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Should atoms be put first in teaching general chemistry and if so how?
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Eric Robert Scerri
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Should atoms be put first in teaching general
chemistry and if so how?
Eric Scerri
Department of Chemistry and Biochemistry UCLA, USA
Chemical education has been in a state of flux
for many years, with many competing suggestions
and programs for how the subject can be made more
appealing and relevant to student. In this article I would
like to concentrate on one recent development, the atoms
first approach (AA) which I believe contains many useful
suggestions but also falls short in some respects that I
intend to outline.
The first article devoted to the AA approach that I
have been able to find in the Journal of Chemical Education
dates from 2013 (Esterling, 2013, Chitiyo, 2018). This was
followed by a couple of other articles that contain the
words “atoms first” in their titles. This situation is a little
surprising given that a far larger number of chemistry
textbooks with “atoms first” in the title, or subtitle,
that are now available (McMurry, 2010, Zumdahl, 2012,
180
Burdge, 2014, Gilbert, 2015, Gilbert, 2018, Tro, 2014,
Zumdahl, 2016).
I believe that the most positive aspect of the AA
approach may be its highlighting of what is wrong with
any traditional or standard approach to the teaching of
chemistry (SA) rather than the notion of atoms first itself.
Proponents of AA point out that standard approaches
are rather disjointed presenting a number of apparently
disconnected topics at the start of a chemistry course.
These topics include such things as the classification of
matter, the difference between chemical and physical
changes, accuracy and precision, significant figures,
dimensional analysis, types of reactions and most tedious
of all, stoichiometric calculations. All these issues are
generally dealt with before even starting to discuss
atoms and molecules. Proponents of AA prefer to start
with subatomic particles, the periodic table and atomic
weights.
The same proponents believe that their approach
makes for less disjointedness which also makes perfectly
reasonable sense. However, such an approach seriously
relegates the role of the historical development of the
field of chemistry. It also risks degenerating into to
a form of naive reductionism, whereby chemistry is
taught as though it was nothing but physics and or that
understanding atoms and molecules leads to a complete
understanding of chemistry (Scerri, McIntyre, 1997).
Whereas the philosophy of chemistry has rejected the
notion of reductionism, especially of this more blatant
form, this has not been the case in the field of chemical
education.
Without wishing to deny the tremendous benefits
that physics of the 20th century has brought to the study
of chemistry, it is essential to maintain contact with
the fact that most branches of chemistry are concerned
181
with macroscopic phenomena and not the microscopic
components that make up chemical substances.
Consequently, placing too much emphasis on the
reducing science of physics may misrepresent the nature
of chemistry.
What is an alternative to AA that retains some of its
recommendations?
As I suggested above, one of the drawbacks of current
atoms first approaches is that they tend to be ahistorical.
What I propose is atoms first but presented as a historical
development, beginning with the laws of chemical
combination that were discovered in the 18th century
before moving to Dalton’s atomic theory of the early
1800s which provided an explanation for them1
.
Dalton’s notion of atomic weight then provides a
natural entry point into a section on the discovery of the
periodic table. As is well known, it was only when atomic
weights became rationalized at the Karlsruhe conference,
and through Cannizaro’s intervention, that it became
possible to order the elements effectively and to see how
their properties recurred approximately after certain
definite intervals or periods (Scerri, 2019).
I believe that a reasonably in-depth introduction to
the periodic table of the elements should be presented
without any mention of electrons or atoms or ions, as
far as possible. There are many reasons why the periodic
table can serve as one of the starting point to a chemistry
course. First of all, it is without doubt the central
paradigm of modern chemistry, that serves to organize
the entire discipline. Secondly, the periodic table has
1
 I am referring to the laws of conservation of mass, of constant
composition and of multiple proportion. The last of these was a
prediction made by Dalton, which was later verified experimentally.
182
become something of a cultural icon which means that
it is already familiar to students, including those who
have never been exposed to any chemistry whatsoever.
