GCSE · Physics · Edexcel · Spec 1PH0

Beta-plus decay process

Fire a particle of antimatter out of a nucleus and the atom turns into a different element. That sounds like science fiction, and it's a real kind of radioactive decay.

Magnesium-23 decays: count the particles

Work through the four stages. At stage 2, make your own count before you open the box.

Your turn

Write the beta-plus equation for magnesium-23

Magnesium-23 (mass number 23, atomic number 12) decays by beta-plus into sodium. Build the nuclear equation, choosing each missing step.

Parent on the left, daughter and positron on the right: ²³₁₂Mg → daughter + positron. Now we need numbers for the positron and the daughter.
missing step

Which line is step 2?

The positron has mass number 0 and atomic number +1.The positron has mass number 0 and atomic number −1.The positron has mass number 1 and atomic number +1.

Two decays, one mirror

Beta-plus decayvsBeta-minus decay

Read across each row: what one decay does, the other does the opposite.

Focus

What changes in the nucleus

Beta-plus decay

A proton changes into a neutron

Beta-minus decay

A neutron changes into a proton

The insight

The same swap in opposite directions. Everything else follows from which way the swap goes.

Particle emitted

Beta-plus decay

A positron

Beta-minus decay

A fast-moving electron (a beta particle), created in the nucleus

Change in atomic number

Beta-plus decay

Goes down by one

Beta-minus decay

Goes up by one

Change in nuclear charge

Beta-plus decay

−1

Beta-minus decay

Most likely in a nucleus with…

Beta-plus decay

Too many protons (magnesium-23 becomes sodium)

Beta-minus decay

Too many neutrons (carbon-14 becomes nitrogen)

Which idea is yours?

What really happens to the nucleus?

Magnesium-23 has just decayed by beta-plus into sodium-23, and a positron has flown out of the nucleus.

Which of these is closest to what you think happened?

The positron was sitting in the nucleus all along, and it has just been let go.A particle left, so the nucleus has lost mass and the mass number must drop.Only a particle was emitted, so it's still magnesium.A proton decayed. A neutron was produced and stayed, a positron was produced and emitted, the nucleon total is the same, and the element has changed.

How sure are you?

I'm sureI think soI'm guessing

The positron was sitting in the nucleus all along, and it has just been let go.

This is a common misconception

It's an easy picture, but the positron isn't stored. It is produced at the moment a proton decays: a neutron is produced (and stays) and a positron is produced (and is emitted).

A particle left, so the nucleus has lost mass and the mass number must drop.

This is a common misconception

Count what changed. One proton became a neutron, so the nucleon total is the same. The positron has mass number 0, so its leaving takes nothing off the mass number.

Only a particle was emitted, so it's still magnesium.

This is a common misconception

The element is decided by the number of protons, and that has dropped from 12 to 11. A different proton number means a different element, so this is now sodium.

A proton decayed. A neutron was produced and stayed, a positron was produced and emitted, the nucleon total is the same, and the element has changed.

This matches the evidence

That's the whole idea. The nucleon total is unchanged because one nucleon simply changed type, and the proton number fell by one, which is why the element changed.

The positron is made, not stored. The nucleon total doesn't change, and the proton number does.

One more thing after the positron

Commit to an answer before you reveal it.

After the positron has gone, the daughter nucleus is often still 'excited': it has spare energy. It loses that energy by emitting something. What, and what does it do to the mass number and atomic number?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

