| MadSci Network: Chemistry |
According to one source, the transition metal groups are labeled IB
to VIIIB with the B simply added to differentiate them from the
representative elements. The representative elements (sometimes
called "main group elements") are labelled I to VIII, and sometimes an A
is added, again just to differentiate them from the transition metals.
Chemicals are put into a given group for two reasons - first because
of atomic number, and second because of chemical properties. It turns out
that the two coincide. Basically, the Aufbau Principle (also called
the "building up" principle) explains chemical properties (the reasons for
the Aufbau Principle are deeply rooted in quantum mechanics). We know
that an element's properties come from its electronic configuration, and
the periodic table takes advantage of that. Elements in a given group
(with the exception of some transition metals) always have the same
oxidation number (or numbers in the case of elements with multiple
oxidation numbers), and we know that electronic configuration determines
an element's oxidation state.
In addition, the periodic table is built up on groups of eight.
There are 8 groups of representative elements, and their are eight groups
of transition metals (the three columns with Fe, Co, and Ni are all
considered group VIIIB). However, the transition metals are labelled in
the following order: IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB (instead of
starting with IB). Looking at oxiation numbers, this makes sense. Sc, Y,
La, and Ac, which are all group IIIB transition metal elements all have an
oxidation number of 3. The same can be said of most other transition
metals (but not all of them) - that is, group number is a good indicator
of oxidation state.
The elements in group VIIIB, which contains three columns, probably
has the most exceptions when it comes to correlating group number to
odixation state. If you look at oxidation numbers, these metals can have
a wide range of oxidation states. However, most of them display the same
main oxidation state. Below are some examples from group VIIIB - the
element is listed with its possible oxidation states. The most common
oxidation state is given in parenthesis.
Fe - 2,3 (3)
Co - 2,3 (2)
Ni - 2,3 (2)
Ru - 2,3,4,6,8 (3,4)
Rh - 2,3,4 (3)
Pd - 2,4 (2)
It is interesting to note that at one time IUPAC (the International
Union for Pure and Applied Chemistry) was considering a revision of the
way groups are numbered. Members considered abandoning the current group
labelling system for one that goes straight from group 1 to group 18. As
far as I know, this new system was never adopted.
Sources:
"Chemistry" by Steven S. Zumdahl (I consulted an old copy from 1986 that
talked about the possibility of a new group labelling system)
"Principles of Modern Chemistry: Third Edition" by David Oxtoby and Norman
H. Nachtrieb
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