Group Numbers and Valence Electrons
Date: November 2007
Relating to the periodic table and valence electrons. The rules
governing this, state that groups 1 and 2 on the periodic table have the
same number of valence electrons as their group number. I have found this
to be untrue in the cases of Rubidium (3), Cesium (5), Francium (5),
Strontium (4), Barium (6), and Radium (6). The number in parentheses
gives the number of valence electrons. I have also noticed that in groups
13-18 it states that they have 10 less valence electrons than their group
number. This is also untrue in several cases? I guess my question is, are
all the Science Books teaching this topic wrong or is there some other
rules that I do not know about.
The source of your dilemma is the stated assumption that: The rules
governing this state that the same number of valence electrons as their group
number. That is not true in general. The "rules" governing the number of valence
electrons is the "Aufbau" (building up) principle.
This is the familiar: 1s2, 2s2, 2p6, etc., ... presented in texts covering the
periodic table. Rb for example has the electron configuration Rb = [Kr], 5s1 where
[Kr] is the closed shell configuration for "Kr" which is:
[Ar], 3d10, 4s2, 4p6 and [Ar] is the closed shell configuration of "Ar".
I think your inquiry raises a larger question. How is the electronic
configuration of atoms without some basic understanding of elementary
quantum theory -- quantum numbers (where they come from), Pauli exclusion
principle, shells and sub-shells, etc. -- writing the electron
onfiguration is a "not-very-useful" number game. For your reference,
often as is the case here, it is useful to consult books that were
closer to the invention of the subject. I refer you to the "classic"
text: "General Chemistry" by Linus Pauling (especially Chapter 5) which
deals with the periodic table and the concepts behind it.
My many years of teaching chemistry has led me to the conclusion that
there is a "too much, too soon" disease that infects chemistry texts.
An analogy from another discipline would be teaching "music theory"
before the student picks up an instrument. The rules governing music
composition have little relevance to 6 to 8 graders -- unless you happen
to have a class of little Mozart's. A better use of time and resources
is to communicate why chemistry is "fun", and how chemicals behave. "Too
much, too soon" will only drive students into other disciplines with a
more enlightened view of their subject.
Check out: http://scifun.chem.wisc.edu/HOMEEXPTS/HOMEEXPTS.HTML and other
web sites of
Prof. Bassam Shakhashiri. His approach is how chemistry should be presented to
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Update: June 2012