The Not So Useless Neutron

We now know that electrons are responsible for the reactions we see in everyday life whether they be rusting, burning, rotting, etc. In the previous article we saw that the reason alchemists were never able to create gold from regular metals was because they were only able to cause chemical reactions to occur. The atoms themselves never change during a chemical reaction, they simply exchange electrons and bond in different ways.

It wasn't until the discovery of radioactive elements and the invention of particle accelerators that we were able to witness and control nuclear reactions in the lab. Nuclear reactions involve changes to the nucleus of an atom. Since it is the nucleus that uniquely defines the type of atom, being able to change the nucleus opens up the possibility of converting one type of atom into another.

To better understand what's going on in the nucleus, let's have a closer look at the two particles that make it up: neutrons and protons. Recall that protons are positively charged and neutrons have no charge. The number of protons in a nucleus is what defines what type of atom it is. For example atoms with 3 protons are lithium atoms. Interestingly enough, the number of neutrons is not always the same for a given atom. For example lithium atoms can be found in nature which have either 3 or 4 neutrons (4 being the most common). Atoms of the same element with different numbers of neutrons are called isotopes. This brings us to two very important questions:

1. What purpose do the neutrons serve?

2. Does it matter how many neutrons an atom has?

To answer this, let's start by having a look at the nucleus of a lithium atom, in this case we'll look at the isotope with 4 neutrons. Lithium with 4 neutrons is referred to as Li-7. "Li" is the chemical symbol for lithium and the "7" shows the total number of particles (also called nucleons) in the nucleus (3 protons + 4 neutrons = 7 particles in total). The isotope with 3 neutrons as you may have guessed is called Li-6.

Nucleus of a Lithium-7 Atom

Nucleus of a Lithium-7 Atom

To understand the purpose of the neutrons, take a really close look at the image above. Something doesn't quite make sense...

Any kid who has ever played with magnets knows that opposite charges attract and like charges repel. If that's the case, then how can all these positively charged protons stay stuck together in the nucleus? Their electrical charges should cause them to push each other away, however there are other forces at play here. Although it may seem like there are all kinds of different forces we encounter in our world, there are actually only 4:

1. Gravity

2. Electromagnetism

3. Weak Nuclear Force

4. Strong Nuclear Force

Gravity is something we're all familiar with, electromagnetism is also familiar to us through magnets, electricity, lightning etc, the weak nuclear force involves forms of radioactive decay that we won't discuss here, and the strong nuclear force is what binds particles in the nucleus of an atom together.

The strong nuclear force is, as the name suggests, very strong but it can only act over very short distances. It is responsible for holding together the quarks that make up protons and neutrons. A side effect of this is that it also causes the quarks in different protons and neutrons to attract each other. Without getting into too much detail, the strong force causes protons to attract both other protons and neutrons and causes neutrons to attract both other neutrons and protons but only when these particles are very close together. If you want more detail on quark interactions, the field which studies these effects is called quantum chromodynamics.

    A name almost as difficult as the subject itself

 

A name almost as difficult as the subject itself

The reason all the positively charged protons in the nucleus don't push each other apart is due to the strong force. Once they are close enough together, the strong force pulling the protons together overwhelms the repulsive force due to their like charges. Since the strong force applies to both neutrons and protons, the neutrons play just as much a role in holding the nucleus together as the protons do. So now we know their purpose: to help hold the nucleus together. However we still haven't answered the question of whether or not it matters how many neutrons there are in the nucleus. In fact we've backtracked a bit because this raises a third question:

3. Does more neutrons mean a more strongly bonded nucleus?

The protons stick together due to the strong force but their positive charges are still trying to push them apart a little bit. The neutrons also stick to each other and to the protons but they have no charge so they don't push apart. It therefore wouldn't be unreasonable to assume more of them means a stronger nucleus right?

