Ask A Scientist© Chemistry Archive

name         Alex M.
status       student
age          15

Question -   Why does electricity flow through metal but not through
plastics?  Please answer in fair detail, sub atomic.
------------------------------------------------
Dear Alex,

Electricity DOES flow through plastics. It just depends on
what plastic you are talking about.

In fact, today in the news I read that a plastic has been
formed which can superconduct (at very, very low temperatures).

Most plastics are insulators, some are semiconductors, and a few
are pretty decent conductors. But stay tuned, this is current
research!

I guess I have not answered your question...sorry...now if you
ask me "why are most plastics insulators, while all metals are
conductors," well, I would say you are comparing apples and oranges.
Metals are made of (more or less) regular arrays of atoms.
Plastics are usually made of (more or less) irregular arrays of
molecules.

Both of these differences (irregularity of arrays and atoms vs.
molecules) are important factors, but they are not the only
important factor. For example, lead is a conductor, germanium
is a semiconductor, and carbon (graphite or diamond) is an
insulator. The main difference between these three is the size of
the energy gap between the highest occupied electron energy state of
the solid and the lowest unoccupied state. For carbon the gap
is very large, for germanium it is modest, and for lead it is virtually
zero. However I know of no simple way to predict this from atomic
properties alone. The size of the atom is important, as is its
electronegativity - but so is the crystal structure, which is not
predictable from atomic properties.

As a rule of thumb, you might say that the molecules in a plastic
have their valences completely satisfied and so there's no overwhelming
tendency for them to share electrons with one another at all, and
therefore no good way for electrons to move from one molecule to the next.
However, this is not true for metals. But this simple answer does not
cover all the bases. Still, I hope it helped.

best, prof. topper
========================================================
The electrons in a metal are free to move in response to a voltage
difference provided by some source == battery, generator etc. In contrast
the electrons in a plastic are covalently bound to constituent atoms are
not free to move in response to an applied voltage -- within reason. Ionic
compounds, like salt, are non-conductors as solids but conduct electricity
as liquids because now the constituent ions are free to move in response to
an applied voltage.

Vince Calder
=========================================================
Alex.

Before you pose questions, always do a search of Newton's archives. You may
find the answer is already there. For example, if you go here

http://146.139.100.40/webpages/askasci/phy00/phy00064.htm

you will find the answer to your question about the electrical conductivity
of metals (conductors) and non-metals (insulators like plastic). In general,
the electrons (charge carriers) in plastics are discretely shared between
atoms. As a result, they are not free to flow under the influence of an
applied potential difference (voltage).

Regards,
ProfHoff
=========================================================
Alex,

You might want to check the issue of nature which
describes research in using plastic as a
superconductor.  Clearly, our physics books will need
ongoing revisions in this area of research.

PLASTIC SUPERCONDUCTOR -- A band of New Jersey
scientistsat Bell Laboratories in Murray Hill has
discovered the world's first plastic superconductor, an
innovation that could provide the essential boost for
next-generation "quantum" computers and other electronic devices.
"This is stunning and truly beautiful work," said
Olle nganas, an authority on organic materials at
Linkoping University in Sweden.  The work on plastics,
described in a Nature essay as an "experimental tour de
force,"also represents the seventh major research paper
on superconductivity by the Bell Labs team in a
year.  Hendrik Schon, a Bell Labs researcher, said the
most promising application for the work would be in
the area of quantum computing, a field that makes
superconductivity seem mundane by comparison.  [Star-Ledger (N.J.),3/8]

Thanks for using NEWTON!

Ric Rupnik
=========================================================