Periodic Table Long Form Page 2
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2
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4Periodic
change
The periodic table, cherished by generations of chemists,
has steadily evolved over time. Eric Scerri is among those
now calling for drastic change
The periodic table has become
recurrences as vertical columns or
In short
something of a style icon while
groups.
remaining indispensable to chemists.
In its original form
The notion of chemical reactivity
Over the years the table has had
the periodic table was
is something of a vague one. To make
to change to accommodate new
relatively simple. Over
this idea more precise, the periodic
elements. But some scientists
the years, extra elements
table pioneers focused on the
propose giving the table a makeover
have been added and the
maximum valence of each element
while others call for drastic changes
and looked for similarities among
layout of the transition
to its core structure.
these quantities (see Mendeleev’s
elements altered
More than 1000 periodic systems
table, p48).
Some call for drastic
have been published since the
The method works very well for
table rearrangements,
Russian chemist Dimitri Mendeleev
the elements up to atomic weight
perhaps placing hydrogen
developed the mature periodic
55 (manganese) after which point
with the halogens.
system – the most fundamental
it starts to fall apart. Although
A new block may be
natural system of classification
there seems to be a repetition in the
needed when chemists
ever devised. (Not to mention the
highest valence of aluminium and
can make elements in
hundreds if not thousands of new
scandium (3), silicon and titanium
the g-block, starting at
systems that have appeared since the
(4), phosphorus and vanadium (5),
element 121
advent of the internet.)
and chlorine and manganese (7),
Such a proliferation prompts
this is not the case with potassium
questions as to whether some tables
and iron. Whereas potassium shows
are more valid than others and
a maximum valence of one, iron
even whether there might be one
shows a maximum valence of three.
optimal periodic system which best
It does not fit properly underneath
reflects the relationship between the
potassium and nor do the next two
chemical elements.
elements, nickel and cobalt. But the
The underlying science embodied
following element, copper, does show
by the periodic system is deceptively
a valence of one and two.
simple. If the elements are placed
Ignoring the valence of two gives at
in a natural sequence according to
least some reason for placing copper
The periodic table’s
increasing atomic number there
underneath potassium. Similarly,
iconic design has taken
appears to be an uncanny repetition
zinc fits perfectly underneath
it outside the realm of
in their properties, albeit an
calcium since both elements show a
science
approximate one.
Using atomic number the
elements can be arranged into a one-
dimensional sequence, which is the
precursor for any periodic system.
H (1), He (2), Li (3), Be (4), B (5)......
But the periodic table is a two-
dimensional display of the elements.
It cuts the sequence at particular
places in order to reflect the
chemical similarities of elements.
The resulting strips can be pasted,
one underneath the other, to
display the approximate chemical
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Chemistry World
March 2009
47
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