A nucleus with too many or too few neutrons for its number of protons is unstable, so over time it can decay.
In beta-plus decay one proton decays: a neutron is produced and stays, and a positron is produced and emitted at high speed.
Have a goImagine counting a nucleus before and after. It starts with 8 protons and 7 neutrons, then one proton makes the swap. Give the new tally.
7 protons and 8 neutrons.
One proton changes into a neutron, so protons go down by one and neutrons go up by one; nothing else changes.
A positron is a particle of antimatter with the same mass as an electron but a positive charge: mass number 0, atomic number +1.
Only one proton changes, so the total number of nucleons stays the same, with one fewer proton and one more neutron.
Have a goZara bets that the nucleon total must drop, because a positron just zoomed out. The nucleus had 15 nucleons before. Who wins the bet, and what is the total afterwards?
Zara loses: still 15 nucleons.
A proton became a neutron, so one nucleon changed type, but none was lost; the positron has mass number 0 and is not one of the nucleons.
One proton fewer means a different element, and the nuclear charge changes by −1.
In the equation the mass number stays the same and the atomic number drops by one; top and bottom rows sum equally on both sides.
Have a goOn the left of an equation sits a made-up parent: 31 on top, 16 underneath. Fill in the daughter's top and bottom numbers across the arrow.
31 on top, 15 underneath.
The top number stays the same and the bottom number falls by one, because one proton has changed into a neutron.
Beta-plus is most likely in a nucleus with too many protons: magnesium-23 becomes sodium, atomic number 12 to 11.
Beta-minus is the reverse swap: a neutron changes into a proton and a fast electron, so the atomic number goes up by one.
Afterwards the daughter nucleus is often excited and loses its spare energy as gamma radiation, which has no mass and no charge.

The big picture

In beta-plus decay one proton in an unstable nucleus decays into a neutron, which stays, and a positron, which is emitted. The nucleon total is unchanged, but there is one fewer proton, so the atomic number falls by one and the element changes. Beta-minus decay is the reverse swap, and gamma radiation often follows.

Key points

1In beta-plus decay one proton changes into a neutron and a positron is emitted.

2The mass number stays the same, the atomic number falls by one, and the nuclear charge changes by −1, so the element changes.

3A positron is antimatter: the same mass as an electron, a positive charge, mass number 0 and atomic number +1.

4Beta-minus is the opposite swap: a neutron changes into a proton and an electron, and the atomic number rises by one.

5Gamma radiation, with no mass and no charge, often follows and changes neither number.

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Worked example

Problem

A made-up nucleus has mass number 30 and atomic number 15. It has too many protons compared with its neutrons and decays by beta-plus. Find the mass number and atomic number of the daughter.

⚠ Watch out

Thinking the mass number drops because a particle flew out. It doesn't: one proton just changed into a neutron, so the nucleon total is the same, and the positron has mass number 0. It's the atomic number that falls by one.

🧠

Memory hook

Beta-PLUS: a PROTON turns into a neutron and a POSITIVE positron flies out. Top number steady, bottom number down one.

✓

Check yourself

Cover the page and explain beta-plus decay to an imaginary friend: the proton, the particle emitted, the top and bottom numbers. Then say what flips in beta-minus.

Flashcards

(12)

When is a nucleus unstable?

When it has too many or too few neutrons compared with its protons. Over time it can decay to become more stable.

What happens inside the nucleus in beta-plus decay?

One proton decays: a neutron is produced and stays in the nucleus, and a positron is produced and emitted at high speed.

What is a positron?

The antielectron: a particle of antimatter with the same mass as an electron but a positive charge. Mass number 0, atomic number +1.

Why does the mass number not change in beta-plus decay?

Only one proton changes, into a neutron, so the total number of nucleons stays the same. The positron has mass number 0.

How do the numbers of protons and neutrons change in beta-plus decay?

One fewer proton and one more neutron.

Why is the daughter of a beta-plus decay a different element?

Because its proton number has changed, so it belongs to a different element from the parent.

How does the charge of the nucleus change in beta-plus decay?

By −1: the nuclear charge goes down, since a proton has gone.

How do the top and bottom numbers behave in a beta-plus equation?

The top (mass) number stays the same and the bottom (atomic) number drops by one. Each row has the same sum on both sides.

Which nucleus is beta-plus decay most likely in? Give an example.

One with too many protons compared with its neutrons. Magnesium-23 becomes sodium, atomic number 12 to 11.

What changes in beta-minus decay?

A neutron changes into a proton and a fast-moving electron (a beta particle) created in the nucleus. Atomic number +1, nuclear charge +1.

Which nucleus is beta-minus decay most likely in? Give an example.

One with too many neutrons. Carbon-14 decays to nitrogen.

What often follows once a positron has been emitted?

The daughter nucleus is often excited, and loses its spare energy by emitting gamma radiation, which has no mass and no charge.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Beta-plus decay process

Four particles, four sets of properties

Write the beta-plus equation for magnesium-23

What really happens to the nucleus?

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