No other scientific discipline that I am aware of shares
this virtue of having one central motif that embodies
the entire subject and brings together so much factual
information as well as allowing for predictions to be
made. Why then not exploit this benefit that chemistry
possesses, through the existence of the periodic table, in
shaping the way that the subject is taught?
In the SA and also the AA approaches to chemistry
the periodic table does of course make an appearance
from time to time, but it is seldom the focus of any
sustained attention. For example, in traditional courses
the periodic table serves as a repository for the atomic
weights of all the elements, something to be consulted
when performing a stoichiometric calculation and little
else until chemical bonding is eventually discussed.
Emphasizing the periodic table maintains a historical
development in the way that chemistry is represented,
given that its discovery occurred some 150 years ago,
as compared with the discovery of modern atomic and
electronic structure which dates to more like 100 years
ago. It is a well-known fact that many discoveries in atomic
and electronic structure, and even quantum mechanics
were motivated by attempts to explain the periodic table,
which remained rather mysterious until talk of electrons
in outer-shells and the like became available. An early
emphasis on the periodic table provides an opportunity
for students to realize that it led the way to atomic and
electronic structure instead of presenting facts about
atoms as though they were axioms in a deductive
science like geometry. Aside from restoring the historical
dimension to the study of chemistry, one can frequently
183
return to the periodic table to anchor many aspects of
a chemistry course or to serve as a major thread while
dealing with topics including atomic structure and
chemical bonding.
Many educators stress the need for science students
to learn about the nature of science in general before
jumping into the details of any particular discipline that
they might be studying. Alternatively, it is important to
continue to emphasize aspects of the nature of science
while mentioning such concepts as theories, models,
hypotheses etc2
.
Interestingly, philosophical aspects of chemistry
have not been studied as extensively as those of physics
and biology. The academic study of the philosophy of
chemistry only began in earnest in the mid 1990s. There is
still only one international academic journal, Foundations
of Chemistry, which is dedicated to this field3
. Chemical
education is therefore in greater need of philosophical
consideration than the other main disciplines in the
natural sciences4
.
How is the periodic table to be introduced and how
can it serve as one of the main threads for a general
chemistry course?
2
 There are entire academic journals devoted to the inclusion of
history and philosophy of science into science education. The best of
these is probably the Springer journal Science and Education. https://
www.springer.com/journal/11191
3
 https://www.springer.com/journal/10698
4
 There are now a number of books and edited collections on
philosophy of chemistry which can help chemical educators to become
more acquainted with the new sub-discipline in the philosophy
of science. They include, Van Brakel, 2000, Baird, Scerri, McIntyre,
2005, Scerri, 2008, Scerri, McIntyre, 2014, Scerri, Fisher, 2016, Scerri,
Restrepo, 2018, Scerri, E. Ghibaudi, 2020, Woody, Hendry, Needham,
2012.
184
My course presents the development of the periodic
table in a historical manner starting with the discovery
of triads of elements. By focusing on this development,
one can begin to convey the notion that there are groups
of elements such as Li, Na and K that behave similarly to
each other and the fact that there are simple numerical
regularities that connect these elements together.
One can then move to the central notion of chemical
periodicity, namely that on arranging the elements in
order of increasing atomic weight there is an approximate
repetition in their chemical and physical properties after
certain intervals (Figure 1).
Figure 1 _ Elements are arranged in increasing order of atomic
number. Elements such as Li, Na and K represent approximate
repetitions after intervals of 8 elements.
At this point it is useful to present the original
8-column table of Mendeleev and other pioneer
discoverers because this format displays periodicity in its
clearest and simplest manner. I like to explain how this
process succeeds very well up to and including element
manganese is reached. The following element, iron, is
not a noble gas of course, and so cannot be placed in the
eighth column of such a table (Figure 2).
185
Figure 2 – The elements in the number line are presented
as a series of rows such as to reflect chemical periodicity in
each column.
The way in which Mendeleev solved this problem was
to ‘exclude’ iron from the main body of the table while
placing it into a miscellaneous group, a move which he also
applied to the following elements cobalt and nickel (Figure
3). After that, things resume as before in that copper and
zinc are placed in groups 1 and 2 respectively since they
commonly display valences of 1 and 2 respectively. I also
take the opportunity of stating that such tables are not
fundamentally incorrect since they do collect together
elements which share a common valence such as Be, Mg,
Ca, Zn, Sr, Ba for example, all of which display valences
of 2.
