Looking Backwards and Forwards at the Development of the

Chemistry International January-March 2019

Looking Backwards and Forwards at

the Development of the Periodic Table

by Eric Scerri

ince the periodic table has

reached the ripe old age of

150 years it may be an appro-

priate time to look back at the devel-

opment of this unique scientific icon.

It is also an opportunity to look

forwards to any changes that the

periodic table may undergo in view

of the ever-growing list of new ele-

ments that continue to be synthe-

sized. The way that the past and

future will be examined in this article

will be to follow a main thread that

focuses on the number of columns in

the periodic table at various stages

in its development.

So, let’s begin with Mendeleev and the others who dis-

covered chemical periodicity in the 1860s and general-

ly presented their ﬁ ndings in the form of an 8-column

table or what has become known as a short-form table

(ﬁ gure 1) [1]. This format has several appealing features

which are worth pausing to consider. The ﬁ rst virtue

is the simplicity of the short-form. It is based on the

notion that chemical and physical properties recur ap-

proximately after eight elements and continue to do

so. Unfortunately, some of the directness of this pre-

sentation is lost on moving to the 18-column format

(ﬁ gure 2) or even wider periodic tables.

A second virtue is that the 8-column table groups

together a wide range of elements that share the same

highest valency. For examples, beryllium, magnesium,

calcium, strontium and cadmium all appear in the sec-

ond column of the short-form table. Not surprisingly,

the 8-column table is still used in certain parts of the

world, most importantly in Russia where its most suc-

cessful version was ﬁ rst discovered by Mendeleev in

1869. The reason why Mendeleev receives the most

credit, even though he was the latest among the six

independent co-discoverers, has been much debated

by historians and philosophers of chemistry.

The usual account is that only Mendeleev made

successful predictions of then unknown elements.

However, another school of thought disputes the claim

that successful predictions are quite so important

and proposes that the suc-

cessful accommodation of

already known data is an

equally good criterion for

the acceptance of scientiﬁ c

theories and concepts [2].

The early periodic tables

were required to literally

accommodate the 60 or so

elements that existed in the

1860s and the relationships

between them, which was

by no means a trivial task.

Today a periodic table must

accommodate the presence

of about twice that number

of elements and their simi-

larity relationships.

Figure 1. Short-form or eight column periodic table as devised by

Mendeleev in 1871.

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No

Fig 2. 18-column or medium-long form table

Chemistry International January-March 2019

18-column tables

The expansion of the periodic table from an 8-col-

umn format to one with 18-columns is not essential but

seems to have been generally made some years after

the initial discovery of chemical periodicity. There are

several reasons why this change was made, some of

them scientiﬁ c and others pragmatic.

First of all, it must be recognized that the periodic

table, an object of enormous utility, is man-made. It is

not given to us directly by Nature even though chem-

ical periodicity is a scientiﬁ c fact. The precise form of

the periodic table is a form of compromise that aims

to serve the majority of scientists and students of

science, but it cannot serve all of them at once. For

example, chemists who choose to focus primarily on

chemical similarities might wish to favor a particular

format; chemical educators or experts focusing on

atomic structure may favor a di erent format. The au-

thor does not claim that there is one optimal periodic

table and yet he believes that it is worth striving to

obtain the best possible compromise version that the

scientiﬁ c community can agree upon.

But let me return to the expansion of the table to

an 18-column format. One reason for expanding the ta-

ble in this way is that upon closer inspection chemical

periodicity does not invariably operate with a constant

repeat distance of eight elements. If one wants to cap-

ture chemical similarities among elements more accu-

rately, one must accept that after two period lengths

consisting of eight elements each, the repeat length

becomes 18 elements. Consider for example the two

metals chromium and molybdenum that are very sim-

ilar chemically but stand 18 rather than 8 elements

apart. The 18-column highlights such similarities more

e ectively than the 8-column version.

