clarify their
meanings.
Chemical equations in The Journal of Chemical Education
The Journal of Chemical Education is a journal that appears to be most closely related
to teaching and learning of chemistry than most. It was, therefore, chosen to
investigate the appearance of chemical equations in the articles it provides for US
high school and college level chemistry. Nine issues from 2002 (January –
September) were examined for the presentation of chemical equations. Many articles
and three complete issues did not present chemical equations at all. Nevertheless, 35
papers provided examples of 116 chemical equations. While this does not constitute a
rigorous or comprehensive survey, it provides a flavour of recent practice. Within this
sample there were (the numbers do not all tally to 116 since there was multiple use in
some equations):
 4 word equations
 50 formulae equations, many concerning molecular reactions that could not be
represented as ionic equations
 9 half equations in ionic form
 57 ionic equations
 only 4 equations that were general ionic or formulae while the rest were
specific
 12 equations were provided with state symbols
 8 equations that were only partial i.e. unbalanced
 a variety of arrows in use (, 86; , 1; , 1; , 20; =, 8). The use of the =
sign was restricted to one paper which also used the ↓ arrow.
 The following pseudo symbols were used: Ac, Et, Ph, R, R’, py (pyridine),
p-O2N-C6H5, Hb (haemoglobin), DNA, L (ligand), NAPDH, NADH,
m-CPBA, t-Bu, M (for metal), e without the minus, e- with the minus.
It would seem that there is a lack of consistency and very little editorial control over
the representation of chemical equations.
7. A timeline of symbols and signs in chemistry
15th C and early16th C elements and compounds represented by icons eg gold as King
1718 Geoffroy's symbols for elements in his table of affinities
1808 Dalton's circle symbols with patterns and letters enclosed
1813 Berzelius letter chemical symbols - abbreviations of names and equivalents
1813 Berzelius proposes barred symbols for multiple letters. This finally disappears
in the second half of the nineteenth century
1820s Symbols adopted as representing atoms
Chemical symbols, signs© John Oversby, October 2002
25
1837 British Association for the Advancement of Science adopts Berzelius system
1830s German and British chemists adopt the subscript convention for multiple
atoms
1856 Wilson uses + on reactant side and 'and' on product side of equation
Wilson uses the = sign for 'reacts with'
Wilson explanation of the order of symbols in a compound
1856 Miller uses the comma as a combination sign.
1864 Wurtz uses the barred symbols to distinguish equivalents. He also uses the
comma for combination
1865 Frankland uses subscripts but symbols in order reverse to the convention of
metals first. Formulae are related to reacting weights. The + sign is explained
as it is today.
1868 Naquet: a symbol represents an atom not a molecule. Barred symbols
represent reacting weights he uses superscripts with words under symbols. Use
of electropositive element first in binary compounds..
1870 Karlsruhe chemical conference to establish the meanings of chemical symbols.
1870 Odling uses multiple symbols for elements rather than coefficients.
1884 Arrhenius' paper on ions - a new particle to be represented. Arrhenius adopted
the same symbols as the elements
1886 Fittig (German) uses superscripts and words under symbols.
1887 Gautier uses superscripts and = signs. He puts words under symbols.
1894 Jago uses + on both sides of the equation.
Jago clarifies symbols as abbreviations for names, and representing both
atoms and macroscopic quantities.
1899 Newth uses + sign but "and also" for different sides of the equation
He characterises atomic and molecular equations.
1900s French chemists begin to convert to the subscript convention from superscripts
1904 Moissan still using superscripts
1900s The use of the = sign begins to be replaced by the  in textbooks. This is not
generally complete until the 1960s
1913 Schmidt uses  and = signs
1920s Use of  for reversible and equilibrium reactions.
1930s Use of the up and down arrows,  and . These generally disappear in the
1960s and are generally replaced by state symbols.
1940s Use of  sign for equilibrium reactions. The  sign is less commonly used in
the 1970s but never disappears entirely.
1959 Emeleus and Anderson using a mixture of  sign and = sign, to differentiate
equilibria and complete reactions.
Chemical symbols, signs© John Oversby, October 2002
26
References
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Brock WH (1992) The Fontana history of chemistry Fontana Press, London
Brown GI (1954) Essentials of Certificate Chemistry Longmans, Green & Company,
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Coulson EH (Ed) (1967) Nuffield Chemistry Handbook for Teachers Longmans
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Duncan IM & Johnstone AH (1973) The mole concept Education in chemistry 10,
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Earl B, K Foulds & Wilford LDR (1996) GCSE Double Award Chemistry John
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Frankland E (1866) Lecture notes for chemical students, embracing mineral and
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Gautier A (1887) Cours de chimie Tome second Organic Savy Paris
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Hall AO (1978) Chemistry for Beginners Heinemann Educational Books Ltd, London
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