What is an alpha particle?

a helium nucleus. Two protons and two neutrons.

What happens to the proton number of a nucleus in β⁻ decay?

It increases by 1. A neutron becomes a proton.

What is given out along with the electron in β⁻ decay?

an antineutrino. It carries away some of the energy.

A nucleus emits a gamma photon. What happens to its nucleon number and proton number?

Neither changes. A photon has no charge and contains no nucleons.

the antiparticle of the electron, with the same mass and the opposite charge. It is emitted in β⁺ decay.

Alpha, beta and gamma decay · Cambridge A Level Physics

In plain words

Some nuclei are unstable, and they settle down by throwing something out. There are three main things they throw: a chunk of two protons and two neutrons, an electron (or its mirror-image twin) made on the spot, or a burst of pure energy. Each changes the nucleus in its own way, and the sums always balance.

6 things to know

Alpha (α): a helium nucleus, two protons and two neutrons, with charge +2e and a mass of about 4 u. The nucleus loses 4 from A and 2 from Z.
Beta-minus (β⁻): a fast electron, charge −e. A neutron turns into a proton, so Z goes up by 1 and A is unchanged. An antineutrino is given out as well.
Beta-plus (β⁺): a positron, which is the electron's antiparticle, with the same mass and charge +e. A proton turns into a neutron, so Z goes down by 1 and A is unchanged. A neutrino is given out as well.
Gamma (γ): a high-energy photon, with no charge and no mass. A and Z do not change.
In every nuclear equation, nucleon number and charge are both conserved.
The alpha particles from one kind of nucleus all have the same energy. Beta particles come out with a whole range of energies, because the neutrino or antineutrino takes a different share each time.

Worked example

Uranium-238 (Z = 92) decays by alpha emission. Find the nucleon number and proton number of the nucleus produced.

An alpha particle has nucleon number 4 and proton number 2.
Nucleon number: 238 − 4 = 234. Proton number: 92 − 2 = 90.
The product is thorium-234.

Tips and tricks

Balance the top numbers and the bottom numbers separately on the two sides of the equation.
In equations an electron is written with A = 0 and Z = −1, and a positron with A = 0 and Z = +1.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Alpha, beta and gamma decay: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is an alpha particle?
- an electron
- a photon
- a helium nucleus (the answer)
- a neutron
Two protons and two neutrons.
What happens to the proton number of a nucleus in β⁻ decay?
- It decreases by 1.
- It increases by 1. (the answer)
- It decreases by 2.
- It stays the same.
A neutron becomes a proton.
What is given out along with the electron in β⁻ decay?
- a neutrino
- an antineutrino (the answer)
- a positron
- a proton
It carries away some of the energy.
A nucleus emits a gamma photon. What happens to its nucleon number and proton number?
- Neither changes. (the answer)
- Both decrease.
- The nucleon number falls by 4.
- The proton number rises by 1.
A photon has no charge and contains no nucleons.
What is a positron?
- the antiparticle of the electron, with the same mass and the opposite charge (the answer)
- a proton with no charge
- a heavy electron
- a photon with a positive charge
It is emitted in β⁺ decay.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 6 marks. Write your answers on paper, then check them.

Alpha, beta and gamma decay

Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org

Name ______________________________ Date ______________

Carbon-14 (Z = 6) decays by β⁻ emission. State the nucleon number and proton number of the nucleus formed.[2]
Show answerHide answer
Nucleon number 14, proton number 7 (nitrogen-14).
I got this one
Sodium-22 (Z = 11) decays by β⁺ emission. State the nucleon number and proton number of the nucleus formed.[2]
Show answerHide answer
Nucleon number 22, proton number 10 (neon-22).
I got this one
Explain why beta particles from one kind of nucleus have a range of energies, while alpha particles have just one.[2]
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In beta decay the energy released is shared between the beta particle and a neutrino or antineutrino, in proportions that vary. In alpha decay only the alpha particle is emitted, so it takes the same energy each time.
I got this one

Answers: Alpha, beta and gamma decay

1. Nucleon number 14, proton number 7 (nitrogen-14).
2. Nucleon number 22, proton number 10 (neon-22).
3. In beta decay the energy released is shared between the beta particle and a neutrino or antineutrino, in proportions that vary. In alpha decay only the alpha particle is emitted, so it takes the same energy each time.