Another motivation for the adoption of the 18-col-

umn table was that Mendeleev’s table displayed certain

awkward looking anomalies. The 8-column table can

only truly display chemical periodicity if certain short

sequences of elements are excluded from the main

body of the table. For example, Mendeleev relegated

iron, cobalt and nickel to what he labelled as group VIII

(ﬁ gure 1). He did this again for ruthenium, rhodium and

palladium as well as osmium, iridium and platinum, all

of which elements he termed as transition elements.

In an 18-column table there is no longer any need

to exclude these elements. This feature would seem

to suggest that an 18-column format shows some ad-

vantages in terms of representing all the elements in

an evenhanded manner, although as we will see be-

low, the 18-column table introduces another set of

‘relegated’ elements.

In historical terms the use of an 18-column format

has followed a complicated path. Interestingly, even

Mendeleev published some medium-long form tables,

although his versions contained 17 rather than 18 col-

umns since the noble gases had not yet been discov-

ered [3]. The advent of quantum mechanics and the

notion that electrons can be regarded as being situat-

ed in distinct shells also seems to have motivated the

widespread adoption of a medium-long or 18-column

format. Simply put, the 18 groups arise from the fact

that, starting with the 3rd main electron shell, elec-

trons occupy s, p and d-orbitals, numbering 9 in all,

each of which can be doubly occupied to make a total

of 18 electrons and hence the atoms of 18 successive

elements, with each one having an additional electron.

Since not all electron shells reach their capacity once

they contain 8 electrons, it makes perfect sense to ex-

pand the periodic table according to the quantum me-

chanical explanation of chemical periodicity.

32-column tables

However, from the 4th electron shell 14 more elec-

trons can now be accommodated in addition to the pre-

vious 18. In the modern terminology we now have f-orbit-

al electrons in addition to the earlier mentioned s, p and

d orbital electrons. So why don’t we expand the periodic

table further to make it into a 32-column format (ﬁ g 3)?

In fact, an increasing number of textbooks are beginning

to show such a long-form periodic table, which again has

some advantages and some disadvantages.

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og

Fig. 3. 32-column or long form table

Chemistry International January-March 2019

Looking Backwards and Forwards

On the plus side, it allows every single element to

be incorporated into the main body of the table. The

odd-looking footnote to the 18-column table which

traditionally houses the f-block elements now disap

pears. This is an analogous change to the one that

occ

urs on moving from an 8 to an 18-column format

that results in the incorporation of certain otherwise

excluded elements into the main body of the table.

Returning to the 32-column table, this also shows ev

ery single element in its correct sequence in terms

of incr

easing a

tomic numbers as one moves through

each period from left to right.

There are some pragmatic downsides, however.

Presenting the periodic table in a 32-column format

requires that the space for each element must be ap

proximately halved. Worse still, the one or two-letter

symbol for each element must now be reduced in size

with the risk of rendering them less legible.

What next?

If we continue to follow this line of thinking regard-

ing the progressive expansion of the periodic table

we notic

e that the table may be due for yet a further

expansion, at least in principle. Rapid advances have

taken place in the synthesis of super-heavy elements

in recent years. The f-block of the table has now been

completely ﬁlled with elements, the most recent ad

ditions being nihonium, moscovium, tennessine and

oganesson. For the very ﬁrst time, and also the last

time in the foreseeable future, the periodic table has

absolutely no missing gaps. At least this state of aairs

is true for the current periodic table that houses 118

elements arranged in seven periods.

There is no reason to believe that the periodic table

has reached its end point and there are several cur

rent initiatives that are aimed at producing elements

119

, 120 and be

yond. The discovery of elements 119 and

120 will be easily accommodated by tagging two new

spaces directly below francium and radium in either

the 18 or 32-column formats. However, as soon as el

ement 121 is synthesized, it will become necessary to

intr

oduc

e a new kind of footnote to the table to house

what will be formally known as the g-block elements.

