The
ninth of April 2010 was a rather special date for the discovery of the
elements. First of all, it was the day on which an article was published
in a physics journal to announce the synthesis of element 117. Second,
and perhaps more important, it represented the completion of the 7th row
in the periodic table, which contains 32 elements. As of that day, the
periodic table of Mendeleev was finally completed in a way that it never
was before. This is because there are now absolutely no gaps in the
periodic table, although there may well be some new elements to follow
in a 8th row that will probably begin to form very soon. Such a
situation has never existed before because in the past there were always
gaps within the boundaries of the elements that had already been
discovered.
To appreciate the full impact of this development,
we need to briefly consider the history of the periodic table. It was
discovered over a period of about nine years from 1862 to 1871.
There were several different versions of the table published, but what
they all had in common was this; if all the elements were arranged in a
sequential fashion based on the weights of their atoms, the elements
showed an approximate repetition after a particular sequence of
elements. In these early periodic tables things appeared to be rather
simple because the repeat distance, or length of each period, was the
number eight throughout the table. Among these short-form tables, the
one designed by the Russian chemist Dimitri Mendeleev is widely
considered to be the most important, displaying Group I to VIII over 8
columns and 12 series (or periods) as shown in figure 1.
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As the table shows, Mendeleev left a number of
gaps in his table. He was more or less forced to do this in order to
make the other elements fall into vertical columns to reflect their
similar chemical and physical properties. The periodic table therefore
began life with many gaps within it.
Mendeleev, unlike some of the other discoverers,
made predictions about the properties of these missing elements. As is
also well known, many of his predictions turned out to be remarkably
accurate.
As time went by, it became increasingly clear that
a better design for the periodic table could be obtained by relaxing
the notion that all periods have the same number of elements. It was
realized that period lengths show a variation and that the 4th and 5th
periods show a length of 18 elements, as shown in the medium-long form
(figure 2). Rather than lumping together say lithium (Li), sodium (Na),
potassium (K), copper (Cu), silver (Ag), and gold (Au) as Mendeleev had
done, it is better to separate the first three from the last three of
these elements to form two different groups. This change was also
applied systematically to a number of other groups in Mendeleev's
original table. As a further example, beryllium (Be), magnesium (Mg),
and calcium (Ca) which Mendeleev initially placed in the same group as
zinc (Zn), cadmium (Cd), and mercury (Hg) now gave rise to two new
groups. The net result of these changes was to produce what is termed
the medium-long form periodic table as shown in figures 2 and 3.
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Notice that there are several gaps in this
periodic table. In 1914, Moseley discovered that it was better to order
the elements according to atomic number rather than atomic weight. This
change resolved a number of "pair reversals" such as the one involving
the elements tellurium (Te) and iodine (I) which were incorrectly
ordered according to the atomic weight criterion.
But the use of atomic numbers did not result in
any profound changes to the form of the periodic table although it did
eventually reveal that there were precisely seven gaps to be filled
within the limits of the old petriodic table, consisting of the elements
ranging between atomic numbers 1 (hydrogen) and 92
(uranium).
(uranium).
Meanwhile, a separate development was taking
shape. As far back as the earliest periodic tables it had been evident
that some elements could not easily fit into the system at all. This
became such a difficult problem for Mendeleev that he handed the task to
a Czech colleague, the chemist Boruslav Brauner who had some partial
success. The elements in question included cerium (Ce), praseodymium
(Pr), and neodymium (Nd) that are so similar that they appear to belong
in the same place in the periodic table. But this would be going against
a basic principle of the periodic table, namely one element, one place.
Another solution was to place these so called rare
earth elements into a separate row at the foot of the main body of the
table as seen in figures 2 and 3. Some chemists realized that this move
necessitated an even longer period consisting of 32 elements, but this
did not have any serious influence on those who designed periodic tables
who stuck with the medium-long format and its 18 columns.
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Starting in the 1940s, new elements began to be
synthesized, thus extending the periodic table beyond the original 92
elements. Soon afterwards, a second period of 32 elements was discovered
by the American chemist Glen Seaborg while he was in the process of
attempting to synthesize more new elements. Seaborg realized that the
elements actinium (Ac), thorium (Th), protactinium (Pa), and uranium(U)
did not belong in the places shown in figure 2, but that they formed
part of a new 14 element series which became known as the actinides.
Now, the case for arranging the elements in a long-form table became
more compelling. Seaborg and others began to publish long-form tables
such as in figure 4.
Curiously though, such long-form designs are still
not the most commonly encountered format of the periodic table in
textbooks and wall charts. This is likely because it is not very
convenient to represent the periodic table in this more correct form.
