Confusion with free electrons in metals

[Jimmy87]

Hi, I'm really confused with how electrons become 'free' in a metal. I have a few questions and would be very grateful if someone could shed some light on them.

1. Are the electrons actually free? In a sense that the atom it is attached would essentially become an ion
2. If you take a copper wire which is made up of copper atoms then each atom has 29 electrons, so is it one of these 29 electrons in the outer shell that is free?
3. What is it about metals that enables it to have these free electrons compared with say an insulator?

 

[mfb]

1. Are the electrons actually free?

Yes.

In a sense that the atom it is attached would essentially become an ion

Well the electrons are still in the metal, so the total charge is still zero.

2. If you take a copper wire which is made up of copper atoms then each atom has 29 electrons, so is it one of these 29 electrons in the outer shell that is free?

Not sure if exactly one, but yes.

3. What is it about metals that enables it to have these free electrons compared with say an insulator?

The band structure, which comes from the energy levels of the electrons in the material, and that is related to the electron structure of the atoms and the crystal structure. And quantum mechanics.

 

[Jimmy87]

Yes.
Well the electrons are still in the metal, so the total charge is still zero.
Not sure if exactly one, but yes.
The band structure, which comes from the energy levels of the electrons in the material, and that is related to the electron structure of the atoms and the crystal structure. And quantum mechanics.

Thanks for the answers! Could you expand on what you said as I'm really interested to know how these energy levels are arranged to enable some electrons to be free. I know a bit of quantum mechanics if that helps.

 

[Drakkith]

Thanks for the answers! Could you expand on what you said as I'm really interested to know how these energy levels are arranged to enable some electrons to be free. I know a bit of quantum mechanics if that helps.

That's fairly complicated. See these articles for more info:

http://en.wikipedia.org/wiki/Band_theory
http://en.wikipedia.org/wiki/Valence_band
http://en.wikipedia.org/wiki/Conduction_electron

 

[Jimmy87]

That's fairly complicated. See these articles for more info:

http://en.wikipedia.org/wiki/Band_theory
http://en.wikipedia.org/wiki/Valence_band
http://en.wikipedia.org/wiki/Conduction_electron

Thank you. I know its a bit of an over simplification but from what I read from these articles is the following sentence acceptable:

The energy level band gap between the valence band and the electron band is closer in metals which means that electrons can make transitions into the conduction band and are then 'free' in a sense whereas in conductors the band gap is too large therefore very few transitions take place between these bands and hence no 'free' electrons.

 

[Drakkith]

I think you have some of your terms mixed up.

In conductors, there is no gap between the valence band and the conduction band. There is no transition, because the electrons in the valence band are already delocalized and shared throughout the lattice. In materials where a band gap exists, such as insulators and semiconductors, the electrons in the valence band are not delocalized throughout the lattice but are bound to their own atoms or to local atoms through bonds. It takes energy to promote them from the valence band to a higher energy state where they are delocalized and explains why semiconductors and insulators don't conduct electricity as well as conductors.

 

[Jimmy87]

I think you have some of your terms mixed up.

In conductors, there is no gap between the valence band and the conduction band. There is no transition, because the electrons in the valence band are already delocalized and shared throughout the lattice. In materials where a band gap exists, such as insulators and semiconductors, the electrons in the valence band are not delocalized throughout the lattice but are bound to their own atoms or to local atoms through bonds. It takes energy to promote them from the valence band to a higher energy state where they are delocalized and explains why semiconductors and insulators don't conduct electricity as well as conductors.

Ok I see. Thanks! So, in metals the valence band lies within the conduction band. Is that right? Whereas in some materials there can be a small gap (semi-conductors) or a large gap (insulators). For insulators, when dielectric breakdown occurs does this essentially mean that electrons have transitioned across the gap?

 

[Drakkith]

Exactly.