As usual, truth is stranger than fiction. If you have read the article on Chemistry, you will recall that electrons organize themselves into "clouds" or "shells" and they fill these shells in a specific order from inside to outside. The protons and neutrons organize themselves in a similar way within the nucleus. As you add protons to a nucleus, they will gradually fill the different shells available. The same is true for neutrons but they do this independently of the protons. In other words, neutrons will not fill proton shells and protons will not fill neutron shells.

Shells are essentially a way of saying different energy levels. Different positions within the nucleus will result in a proton or neutron achieving a different energy level. Just like a ball falling from a table to reach a lower energy state within the Earth's gravitational field, a proton or neutron will "fall" into a lower energy state within the nucleus if it has the chance. As I said before, protons and neutrons each have their own energy levels or shells within the nucleus, however something very interesting will happen when the number of neutrons and protons is not properly balanced.

Let's take a look at our Lithium-7 atom again, but this time we'll add an extra neutron to see what happens:

Adding an Extra Neutrons to Li-7

Adding an Extra Neutrons to Li-7

Once we add the neutron, it must position itself in a higher energy spot than the neutrons already there. You'll notice though that there's a really nice low energy proton position just begging to be filled. If the difference between the highest available neutron energy level and lowest available proton energy level is large enough, something remarkable will happen:

Decay of Li-8 Into Be-8

Decay of Li-8 Into Be-8

As you may have guessed, the neutron will transform itself into a proton and move into the lower energy level. In doing so it will release an electron (which we're familiar with) and an electron antineutrino (which is a more exotic particle that we won't discuss here). The resulting nucleus will now have one extra proton but the same total number of nucleons (4 protons + 4 neutrons = 8 particles in total). An atom with 4 protons is called Beryllium so this new isotope is called Beryllium-8.

For atoms with a high number of protons but relatively few neutrons, a proton can also transform into a neutron. Neutrons and protons will transform into each other only if it puts them into a low enough energy state to justify the transformation. Nucleii who could lower their overall energy by making such a transformation are called radioactive. The name comes from the fact that these transformations release radiation. Once a nucleus achieves an energy state that it cannot reduce, it is called stable. It turns out that of all the possible combinations of neutrons and protons, there are very few that are actually stable. You can see this on the chart of the nuclides below. The stable nucleii are in black, the rest of them are radioactive.

Image courtesy of Brookhaven National Laboratory   Chart of the Nuclides

Image courtesy of Brookhaven National Laboratory

Chart of the Nuclides

The chart shows all the possible isotope combinations. The number of neutrons is plotted along the x-axis and number of protons along the y-axis. For the most part, the stable nucleii have about the same number of neutrons and protons. However you'll notice the line of black bends down a bit as you move up the graph.This is because in larger nucleii, most of the protons are too far apart to be attracted to each other by the strong force. It therefore takes more neutrons to overcome the repulsion of these distant protons.

Now we finally have the answer to both our original questions plus the third question that we came across while trying to answer the first two:

1. What is the purpose of the neutrons? 

The strong force applies to neutrons and protons, causing them to attract each other so neutrons help hold the nucleus together.

2. Does it matter how many neutrons there are?

Absolutely. Neutrons and protons both have energy levels within the nucleus. If there is too many of one or the other, a transformation will occur to achieve a stable balance.

3. Do more neutrons mean a more strongly bonded nucleus?

Yes and no. If there are too many neutrons then a neutron will typically transform into a proton to put itself and the nucleus into a lower energy state. For small atoms, a stable nucleus has about the same number of neutrons and protons. However larger nucleii require some extra neutrons to compensate for the fact that there are so many protons trying to push each other apart. The most stable mix of neutrons and protons in a heavy nucleus will therefore be when there are a few more neutrons than protons.

I will close by saying that we discussed two types of nuclear reactions here: transformation of neutrons into protons and transformation of protons into neutrons. As it turns out, there are other reactions that nucleii can use to achieve stability. These other reactions hold the secret to how stars shine and how nuclear power plants make electricity.