Such tables stretch a little too far horizontally and so tend to be
avoided by designers of periodic tables, even though everyone agrees
that they are scientifically more correct. One clear advantage that the
long-form table has is that it lists all the elements in sequential
order of increasing atomic number whereas the medium-long form displays a
couple of anomalous jumps which occur between barium (Ba) and lanthanum
(La) and another between radium (Ra) and actinium (Ac).
In any case, whether the medium-long or long-form
table was used, there were still several gaps that remained to be filled
in the seventh row of the table. As more and more elements were
synthesized these gaps were reduced until the last piece in the jig-saw
puzzle was filled on 9 April 2010 with the announcement of the discovery
of the elusive element 117.
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| Figures 4–6 (top to bottom): Three different
long-form, or 32-column, periodic tables with differences highlighted.
Figure 4 (top): Version with group 3 consisting of Sc, Y, Lu, and
Lr. Figure 5 (middle): 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 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, while grouping together Sc, Y, La and Ac but at the expense of breaking-up the d-block into two highly uneven portions. |
The Group 3 Question
In an article in the Jan-Feb 2009 issue of Chemistry International,
page 5, Jeffrey Leigh correctly pointed out that IUPAC does not take a
position on what should be regarded as the correct periodic table.
There is no such thing as an IUPAC-approved table, contrary to the
label "IUPAC periodic table" that one might see in some books or on
certain websites.
Leigh was responding more specifically to the
debate that had been conducted, mostly in the chemical education
literature, concerning the membership of group 3 of the periodic table. In this article, I propose that IUPAC should
in fact take a stance on the membership of particular groups even if
this has not been the practice up to this point. This would not of
course amount to taking an official position on an optimal periodic
table since it would concern the placement of elements into groups
rather than any other aspect of the periodic table such as what shape or
form it should take or whether it should be two or three dimensional.
Some years ago, following some work by physicists,
it was pointed out that the elements lutetium (Lu) and lawrencium (Lr)
show greater similarities with scandium (Sc) and yttrium (Y) than do
lanthanum (La) and actinium (Ac).
As a result, many textbooks and websites, but by no means all of them,
have adopted this new version of group 3 (as depicted in figure 3). This
has led to a situation in which chemistry students and professionals
alike are often confused as to which version is more "correct" if any.
Quite apart from arguments based on electronic configurations, chemical
and physical properties, which are not completely categorical, I will
present an argument here that I believe renders the newer grouping of
Sc, Y, Lu, and Lr rather compelling.
In addition to arranging all the elements in a
more correct sequence of increasing atomic numbers, the decision to move
to a long-form or 32-column table forces the periodic table designer
towards just one possible option regarding the question of which
elements to place in group 3. The natural choice, turns out to be the
placement of Lu and Lr into group 3, as seen in figure 4, because the
other option fails to maintain an orderly increasing sequence.
I suggest that any reluctance to accept this
grouping as opposed to the more frequently seen grouping of Sc, Y, La,
and Ac (as shown in figure 5) stems entirely from a reluctance to
display the periodic table in its 32-column format. If this obstacle is
removed and the rare earths are taken up into the main body of the table
the choice of how to do so is almost entirely in favor of a group 3
consisting by Sc, Y, Lu, and Lr.
I say "almost entirely" because there does exist a
third option, although this can be dismissed on the grounds that it
represents a very asymmetrical possibility. As seen in figure 6, the
third option requires that the d-block elements should be broken into
two very uneven portions consisting of one group, followed by the
insertion of the f-block elements and continuing with a block of nine
groups that make up the remainder of the d-block elements. Indeed, this
form of the periodic table is also sometimes encountered in textbooks
and articles, although this fact does not render it any more legitimate.
Of course, there may still be a preference for an
18-column table among many authors, in which case one can easily revert
to the form in figure 3, but with the knowledge that the group 3 issue
is now resolved. At the risk of repeating myself, it is this question
which I believe is in need of resolution and not the issue of the best
shape for the periodic table, or indeed, whether it should be presented
in a medium-long or long form. I am not, therefore, suggesting a change
of IUPAC policy regarding a commitment to an "optimal format" for the
periodic table. The latter must remain as a choice for textbook authors
and individual periodic table designers.
Finally, given that the periodic table is now
complete for the first time, and probably not for long, would it not be
an occasion for IUPAC to turn its attention to the central icon and
framework of chemistry in order to resolve a remaining issue that
continues to confuse seasoned practitioners and novices alike? And who
knows what discoveries might lie ahead if a more precise grouping of
elements in group 3 were to be established after all the available
evidence has been suitably weighed by the relevant IUPAC committees.
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