Periodic Table Long Form Page 3
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3
4Periodic table
maximum valence of two.
of the main body of his compact or
group. He called these ‘transition
Mendeleev’s table of
The question is what to do with
so-called short-form periodic table.
elements’, because they represented
1871 contained a new
iron, cobalt and nickel? Mendeleev
A similar situation occurs with
transitions in the sequences of
group of elements:
solved this problem by creating an
ruthenium, rhodium and palladium,
elements which showed similarities
group VIII
anomalous new group, or rather a
and later with osmium, iridium and
in their maximum valences, before
group containing three elements in
platinum, which Mendeleev likewise
such behaviour was taken up again as
each place. He placed the new group,
expelled from the main body of the
in the case of copper and zinc, silver
which he labelled VIII, to the right
table and placed into the anomalous
and cadmium, and finally gold and
mercury.
This simple act of removing certain
elements from the main body of the
periodic table is key to understanding
why it has been represented in so
many different forms. Nowadays the
most commonly encountered form
of the periodic table is the medium–
long form (left).
Today’s transition elements are
not the same as Mendeleev’s – the
sense in which they are ‘transitional’
A question of taste: the
has changed. In order to appreciate
medium-long-form table
this change we must consider
(top) or the long-form
the electronic configuration of
table (bottom)?
atoms, none of which was known
at the time of Mendeleev and his
contemporaries.
An atom of potassium has a
2
2
6
2
configuration of 1s
, 2s
, 2p
, 3s
,
6
1
3p
, 4s
followed by calcium, which
2
2
6
2
6
2
is 1s
, 2s
, 2p
, 3s
, 3p
, 4s
. But then
something strange happens – the
following 10 elements after calcium
have a configuration involving the
occupation of the 3d orbitals. It is
only when this process is completed
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Chemistry World
March 2009
48
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