Figure 3. A version of the 8-column periodic table published
by Mendeleev. This is a slightly modified version of Mendeleev’s table of 1871.
186
By contrast, the modern periodic table (Figure 4) solves
the problem of the placement of Fe, Co and Ni by creating
a set of ten new vertical groups in the periodic table, in
what we now call the d-block of the table. It is pointed
out that this format, which is almost ubiquitous these days,
appears to commit an analogous and perhaps more serious
exclusion of some elements, since as many as 28 elements
are relegated to a footnote known as the f-block elements
that lies below the main table5
.
Figure 4 - Modern periodic table.
This provides a perfect opening for presenting the
32-column presentation of the periodic table (Figure 5),
which very naturally incorporates all of these ‘excluded
elements’ into the main body of the table and which
some textbooks are starting to display as what is perhaps
a more coherent periodic table.
This section is developed while carefully not
mentioning electrons, since they had not yet been
discovered when the first periodic tables were published
5
 There is an element of Whiggism at play here. It is only in modern
times that all the f-block elements have been discovered and that the
need for a 32-column display becomes more pressing.
187
in the 1860s. Secondly, I delay presenting the underlying
explanation of the periodic table until we have covered
quantum theory, quantum numbers, the aufbau etc. Too
many modern chemistry courses “put the cart before the
horse” by first explaining the structure of atoms and then
leaping straight into the explanation for the periodic
table in terms of electronic configurations (Burge, 2019,
Gilbert, 2020). Taking such an approach completely
misrepresents the historical sequence and in is therefore
educationally unsound.
Figure 5
Before completing this introduction to the periodic
table, there is an opportunity to present some of the most
recent thinking about this topic, in order to avoid creating
the impression that the periodic table is a finished subject
or a ‘closed book’. This can be carried out by introducing
the left-step periodic table. This representation involves
moving the element helium to the top of group 2 of the
table and then relocating this now modified s-block of
elements to the right edge of the table (Figure 6)6
.
6
 The motivation for wanting to move helium into group 2 of the
periodic table is the fact that its atoms have two electrons while
the members of group 2 have two outer-electrons. But such talk of
electrons was supposed to be excluded from this brief introduction
to the periodic table, which aims to put chemistry and qualitative
similarities among the elements to the fore. However, there is little
188
Figure 6
The advantage of this table, apart from its far more
regular appearance compared with the conventional 18
and 32-couln tables, is that each period, including the
very first one is seen to be repeated in terms of its length
(Scerri, 2022). Needless to say, the field of chemistry is
full of anomalies, although the proponents of this leftstep table do not believe that the existence of just one
period length that does not repeat may be one such
genuine anomaly.
Modern atomic structure
The teaching of this topic can begin with the three
momentous discoveries that were made at the turn of the
20th century, namely X-rays in 1895, radioactivity in 1896
and the electron in 1897.
One then turns to the historical development of
successive models of the atom, starting with Thomson’s
plum pudding model, to Rutherford’s nuclear atom.
Any discussion of the next model due to Bohr should be
preceded with a section that introduces the old quantum
doubt that beginning undergraduate chemistry students are already
well aware of the fundamental explanation for the periodic table
in terms of outer-shell electrons, even if the college course being
presented here aims to provide a deeper understanding of this
explanation.
189
theory of Planck and Einstein. I find it rather hopeless
to try to explain the problems surrounding black-body
radiation in general chemistry due to the complexity of
the subject matter. I prefer to state the result that Planck’s
work culminated in the assumption that the energy of
electromagnetic radiation must be thought of as being
quantized in order to make sense of the experimental
data on glowing hot bodies.
