Chapter 1 of 8

All 8 chapters
  1. 1Unstable nuclei, readunderstood
  2. 2Alpha, beta and gamma, readunderstood
  3. 3Half-life, readunderstood
  4. 4Background radiation, readunderstood
  5. 5Radiation and safety, readunderstood
  6. 6Uses of radiation, readunderstood
  7. 7Fission and fusion, readunderstood
  8. 8Weighing up nuclear power, readunderstood

S4.11.1 Chapter 1

Unstable nuclei

Carbon 12 stays the same for ever, but carbon 14 does not. What is different about them?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain why some isotopes are unstable and decay
  • Explain that decay is random and cannot be predicted for one atom

Most atoms stay as they are for ever. A few do not. They change by themselves, and this book is about those few. They matter because of what they give out, and because we can use it and must respect it.

Remember the atom. At its centre is a tiny nucleus of protons and neutrons. Protons have a positive charge and neutrons have none. The electrons are outside it. The mass number is the number of protons plus neutrons.

Protons push each other apart, because like charges repel. Something stronger must hold the nucleus together. It is called the strong nuclear force. It pulls protons and neutrons together, but only over a very short distance.

Neutrons help. They add to the pull and add no push. A nucleus holds together when it has the right mix of protons and neutrons. We call that nucleus stable: it stays as it is for ever.

Two nuclei drawn as touching circles. Carbon 12 has 6 hatched circles marked with a plus sign, the protons, and 6 plain circles, the neutrons. Carbon 14 has 6 protons and 8 neutrons. A key names proton and neutron.carbon 12protonneutroncarbon 14protonneutron
Both have 6 protons, so both are carbon. Carbon 14 has two more neutrons, and its nucleus is unstable.

A nucleus is unstable when the mix is wrong. It may have too many neutrons, or too few, for its protons. Or it may be very large, like uranium. An unstable nucleus can change.

We call an unstable nucleus radioactive. When it changes, we say it decays. It gives out radiation, which is energy or fast particles from the nucleus. The nucleus turns into a new, more stable one, sometimes of another element.

Three boxes joined by arrows going down. The first box says an unstable nucleus. The second says it decays and gives out radiation. The third says a new nucleus, more stable.an unstable nucleusit decays and gives outradiationa new nucleus, more stable
One decay. The nucleus changes once. It does not wear out little by little.
Two small tables of real nuclei, each with the columns Nucleus, Protons and Neutrons. Under the label stable: He 4 has 2 and 2, Ne 20 has 10 and 10, Pb 206 has 82 and 124. Under the label unstable: C 14 has 6 and 8, K 40 has 19 and 21, U 235 has 92 and 143.stableNucleusProtonsNeutronsHe 422Ne 201010Pb 20682124unstableNucleusProtonsNeutronsC 1468K 401921U 23592143
Some real nuclei, with the symbol and mass number. He is helium, Ne neon, Pb lead, C carbon, K potassium and U uranium. No rule on neutrons alone tells the two lists apart.

Worked example

A nucleus of carbon 14 has 6 protons and mass number 14. How many neutrons does it have?

  1. Mass number = protons + neutrons.
  2. neutrons = mass number − protons.
  3. neutrons = 14 − 6 = 8.

Answer: 8 neutrons

Try it

Draw a nucleus

You need: paper, a pencil.

  1. Draw 5 small circles and put a plus sign in each. These are protons.
  2. Draw 6 plain circles beside them. These are neutrons.
  3. Count all 11 circles. This number is the mass number.
  4. Write the name of the element: 5 protons make boron. Then write boron 11.
  5. Now draw the nucleus of boron 10 in the same way. How many neutrons does it have?

What you will see: You can draw any mix of circles you like. Nature does not. Only some mixes hold together, and the rest are unstable. Boron 11 and boron 10 are both stable, but a boron nucleus with many more neutrons is not.

If you cannot try it, this is what happens.
A nucleus of boron 11 drawn as touching circles. Five hatched circles marked with a plus sign are protons and six plain circles are neutrons. A key names proton and neutron.protonneutron

Now a puzzle. Think of a large sample of one unstable kind of nucleus. All the nuclei are identical. Yet they do not all decay together. One decays now, and its neighbour waits for years. Why?

The answer is that decay is random. Nothing makes a nucleus decay at a certain time. Each nucleus has a certain chance of decaying in each second. This chance is the same for every nucleus of that kind. We cannot say when it will happen to one of them.

Random does not mean that nothing can be said. A coin is random when tossed once. Yet 1 000 tosses give about 500 heads. In the same way, we cannot predict one nucleus, but a huge number of them behave in a steady, predictable way. The next chapters use this.

A bar chart of five counts from one source, each taken over 10 s. The scale is counts in 10 s, from 0 to 40 in steps of 10. The bars are labelled 1, 2, 3, 4 and 5 and read 31, 27, 35, 29 and 33. They are not all the same.01020304012345counts in 10 s
The same source, counted five times for 10 s. Bars 1 to 5 are the five counts. The counts differ because each decay is random.