On the other hand, if we insist that all elements be

placed together in the main body of the table and that

all elements are numbered sequentially we will have no

choice but to introduce a 50-column wide table! But

this will only be the formal beginning of the g-block

since theoretical calculations predict that the ﬁrst el

ement with a true g-orbital electron will be approxi-

mately element number 125 [4].

Interesting issues connected with the onset of

new blocks of the table

Each time that a new kind of orbital occurs in the

Aufbau and the sequence of increasing atomic num-

bers, a new kind of problem also seems to arise.

he ﬁrs

t time that a d-orbital electron appears is

in the atom of scandium, or element 21. In this case

the claim that the atom contains a d-electron is not

merely formal but is supported by much spectroscopic

evidence. The problematical aspect concerns the fact

that 3d orbital electrons only begin to appear after the

4s orbital has been occupied in the case of the atoms

of potassium and calcium.

The vast majority of textbooks state that in the

case of scandium the ﬁnal electron to enter the atom,

in terms of the ﬁctitious but useful Aufbau scheme, is a

3d electron. This view immediately creates a problem

when it comes to explaining the ionization behavior

of the scandium atom. Experimental evidence clearly

shows that the 4s electrons are preferentially ionized

in scandium. If the 3d orbital had really been the ﬁnal

one to enter the atom it ought to be the ﬁrst to be

ionized, which runs contrary to the experimental facts.

Almost every textbook proceeds to simply fudge the

issue, in order to maintain that 4s electrons enter the

atom ﬁrst but are also the ﬁrst to depart during the

ionization process, something that clearly makes no

sense in energetic terms [5].

The problem was clariﬁed relatively recently by the

theoretical chemist Eugen Schwarz who pointed out

that in fact the 3d orbital electrons are preferential

ly occupied in scandium, followed by the 4s electrons

and thus explaining perfectly why it is that 4s electrons

are the ﬁrst to be ionized [6]. However, it appears that

Schwarz wants to throw out the “Aufbau baby with

the bathwater.” Schwarz correctly points out that the

Madelung rule fails for all except the s-block elements.

This is the rule that purports to show the relative en

ergies of all the orbitals, and is part of the staple diet

of high school and ﬁrst-

year undergraduate chemistry

courses. However, any dismissal of this well-known

mnemonic would be rather unfortunate since it still

succeeds in listing the dierentiating electron in all but

about 20 atoms in the entire periodic table.

My reason for saying this is that as we move through

the periodic table there is no denying that the dier

entiating electrons in potassium and calcium are 4s

electr

ons while f

or scandium and most of the following

transition metal atoms the dierentiating electron is of

the 3d variety. The Madelung rule therefore still rules

when it comes to discussing the periodic table as a

Chemistry International January-March 2019

at the Development of the Periodic Table

Figures 4–6 (top to bottom): Three different long-form periodic tables with differences highlighted. Figure 4

(top): Version with group 3 consisting of Sc, Y, La, Ac. The sequence of increasing atomic number is anomalous

with this assignment of elements to group 3, e.g., Lu (71), La (57), Hf (72). Figure 5 (middle): Second option for

incorporating the f-block elements into a long-form table. This version adheres to increasing order of atomic

number from left to right in all periods, but with lanthanum located at the start of a 15-element block. Figure

6 (bottom): Third option for incorporating the f-block elements into a long-form table. This version adheres to

increasing order of atomic number from left to right in all periods, and groups Sc, Y, Lu and Lr together as group 3.

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

100

101

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105

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118

100

101

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118

Chemistry International January-March 2019

Looking Backwards and Forwards

whole, as opposed to the occupation and ionization

behavior of a single element such as scandium as dis-

cussed above [7].

First appearance of an f-electron

In principle, or using the Madelung rule, we ﬁ nd

that f-orbital electrons begin to appear in the atom of

lanthanum or element 57. However, according to ex-

perimental evidence this event occurs at the next ele-

ment cerium (Z = 58). Notice how this delayed onset is

analogous to the delayed onset of g-electrons that was

described above.

