Lesson 10: Changes to the nucleus
In this lesson we’ll look at how alpha, beta and gamma radiation change the arrangement of protons and neutrons in a nucleus. We’ll also see how protons and neutrons are made of smaller particles called quarks.
Let’s start by looking at a quick way of describing a nucleus.
If we want to know what element we’re dealing with we have to count the protons in the nucleus. This is called the ‘proton number’. The ‘proton number’ is also called the ‘atomic number’.
The periodic table shows you the proton number for each element. The proton number for hydrogen is 1.
Different isotopes of hydrogen have different numbers of neutrons. One way of defining the isotope would be to write down the number of these neutrons.
But in fact it’s more normal to write down the total number of protons and neutrons. The particles that make up a nucleus are called ‘nucleons’. Protons and neutrons are ‘nucleons’. So the total number of protons and neutrons in a nucleus is called the ‘nucleon’ number. The nucleon number is sometimes called the ‘mass number’.
Nucleon number defines an isotope
It’s the nucleon number we use when we talk about an isotope. Strictly speaking ‘isotope’ refers to a whole atom; nucleus, electrons and all. If we’re talking about different forms of the nucleus we should use the term ‘nuclide’. But for simplicity we’ll just refer to isotopes.
Hydrogen is unusual in having special names for the atoms of its isotopes: protium, deuterium and tritium for hydrogen-1 (no neutrons), hydrogen-2 (1 neutron) and hydrogen-3 (2 neutrons).
Only about one in ten thousand hydrogen atoms on Earth are deuterium. Tritium is not found naturally on Earth and has to be made in a nuclear reactor.
We can use proton and nucleon numbers to describe nuclear radiation.
How alpha particles change proton and neutron number
An alpha particle is the nucleus of a helium atom. It consists of two protons and two neutrons. So its proton number is 2 and its nucleon number is 4. You may see it written as either the symbol for helium, He, or the α symbol.
Let’s see how we can use this information to write a nuclear equation.
Americium-241 is the isotope that’s used in smoke alarms in people’s homes. It has a proton number of 95. This particular isotope has a nucleon number of 241. This means there are a total of 241 protons and neutrons in the nucleus.
An americium nucleus decays by giving off an alpha particle. So two protons and two neutrons leave the americium nucleus. How can we work out what kind of nucleus we’re left with?
We know that the alpha particle has a proton number of 2 and a nucleon number of 4. We know that no protons have disappeard into nothingness or appeared from nowhere. We started off with 95 so we must end up with 95. The alpha particle accounts for 2 of them. So the new nucleus must have 93 protons. But what element has 93 protons?
We use the periodic table to find the element with proton number 93. It’s an element called neptunium, two places lower than americium.
We also know that we haven’t lost or gained any neutrons. The total number of protons and neutrons we started with was 241. So we must end up with 241 protons and neutrons. The alpha particle accounts for 4 of them. So that leaves a total of 237 protons and neutrons making up the neptunium nucleus.
So the isotope we end up with when americium-241 decays is neptunium-237.
With nuclear equations we tend not to worry about electrons
An americium atom has 95 protons in its nucleus. There are also 95 electrons surrounding the nucleus. This makes the atom neutral.
But when the alpha particle was given off the number of protons in the nucleus was reduced to 93. So the new atom has 93 protons and 95 electrons. Doesn’t this mean it has two extra electrons?
The answer is yes it does. But the extra electrons are quickly lost to nearby atoms. These pass them on and so on. Radioactivity is all about the nucleus so we just tend not to worry about what happens to the electrons. They’ll look after themselves.
How beta particles change proton and neutron numbers
Now let’s look at a beta particle. A beta particle is just an electron.
An electron isn’t made up of protons and neutrons so it has a nucleon number of zero.
What about the proton number? A proton has a positive electric charge but an electron has a negative one. So we say that the beta particle has a proton number of minus one.
But where does this electron come from?
Beta particles are electrons from changes in the nucleus
Very bizarrely, it’s possible for a neutron suddenly to turn into a proton.
When it does this it emits an electron. This is what a beta particle is. It’s an electron given off when a neutron turns into a proton.
It’s best not to think of a neutron being ‘made from’ a proton and electron. Just think that the neutron can turn into a proton and when this happens an electron is created at the same time.
So a beta particle comes from the nucleus of an atom, NOT from the electron clouds.
A nuclear equation for beta decay
We can write a balanced nuclear equation for this process.
We start with a nucleon number of one so we must end up with a total of one. We start with a proton number of zero and this is also what we have to end up with.
Carbon has a proton number of 6. In a nucleus of carbon-14 one of the neutrons can turn into a proton and a beta particle is given off. But what kind of nucleus do we end up with?
This time it’s easier to start with the nucleon numbers. The nucleon number of the new nucleus must be 14. The proton number must be 7 because 7 minus 1 is 6.
We use our periodic table to look up the element with atomic number 7 and find that it’s nitrogen, one place higher than carbon. So carbon-14 emits a beta particle and decays into nitrogen-14.
Notice that with beta decay the nucleon number doesn’t change. It remains 14. This is because a neutron just turns into a proton. So the proton number increases by 1 because there’s this new proton where once there was a neutron.
Protons and neutrons are made from quarks
It turns out that protons and neutrons aren't as small as you can go. They are made up of more fundamental particles called quarks which were first studied experimentally by a process called deep inelastic scattering.
Beta decay can also be explained in terms of changes to quarks.
Gamma radiation doesn't change proton or nucleon number
Finally, gamma radiation is a kind of very high energy light. It has a proton number of zero and a nucleon number of zero.
Gamma radiation doesn’t change the number of protons or neutrons in a nucleus but it does change the way they are arranged. We’ll look at this in more detail in lesson 12.