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From: matt@oddjob.uchicago.edu (Matt Crawford)
Newsgroups: sci.physics,sci.chem,sci.electronics
Subject: Re: Valence electrons & metalic hydrogen
Message-ID: <5769@tank.uchicago.edu>
Date: 11 Oct 89 16:28:54 GMT
References: <1017@mgse.UUCP>
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Reply-To: matt@oddjob.uchicago.edu (Matt Crawford)
Followup-To: sci.physics
Organization: Up against the wall of SCIENCE
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In-reply-to: marks@mgse.UUCP (Mark Seiffert)

In article <1017@mgse.UUCP>, marks@mgse (Mark Seiffert) writes:
) I just started an electronics course today ...
) If there was one valence electron, the element
) was a conductor, if there was four, the element was a semiconductor,
) and if there was 8 valence electrons, the element was an insulator. 

Oh, ghods!  I hope that's a high school course and not a college one!
I can sort of excuse a high school teacher being that ignorant, since
my high school's electronics teacher was mainly a football coach.

This categorization of conductors, semiconductors and insulators is
just not correct.  The modern description is not in terms of valence
electrons but in terms of filled and empty energy levels for electrons
in the bulk material.

When you put some material together in a macroscopic quantity, there are
formed some possible states in which electrons can exist in a
"delocalized" condition.  That is, an electron in such a state can
freely travel around.  (Electron states which are bound to individual
atmos continue to exist, and the distinction between core electrons and
valence electrons is still useful.)  Because the sample has an enormous
number of atoms in it, these states exist in "bands" of states with
similar energies.  For instance, there may be a set of possible states
for electrons with energies varying continuously from, say, 0.3 to 0.4
eV (electron-volts).  (I'm just making these numbers up -- I have no
reference book handy.)  Then there is generally a "gap" in the energy
range in which there are no allowed states for electrons, and then
another band of possible states.  For instance, there may be no allowed
states from 0.4 to 0.6 eV, then another band from 0.6 to 0.8 eV.

Which of these states will be occupied?  Well, since room temperature
corresponds to an energy per electron of only about 0.025eV, electrons
will mainly fill all the lowest possible states, with very little
thermal energy available to move up.  Although the different energy
states in one band are clustered together in energy so closely as to be
essentially a continuum of states, each state can hold only two
electrons (one with "spin up" and one with "spin down") -- just as each
orbital in an isolated atom can contain two electrons.

A completely-filled band of energy states cannot contribute to
conductivity, because for each electron in the energy band which is
moving to the left, there will be another electron of equal energy which
is moving to the right.  Hence, no net current can be present in a
completely filled band.

So in a sample of some substance, the distribution of electrons will be
as follows.  The innermost electrons will still be tightly bound to
specific atoms.  Other electrons will be in the delocalized energy
states, filling them from lowest energy to highest, using as many bands
as needed to accommodate all the electrons.

What distinguishes conductors from insulators is how the bands are
occupied.  If the highest occupied band is completely filled, the
material is an insulator because it can support no net flow of current.

If the highest band occupied is only *partly* filled, then the material
is a conductor.  Even though equal numbers of electrons are traveling
left and right, it takes only a tiny amount of energy to lift a
leftward-moving electron up into an unfilled rightward-moving state.

If the highest occupied band is completely filled, but the energy gap
between it and next band above it is very small, then the material is a
semiconductor.  (This is why silicon is opaque to visible light but
transparent to the lower-energy photons of infrared light.

I simplified this quite a bit.  If you want to know more, look for an
introductory book on solid state (aka condensed matter) physics.

) They question i have is what about Hydrogen. It has one valence
) electron, is Hydrogen gas conductive?

In ordinary hydrogen gas, the molecules aren't packed closely enough to
create the delocalized states.  You'd have to put the hydrogen under a
huge pressure to get it to form a metal.
________________________________________________________
Matt Crawford	     		matt@oddjob.uchicago.edu
(I don't mean to run down high school teachers -- I had
some good ones.  But the electronics `coach' was not
among them.)