the largest Category of course other metals
Semi metals
understand how
the electrons behave in those substances and
Determine their conductivity
Now there's a range of substances
In the metals category and some are more conductive than others gold is a better conductor than iron for example
And of course in our nonmetals, we clearly have substances that are very good insulators. They don't conduct any electricity at all
In this video. I want to talk specifically
what the factor is that determines their conductivity and I'm going to introduce the concept [of] the band theory and
hopefully in this video you come away with [a] better understanding of
What it is that [is] occurring at the molecular level that?
as you know the atom consists of a nucleus with electrons that are in orbit around it and
These orbits are in discrete shells
So let's take atoms such as sodium. We know it's sodium that as long as it's neutral it has 11 protons and 11
electrons and
These are arranged in
Depending on how many electrons can be held in each shell
So we have two in the inner shell we have 8 in the next shell, and then we have one in the outermost shell
This electrode here which is
Very loosely held in fact and it actually contributes to the reactivity [of] sodium
Actually also it is referred [to] as the valency shell
so the valency Shell is simply the shell that contains some or
electrons in the Outermost Shell
Now because this particular shell
Isn't a is that the atom [os] and the electrons aren't tightly held this electron actually
under a potential difference is free to move and
so this particular area or
Back Shell is actually [also] referred to as a conduction band
In other words. It doesn't require this electron doesn't require any
energy to
become conductive it is already in an energy state that allows it to become conductive, but
[what] happens if we have a different atom in?
This case we have chlorine
now chlorine has
Seven electrons in its outermost shell [and] as a result it actually these electrons actually quite tightly held
the outermost shell is still called the valency band it is the actual atom ashed Shell and
Therefore it contains the Valence electrons seven in this case
But these electrons are held tightly
They do not have enough energy to become conductive staying in this position in
Order to become conductive they actually have to move to the conduction band
But is now in a new shell
There are no electrons in this case for chlorine in this conduction, band
however for an electron to go from the valency band to the conduction
Band in order to become used to conduction. It needs to cross a forbidden gap
According to balls balls theories an electron can have enough energy to be in this shell
You can have a certain amount of energy to be in this show. I am out of energy, but cannot exist in between
So in order to get from this cell to this show. It needs a very specific amount of energy and
In terms of insulators. This is actually quite a large gap to overcome
valency and our conduction bands
represented by two boxes
conductor
so here's a conductor and [a] conductor has a a
the
valency band such as this
As you can see it's represented by an increasing amount of energy as we go up
So because they overlap any electrons that exist in the valency band
is
Automatically in the conduction band there is no gap for them to have to
Traverse in order to become conductive in other words outermost electrons are free to conduct
All insulator also has a valency band in a conduction band
In in this case for the insulator for the electrons in the valency band to give to the conduction band
Electron Volts
Then we say these electrons because [they're] unable to get into the conduction band are
Therefore unable to become conductive and in it is an insulator
But what about the third to right situation?
How about a semi conductor?
Well a semi conductor also has a valency band and a conduction band
There is also a gap
however the forbidden gap is much smaller and
so for an electron to [move] from the valency band to the conduction band [it] does require a
Amount of energy to move from the [valency] band to conduction band, but the energy can be gained through thermal energy
So for example if I have a semiconductor such as germanium
The it only needs about [1.1]. Electron volts to traverse it. How do I manage to do that well?
I simply heat up the substance and as long as it is above zero degrees Kelvin
then there may be enough energy for electrons to move from the [valency] band to the conduction Band and
Of course once they are in the conduction band
They're free to become a conductive and they will conduct if I apply electric [field] to them or apply potential difference across them I?
Now want to give you a bit of an analogy to help you understand the band theory
And in this analogy, I'm going to model
But the band theory by using this piece of metal and you can see that this metal consists basically of a smooth surface
small divots here, and then a large divots here and
The surface of the [Metal] is going to represent our
Conduction band it [is] the place where the electrons are free to become?
Conductive, and that's going to be represented by our marbles
And they're free to move and of course you don't want to apply a potential difference
I simply have to tip it and so as you can [see] by tipping it I can clearly have the electrons
moving in One Direction
So let's go and discuss how these divots represent our various substances
If I place my marbles in the largest [divot] you can see that these electrons are really fixed and no amount of potential difference is
Going to make them move shaking them isn't going to cause them to move
What that means is is that the amount of energy required for these?
electrons, so to speak to pop up to this conduction band is so large and that the electrons are not able to do so and
therefore stay in the Valency Band and
So we classify this area as our
insulator the Gap between the valency Band and the conduction band is just [to] large to overcome and
Similarly it is clear to understand that this ball bearing or marble clearly either sits on the surface
Or is in the actual divot and that of course represents that that electron does not exist somewhere in between
The two bands in other words. That's why we refer to it as the forbidden
gap
But what if I have my electrons in these divots here?
Now clearly again, they're fixed and as long as there's no energy in this case the energy is
vibrational energy and that is going to be temperature then the electrons are not free to move they stay in the valency band and
if I apply [a] potential difference they stay put
But as soon as I start to vibrate it you can see that. They are free to move in other words
I've provided them enough energy to pop out of the divot and as a result they are
Going into the conduction band enough free to move
Similarly speaking these divots here do the same job again if there is no energy to the actual substance
They're fixed then are going to move
however, if I produce a small amount of energy in this case
Vibrating it which represents an increase in temperature?
they are free to move and if I apply a potential difference you can see that they are moving off in One direction and
So this region here these two divots represents our semiconductors
you could argue that this having a larger divot means a larger forbidden gap and that will be something like
Silicon which is about 1.1
Electron Volts fitting get bitten Gap and germanium only 0.7
And of course clearly in this case the electrons
Along this surface of free to move there is a need any energy whatsoever
To move from their position and so this area here represents the metal so we have our insulator. We have a
semiconductor and we have our metal and each have their
differences due to the place where the electrons are
and the
Circumstances that are required for the electrons to become conductive [if] they are in the conduction, band
now one word of caution
This is clearly a model for band theory
although [it] is simplistic and it explains aspects of band theory it is a
Model nonetheless [which] means it does have some [limitations], and that's an important thing to consider
that models are useful in science and
therefore can
explain complex concepts in simplistic terms
But they have their limitations. They do not explain everything and in this case my metal sheet has its limitations
It does not explain everything about band theory
But hopefully with the animation and this particular demonstration you'll have a better understanding of van theory