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Name: Zoe Boyle
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Around 1999


Question:
Why is metal a good conductor of electricity?


Replies:
Hi Zoe,

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 completely.

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 the other.

Bradburn


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.

Ross


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.

Felix



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