Although Planck himself is said to have doubted
the value of such a revolutionary idea it took the young
Albert Einstein to make the first significant application
of quantization of light energy in the context of the
photoelectric effect and to thereby put quantization on
a more secure foundation, as well as establishing the
particulate nature of radiation and initiating the notion of
photons of light. What Bohr did in introducing his model
of the atom was to essentially apply quantization to the
energy and angular momenta of the electron orbiting
the nucleus of the hydrogen atom. I have discussed a
conceptual treatment of this topic in a recent publication
in Chimica Nella Scuola and will not therefore elaborate
too much further here (Scerri, 2021).
Talking of the Bohr model, it is interesting to note
that some chemistry textbooks now recommend skipping
this episode altogether on the grounds that the model was
soon replaced by Schrodinger’s model7
. I have to say that
I believe it is a mistake to omit the Bohr model, in view
of its iconic status and the fact that it was an important
stepping stone towards the fully quantized model that
incorporated the wave nature of the electron.
Proceeding historically, I like to then discuss De
Broglie’s speculation that the electron might have a wave
7
 For example, the well-known textbook by Atkins, Jones and
Laverman omits any discussion whatsoever of the Bohr model.
190
nature and its experimental confirmation that took place
soon afterwards. At this point Schrödinger was able to
write a wave equation for the electron in the hydrogen
atom. The solution of this equation featured 3 quantum
numbers which had to be attributed to the electron in
it’s various energy levels, later augmented by a fourth
quantum number following Pauli’s suggestion. Armed
with the relationship between these four quantum
numbers one could finally explain the periodic table in
a fundamental manner, namely the possible lengths of
periods as 2, 8, 18 or 32. This is a major success which
is not sufficiently emphasized in chemistry courses. It
is worth noting that this explanation is achieved with
an equation which was initially applied just to the
hydrogen atom and which can only be solved exactly
for this case.
The course continues with the obligatory section
on writing electronic configurations for many-electron
atoms including the controversial issues such as the
relative occupation ad ionization of the 3d and 4s
orbitals in the first transition metal series (Scerri, 2019)8
.
Turning finally to chemical bonding one now needs to
break with the historical sequence by covering classical
bonding concepts such as Lewis structures, VSEPR and
dipoles only to afterwards resume the quantum story to
see how it was applied to chemical bonding. I suppose
it would be possible to maintain a strictly historical
account and teach classical bonding concepts before
any quantum concepts have been mentioned but I have
never attempted to do so.
In teaching the above-mentioned classical bonding
concepts I like to stress their limitations in order to
8
 This article also discusses several problematical issues in chemical
education such as Le Châtelier’s Principle.
191
motivate the need for quantum theories of bonding. For
example, many Lewis structures such as that of benzene
need to be augmented with the notion of resonance but
even this is not a fully satisfactory explanation because
it still assumes the presence of distinct single and double
bonds. Similarly, there are many molecules whose shape
or bond angles cannot be explained using the VSEPR
model. I am thinking of cases such as H2
S or PH3
 which
would be expected to have bond angles similar to water
and ammonia respectively but which actually have
angles of 920 and 930 respectively.
The two quantum theories of bonding that are
presented in general chemistry consist of valence
bond/hybridization and molecular orbital theory. Both
are treated in a qualitative fashion featuring diagrams
rather than mathematically. Molecular orbital theory
is introduced with energy diagrams that allow one to
predict or at least rationalize whether any particular
diatomic molecule may be stable or not according to
the relative occupation of bonding and anti-bonding
orbitals.
Finally, a general chemistry course would typically
also include sections on thermodynamics, equilibrium
theory, kinetics and redox, although discussion of these
are topics that will have to wait for a future article. To
conclude, the proposed modification to the atoms first
approach consists in teaching the material in a historical
manner as far as possible. In this way the student is
relieved of the tedium of the traditional beginning topics
that were mentioned in my introduction. In addition,
the study of chemistry is enriched and humanized by
exposing the student to the gradual development of
ideas and discoveries, rather than presenting chemistry
as a jumble of somewhat unrelated sub-sections.
192
Acknowledgement
I thank Professor Laura Orian for inviting me to the
University of Padova and for suggesting that I contribute
to this volume of articles.
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