C hapter 4 Material Classes, structure, and properties
90
9.11 × 10
−31 kg that revolve around the nucleus in discrete orbits.
Thus, the nucleus is positively charged and very heavy and the
electrons are negatively charged and light.
Let’s now think about another aspect: What is particular about particles such as electrons, protons, neutrons, and photons? Newton
thought that light was made up of particles and therefore should
behave the way particles do. And in fact light does behave like particles. However, this is not the whole story. Light also behaves like a
wave. How can particles such as electrons and light exhibit this dual
behavior? We don’t know. For the time being we need to accept
this, keeping in mind that on a small scale the world behaves in a
very different way. It is hard for us to imagine this because we have
evolved in a different kind of world. However, we can still use our
imagination. This is the field of quantum mechanics.
Let’s explore the first idea of quantum mechanics. This idea claims
that we are not allowed to know simultaneously the definite location and the definite speed of a particle. This is called the Heinserberg Uncertainty Principle. In other words, we can only say that
there is a probability that a particle will have a position near some
coordinate x. This is akin to watching Shaquille O’Neal throw a
basketball to the basket. You can’t say that he is going to hit the
basket for sure! There is also a certain probability. This explains a
very mysterious paradox, which is this: If the atoms are made of
plus and minus charges, why don’t the electrons get closer? Why
are atoms so big? Why is the nucleus at the center with electrons
around it? What keeps the electrons from simply falling in? The
answer is that if the electrons were in the nucleus, we would know
their position and then they would have to have a very high speed,
which would lead to them breaking away from the nucleus.
So far, when we have been talking about atoms, we have considered
their lowest possible energy configuration. But it turns out that electrons can exist in higher-energy configurations. Are those energies
arbitrary? The answer is no. In fact, atoms interchange energy in a
very particular away. An analogous idea is to have people exchange
paper currency. Imagine that I want to buy a CD that costs $17 and
that I only have $5 bills. Further imagine that the CD store only
has $5 bills in the cash register. In this case, the CD I want to buy
will cost either $15 or $20, depending on which party wants to
assume the loss. Atoms are very similar. They can only exchange
certain “dollar bills.” For simplicity, let’s look at the hydrogen atom
(see Figure 4.3). As shown in this figure, the ground energy for the
hydrogen atom is −13.6 eV (electron volts). Why is it negative?
Figure 4.3
Electron energy states for a hydrogen atom.
Energy (eV)
3d
3p
2p
3s
2s
1s
-10
0
-5
-3.45
-1.55
n = 1
n = 3
n = 2
-13.6
90
9.11 × 10
−31 kg that revolve around the nucleus in discrete orbits.
Thus, the nucleus is positively charged and very heavy and the
electrons are negatively charged and light.
Let’s now think about another aspect: What is particular about particles such as electrons, protons, neutrons, and photons? Newton
thought that light was made up of particles and therefore should
behave the way particles do. And in fact light does behave like particles. However, this is not the whole story. Light also behaves like a
wave. How can particles such as electrons and light exhibit this dual
behavior? We don’t know. For the time being we need to accept
this, keeping in mind that on a small scale the world behaves in a
very different way. It is hard for us to imagine this because we have
evolved in a different kind of world. However, we can still use our
imagination. This is the field of quantum mechanics.
Let’s explore the first idea of quantum mechanics. This idea claims
that we are not allowed to know simultaneously the definite location and the definite speed of a particle. This is called the Heinserberg Uncertainty Principle. In other words, we can only say that
there is a probability that a particle will have a position near some
coordinate x. This is akin to watching Shaquille O’Neal throw a
basketball to the basket. You can’t say that he is going to hit the
basket for sure! There is also a certain probability. This explains a
very mysterious paradox, which is this: If the atoms are made of
plus and minus charges, why don’t the electrons get closer? Why
are atoms so big? Why is the nucleus at the center with electrons
around it? What keeps the electrons from simply falling in? The
answer is that if the electrons were in the nucleus, we would know
their position and then they would have to have a very high speed,
which would lead to them breaking away from the nucleus.
So far, when we have been talking about atoms, we have considered
their lowest possible energy configuration. But it turns out that electrons can exist in higher-energy configurations. Are those energies
arbitrary? The answer is no. In fact, atoms interchange energy in a
very particular away. An analogous idea is to have people exchange
paper currency. Imagine that I want to buy a CD that costs $17 and
that I only have $5 bills. Further imagine that the CD store only
has $5 bills in the cash register. In this case, the CD I want to buy
will cost either $15 or $20, depending on which party wants to
assume the loss. Atoms are very similar. They can only exchange
certain “dollar bills.” For simplicity, let’s look at the hydrogen atom
(see Figure 4.3). As shown in this figure, the ground energy for the
hydrogen atom is −13.6 eV (electron volts). Why is it negative?
Figure 4.3
Electron energy states for a hydrogen atom.
Energy (eV)
3d
3p
2p
3s
2s
1s
-10
0
-5
-3.45
-1.55
n = 1
n = 3
n = 2
-13.6