Name: Zoe Boyle
Date: Around 1999
Why is metal a good conductor of electricity?
There are two requirements for conducting electricity: 1) there must be
empty valence band orbitals (see below) that can transport the
electrons, and 2) you must apply an external electric field.
The electrons of an individual metal atom (as with any other type of
isolated atom) occupy orbitals that stay close to that atom (the
orbitals are labelled s, p, d, f, etc.). In fact, the orbitals exist
whether or not they are occupied. When a covalent molecule is formed,
the electron orbitals are modified. Orbitals from each atom mix
together in such a way that the electrons from those orbitals are shared
between the two (or more) atoms involved in the bond and, since two
atomic orbitals are involved (one from each atom), two 'molecular'
orbitals are formed. The energies of these new molecular orbitals are
slightly different from the energies of the atomic orbitals, one being
slightly higher in energy than either atomic orbital and one being
slightly lower in energy than either atomic orbital. Each orbital can
accept two electrons with opposite spins.
A lump of metal is essentially one big molecule. There are so many
atomic orbitals involved in the individual bonds that the molecular
orbital spreads out over the entire lump of metal, meaning that an
electron put into one end of the lump (or wire) can be taken out at the
other end because the molecular orbital allows it to be anywhere in the
wire. There are as many molecular orbitals as there are electrons in
the wire and they each have slightly different energies. In metals,
semiconductors, and insulators these VERY closely spaced orbitals are
collectively called 'bands'. There is a valence band and a conduction
band and the energy separation of the bands will impact the properties
of semiconductors -- but that's the answer to another question. The
valence band is lower energy than the conduction band and so is filled
with electrons before any electrons go into the conduction band. In
metals, there are not enough electrons to fill the valence band
Because there are so many orbitals the energy levels are so closely
spaced that it is very easy for electrons to shift between molecular
orbitals and it is very easy to add electrons to the bands (from an
external source), if there are any low energy (valence band) orbitals
that are not fully occupied. Applying a potential across the wire
(e.g., - charge on the left and + charge on the right) will modify the
energies of the orbitals slightly. The energies of orbitals that favor
putting the electrons on the right end of the wire will be slightly
lower than for orbitals that put more charge on the left end of the
wire. As a result, more electrons will be in orbitals that concentrate
charge on the right end of the wire. If you add an electron at the left
end of the wire it will go into the lowest energy level orbital that
does not already have two electrons. In a metal this will be be a
valence band orbital that favors the electron being on the right end of
the wire (in our example). If you remove an electron from the right end
of the wire you have completed the circuit and have conducted an
electron (although not the same electron) from one end of the wire to
This can get pretty complicated.
Atoms are made of electrons and a nucleus. The electrons are "negative"
charged electtricity, the nucleus is positive. So the negative and the
positive charges hold to each other and the atom stays together. But
some atoms have alot of electrons, and some of these atoms also have
some outer electrons that are not held too tightly. When you start
putting alot of these atoms together, there begiins tobe a collection of
electrons which are not really "owned" by any one nucleus, but sort of
swim thru a sea of nuclei, and other electrons. Keep in mind that MOST
of the electrons are tightly held to their particular nucleus. But some
are freeer, to swim thru the entire collection. This would mean that
this material could conduct electricity. If however very few of these
swimming around electrons can exist (because each of their nuclei are
very reluctant to let go of even one electron to contribute), then there
is no way to move electrons freely thru the material, and we would call
this an insulator of electricity.
Zoe, electricity is the movement of electrons, so any element with only 1 or
2 electrons in its outer shell would be a good candidate for a conductor.
Check your periodic table and look at the electron shells of elements which
are metals and transition metals, and you'll see what I mean.
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Update: June 2012