If one consults current versions of the periodic

table one ﬁ nds that there are at least three versions

that are on o er. In the majority of textbooks and wall-

chart periodic tables we ﬁ nd lanthanum located in

the d-block directly below the atom of yttrium (ﬁ gure

4). In a smaller number of currently available periodic

tables one ﬁ nds lanthanum located at the start of a

15-element wide f-block (ﬁ gure 5); and yet a third ver-

sion places lanthanum at the start of a 14-element wide

f-block (ﬁ gure 6).

As a result of these alternative tables there are

three di erent ways of regarding group 3 of the pe-

riodic table. According to the ﬁ rst option group 3

consists of scandium, yttrium, lanthanum and actin-

ium (ﬁ gure 4). In the second option, which features

a 15-element wide f-block, group 3 contains a mere

2 elements, namely scandium and yttrium (ﬁ gure 5).

Finally, the third form of the periodic table implies that

group 3 should be regarded as containing scandium,

yttrium, lutetium and lawrencium (ﬁ gure 6). What is a

student of chemistry, or even a professional chemist

to make of all of this?

A further complication is that neither chemical and

physical evidence on the elements concerned, nor mi-

croscopic evidence in the form of electronic conﬁ g-

urations, provide an unambiguous resolution of the

question. One possible way to try to resolve the issue

is to consider a 32-column table representation, and

return to the main theme of this article. It turns out

that in a 32-column table that also maintains all the el-

ements in their correct sequence of increasing atomic

number, the 3rd option would seem to be the most

reasonable choice [8].

Needless to say, it is important for IUPAC to be in

a position of recommending a compromise periodic ta-

ble that most e ectively conveys the largest amount of

information to the largest group of users. Since the pe-

riodic table is a human construct there is no absolutely

correct version of the periodic table. My own personal

recommendation is that group 3 should be considered

as consisting of scandium, yttrium, lutetium and lawren-

cium and that the f-block should formally begin at lan-

thanum even though the atom of lanthanum does not

actually contain an f-electron. It remains to be seen what

the recommendations of the working group will be [9].

What does not seem to be well known, even though

Je ery Leigh has written an article on the subject in

this very magazine, is that there is currently no o -

cially recommended IUPAC periodic table even though

it regularly publishes one [10]. Now that the periodic

table has reached 150 years it may be time for IUPAC

to take the plunge and go ahead and recommend one

o cial table.

References

1. E. Scerri, The Periodic Table, Its Story and Its Significance,

Oxford University Press, New York, 2007.

2. S. G. Brush, Making 20th Century Science: How Theories

Became Knowledge, Oxford University Press, New York,

2015 ; E. Scerri, J. Worrall, Prediction and the Periodic

Table, Studies in History and Philosophy of Science Part

A, 32, 407-452, 2001.

3. E. Scerri, A Very Short Introduction to the Periodic Table,

Oxford University Press, 2011.

4. P. Pyykkö, Chem. Rev., 88, 563-594, 1988.

5. E. Scerri, Education in Chemistry, 24-26, November, 2013

6. W.H. E. Schwarz, J. Chem. Educ., 87, 444-448, 2010.

7. S. Salehzadeh, F. Maleki, Foundations of Chemistry, 18,

57-65 (2016).

8. E.R. Scerri, W. Parsons, What Elements Belong in Group

3 of the Periodic Table? in E. Scerri, G. Restrepo, (eds.),

From Mendeleev to Oganesson, Oxford University Press,

New York, 2018, P. 140-151.

9. The Constitution of Group 3 of the Periodic Table, IUPAC

project 2015-039-2-200, E. Scerri, task group chair,

https://iupac.org/project/2015-039-2-200

10. J. Leigh, Chem Int. 31(1), Jan-Feb 2009, 4-6, http://www.

iupac.org/publications/ci/2009/3101/1_leigh.html

Eric Scerri <[email protected].edu> is in the Chemistry & Biochemistry

Department at UCLA. www.ericscerri.com, ORCID: https://orcid.org/0000-

0001-9775-5829