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O LevelPhysicsCambridge (CIE)Revision Notes5. Nuclear Physics5.2 RadioactivityRadioactive Decay

Radioactive Decay (Cambridge (CIE) O Level Physics): Revision Note

Ann Howell

Last updated

Radioactive Decay

  • Some atomic nuclei are unstable

  • This is because of an imbalance in the forces within the nucleus

    • Forces exist between the particles in the nucleus

    • This is commonly due to the nucleus having too many protons or neutrons

  • Carbon-14 is an isotope of carbon which is unstable

    • It has two extra neutrons compared to stable carbon-12

Isotopes of Carbon-12 and Carbon-14

Unstable nucleus, downloadable IGCSE & GCSE Physics revision notes

Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons

 

  • Some isotopes are unstable because of their large size or because they have too many or too few neutrons

  • Unstable nuclei can emit radiation in the form of α-particles to become more stable

An Unstable Nucleus Emitting an Alpha Particle

Radioactive decay, downloadable IGCSE & GCSE Physics revision notes

Unstable nuclei decay by emitting α-particles

  • As the α-particle moves away from the nucleus, it takes some energy with it

    • This reduces the overall energy of the nucleus

    • It makes the nucleus more stable

  • The process of emitting α-particles is called radioactive decay

  • Radioactive decay is a random process

    • There is an equal probability of any nucleus decaying

    • It cannot be known which particular nucleus will decay next

    • It cannot be known at what time a particular nucleus will decay

    • The rate of decay is unaffected by the surrounding conditions

    • It is only possible to estimate the probability of a nucleus decaying in a given time-period

  • Therefore, the emission of radiation is:

    • Spontaneous 

    • Random in direction

Worked Example

Which of the following statements is not true?

A. Isotopes can be unstable because they have too many or too few neutrons

B. The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay

C. Scientists can predict when a nucleus will decay

D. Radiation refers to the particles or waves emitted from a decaying nucleus

Answer: C

  • Answer A is true. The number of neutrons in a nucleus determines the stability

  • Answer B is true. This is a suitable description of radioactive decay

  • Answer D is true. Radiation is about emissions. It is different to radioactive particles

  • Answer C is not true

    • Radioactive decay is a random process

    • It is not possible to predict precisely when a particular nucleus will decay

Examiner Tips and Tricks

The terms unstable, random and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!

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Decay Equations

  • Radioactive decay events can be shown using a decay equation

  • A decay equation is similar to a chemical reaction equation

    • The particles present before the decay are shown before the arrow

    • The particles produced in the decay are shown after the arrow

  • During decay equations, the sum of the mass and atomic numbers before the reaction must be the same as the sum of the mass and atomic numbers after the reaction

Alpha Decay Equations

  • When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes

    • The mass number decreases by 4

    • The atomic number decreases by 2

The Alpha Decay Equation

Alpha decay equation showing the change in mass and atomic numbers

  • The following decay equation shows Polonium-212 undergoing alpha decay

    • It forms Lead-208 and an alpha particle

    • An alpha particle can also be written as a helium nucleus (Symbol He)

e.g.  Po presubscript 84 presuperscript 212 space rightwards arrow space Pb presubscript 82 presuperscript 208 space plus space straight alpha presubscript 2 presuperscript 4

Beta Decay Equation

  • During beta decay, a neutron changes into a proton and an electron

    • The electron is emitted and the proton remains in the nuclei

The Beta Decay Equation

Beta decay equation showing the change in atomic number

Gamma Decay

  • The gamma ray that is emitted has a lot of energy, but no mass or charge

  • Here is an example of Uranium-238 undergoing gamma decay

    • Notice that the mass number and atomic number of the unstable nuclei remain the same during the decay

The Gamma Decay Equation

Gamma decay equation showing no change in mass or atomic number

Worked Example

A nucleus with 84 protons and 126 neutrons undergoes alpha decay. It forms lead, which has the element symbol Pb.

A. Pb presubscript 82 presuperscript 206

B. Pb presubscript 82 presuperscript 208

C. Pb presubscript 84 presuperscript 210

D. Pb presubscript 86 presuperscript 214

Which of the isotopes of lead listed is the correct one formed during the decay?

Answer: A

Step 1: Calculate the mass number of the original nucleus

  • The mass number is equal to the number of protons plus the number of neutrons

  • The original nucleus has 84 protons and 126 neutrons

84 + 126 = 210

  • The mass number of the original nucleus is 210

Step 2: Calculate the new atomic number

  • The alpha particle emitted is made of two protons and two neutrons

  • Protons have an atomic number of 1, and neutrons have an atomic number of 0

  • Removing two protons and two neutrons will reduce the atomic number by 2

84 – 2 = 82

  • The new nucleus has an atomic number of 82

Step 3: Calculate the new mass number

  • Protons and neutrons both have a mass number of 1

  • Removing two protons and two neutrons will reduce the mass number by 4

210 – 4 = 206

  • The new nucleus has a mass number of 206

Worked Example

A nucleus with 11 protons and 13 neutrons undergoes beta decay. It forms magnesium, which has the element symbol Mg.

Worked example beta decay, downloadable IGCSE & GCSE Physics revision notes

Which is the correct isotope of magnesium formed during the decay?

Answer: D

Step 1: Calculate the mass number of the original nucleus

  • The mass number is equal to the number of protons plus the number of neutrons

  • The original nucleus has 11 protons and 13 neutrons

11 + 13 = 24

  • The mass number of the original nucleus is 24

Step 2: Calculate the new atomic number

  • During beta decay a neutron changes into a proton and an electron

  • The electron is emitted as a beta particle

  • The neutron has an atomic number of 0 and the proton has an atomic number of 1

  • So the atomic number increases by 1

11 + 1 = 12

  • The new nucleus has an atomic number of 12

Step 3: Calculate the new mass number

  • Protons and neutrons both have a mass number of 1

  • Changing a neutron to a proton will not affect the mass number

  • The new nucleus has a mass number of 24 (the same as before)

Examiner Tips and Tricks

You are not expected to know the names of the elements produced during radioactive decays, but you do need to be able to calculate the mass and atomic numbers by making sure they are balanced on either side of the reaction.

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    Ann Howell

    Author: Ann Howell

    Expertise: Physics Content Creator

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.