Worked example

A counter gave these counts in five periods of 10 s from one source: 31, 27, 35, 29, 33. What was the mean count in 10 s?

  1. mean = total ÷ number of readings.
  2. total = 31 + 27 + 35 + 29 + 33 = 155.
  3. mean = 155 ÷ 5 = 31 counts.

Answer: 31 counts

One more fact. Heating a sample does not change how likely its nuclei are to decay. Nor does cooling it, squeezing it, or joining its atoms into a compound. The cause is inside the nucleus, and ordinary changes outside it do not reach there. A very small effect has been measured for a few rare kinds of decay. It does not matter here.

A wrong idea many people have

A substance is radioactive because it has been heated or charged with energy. It will calm down if it is cooled.

Why it feels right: A hot stove glows and gives out heat, and it stops when it cools. It seems radiation works the same way.

The test that breaks it: A radioactive sample kept in a very cold place still decays at the same rate. So does the same sample heated. Neither changes the nucleus.

What is true: Radiation comes from the nucleus. It happens because the nucleus is unstable. Heat, cold and chemistry do not switch it on or off.

A wrong idea many people have

Each unstable nucleus has its own timer, so it decays at a fixed time that we could work out.

Why it feels right: A big sample decays in a steady way, so each nucleus seems to follow a plan.

The test that breaks it: Scientists count the decays from one source several times, for the same length of time. The counts differ each time, as in the bar chart. A plan would give the same count.

What is true: Each decay is random. We can give the chance, and for a huge number of nuclei the total is steady. We cannot say which nucleus goes next.

Key words

nucleus
the tiny centre of an atom, made of protons and neutrons.
stable
a nucleus that stays as it is for ever.
unstable
a nucleus that may change by itself.
radioactive
having unstable nuclei that decay and give out radiation.
decay
the change of an unstable nucleus into a more stable one.
radiation
energy or fast particles given out by an unstable nucleus.
random
not possible to predict for one nucleus, only as a chance.

Back to your guess

The answer is A: Carbon 14 has two more neutrons in its nucleus.

Both are carbon, so both have 6 protons and 6 electrons. Carbon 14 has 8 neutrons and carbon 12 has 6. That mix makes the nucleus of carbon 14 unstable.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which statement about an unstable nucleus is true?

    Tap your answer.

    1. Not this one. That describes a stable nucleus. An unstable nucleus may change. Try another.

    2. Not this one. Neutrons are not the problem. Carbon 14 is unstable and has 8 of them. It is the mix of protons and neutrons that matters. Try another.

    3. Yes, that is right. An unstable nucleus can decay. When it does, it gives out radiation and becomes a new, more stable nucleus.

    4. Not this one. Electrons are outside the nucleus. The instability is inside the nucleus, and electrons do not cause it. Go to S4.6.1. Try another.

    See the answer

    It may change by giving out radiation.

    An unstable nucleus can decay. When it does, it gives out radiation and becomes a new, more stable nucleus.

    • If you chose A: That describes a stable nucleus. An unstable nucleus may change.
    • If you chose B: Neutrons are not the problem. Carbon 14 is unstable and has 8 of them. It is the mix of protons and neutrons that matters.
    • If you chose D: Electrons are outside the nucleus. The instability is inside the nucleus, and electrons do not cause it. Go to S4.6.1.

  2. Carbon 12 and carbon 14 both have 6 protons. Why is only carbon 14 unstable?

    Tap your answer.

    1. Yes, that is right. Carbon 14 has 8 neutrons for 6 protons. This mix is not stable, so the nucleus can decay. Carbon 12 has 6 and 6, which is stable.

    2. Not this one. Both have 6 electrons, and the electrons are not in the nucleus. The cause is in the nucleus. Go to S4.6.3. Try another.

    3. Not this one. Both have 6 protons. That is why both are carbon. Only the neutrons differ. Go to S4.6.3. Try another.

    4. Not this one. Many heavy nuclei, such as lead 206, are stable. A large mass number alone does not make a nucleus unstable. Try another.

    See the answer

    Its nucleus has too many neutrons for its protons.

    Carbon 14 has 8 neutrons for 6 protons. This mix is not stable, so the nucleus can decay. Carbon 12 has 6 and 6, which is stable.

    • If you chose B: Both have 6 electrons, and the electrons are not in the nucleus. The cause is in the nucleus. Go to S4.6.3.
    • If you chose C: Both have 6 protons. That is why both are carbon. Only the neutrons differ. Go to S4.6.3.
    • If you chose D: Many heavy nuclei, such as lead 206, are stable. A large mass number alone does not make a nucleus unstable.

  3. A scientist wants a radioactive sample to decay faster. Which of these changes how fast its nuclei decay?

    Tap your answer.

    1. Not this one. Heat acts on the atoms from outside. It does not reach the nucleus, so it does not change the decay. Try another.

    2. Not this one. Pressure does not reach the nucleus either. The decay is the same. Try another.

    3. Not this one. A chemical change moves electrons. It leaves the nucleus alone, so the decay is the same. Go to S3.6.1. Try another.

    4. Yes, that is right. Decay happens inside the nucleus. Heat, pressure and chemical changes happen outside it, so they do not change how likely a nucleus is to decay.

    See the answer

    None of these. They do not change it.

    Decay happens inside the nucleus. Heat, pressure and chemical changes happen outside it, so they do not change how likely a nucleus is to decay.

    • If you chose A: Heat acts on the atoms from outside. It does not reach the nucleus, so it does not change the decay.
    • If you chose B: Pressure does not reach the nucleus either. The decay is the same.
    • If you chose C: A chemical change moves electrons. It leaves the nucleus alone, so the decay is the same. Go to S3.6.1.

  4. A sample holds a million identical unstable nuclei. Which statement is true?

    Tap your answer.

    1. Not this one. There is no fixed order. Each nucleus has only a chance of decaying, and the chance is the same for all. Try another.

    2. Yes, that is right. Decay is random. We can give the chance for each nucleus, and the total for a huge number is steady. We cannot say which nucleus goes next.

    3. Not this one. They do not. The decays are spread out over time, at random. Try another.

    4. Not this one. Position does not matter. Every nucleus of the kind has the same chance at any moment. Try another.

    See the answer

    No one can tell which nucleus will decay next.

    Decay is random. We can give the chance for each nucleus, and the total for a huge number is steady. We cannot say which nucleus goes next.

    • If you chose A: There is no fixed order. Each nucleus has only a chance of decaying, and the chance is the same for all.
    • If you chose C: They do not. The decays are spread out over time, at random.
    • If you chose D: Position does not matter. Every nucleus of the kind has the same chance at any moment.

  5. A counter reads 44, 38, 47, 41 and 45 counts in five periods of 10 s from one source. Bars 1 to 5 in the picture are the five counts. What do the different readings show?

    A bar chart of five counts from another source, each taken over 10 s. The scale is counts in 10 s, from 0 to 50 in steps of 10. The bars are labelled 1, 2, 3, 4 and 5 and read 44, 38, 47, 41 and 45.0102030405012345counts in 10 s
    Tap your answer.

    1. Not this one. The counts go up and down, not steadily up. A source does not get stronger as it decays. Try another.

    2. Not this one. A working counter gives different readings too, because decay is random. A broken counter would be far off, or stuck. Try another.

    3. Not this one. That may be true, but it does not explain readings that differ. Even a pure source shows this. Try another.

    4. Yes, that is right. The number of decays in a short time varies by chance. The readings stay close to a steady mean but are not equal.

    See the answer

    Decay is random, so the count changes from one period to the next.

    The number of decays in a short time varies by chance. The readings stay close to a steady mean but are not equal.

    • If you chose A: The counts go up and down, not steadily up. A source does not get stronger as it decays.
    • If you chose B: A working counter gives different readings too, because decay is random. A broken counter would be far off, or stuck.
    • If you chose C: That may be true, but it does not explain readings that differ. Even a pure source shows this.

  6. A nucleus of uranium 238 has 92 protons. How many neutrons does it have?

    Tap your answer.

    1. Yes, that is right. neutrons = mass number − protons = 238 − 92 = 146.

    2. Not this one. That is the mass number. It counts the protons too, so take them away. Go to S4.6.1. Try another.

    3. Not this one. You added the protons. The mass number already includes them, so subtract. Go to S4.6.1. Try another.

    4. Not this one. That is the number of protons. The neutrons are the rest of the mass number. Try another.

    See the answer

    146

    neutrons = mass number − protons = 238 − 92 = 146.

    • If you chose B: That is the mass number. It counts the protons too, so take them away. Go to S4.6.1.
    • If you chose C: You added the protons. The mass number already includes them, so subtract. Go to S4.6.1.
    • If you chose D: That is the number of protons. The neutrons are the rest of the mass number.

  7. How many neutrons does the nucleus in the picture have?

    A nucleus of lithium drawn as touching circles. Hatched circles marked with a plus sign are protons and plain circles are neutrons. A key names proton and neutron.protonneutron
    See the answer

    4 neutrons

    The plain circles are neutrons. Count the circles that have no plus sign: there are 4.

  8. A nucleus of cobalt 60 has 27 protons. How many neutrons does it have?

    See the answer

    33 neutrons

    neutrons = mass number − protons = 60 − 27 = 33.

  9. A counter gave 52, 48, 55 and 49 counts in four periods of 10 s from one source. What was the mean count in 10 s?

    See the answer

    51 counts

    total = 52 + 48 + 55 + 49 = 204. mean = 204 ÷ 4 = 51.

Keep it

Copy this into your exercise book.

A nucleus is stable or unstable. An unstable one decays and gives out radiation. The wrong mix of neutrons and protons, or a very large nucleus, can cause it. Decay is random. Heat does not change it.

Draw: Carbon 12 and carbon 14 side by side as circles: 6 protons and 6 neutrons, then 6 protons and 8 neutrons.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (788 KB). It opens on any phone.