GCSE · Physics · Edexcel · Spec 1PH0

Beta-minus decay process

An electron shoots out of a nucleus with no electrons in it, and the atom quietly turns into a different element. How is that even possible?

Follow one neutron inside a carbon-14 nucleus

Stage 1 of 4: Unstable nucleus. Protons in the nucleus: 6. paused

Unstable nucleus · 1/4Scrub left and right. The proton count only changes at one stage. Which one?

Carbon-14: 6 protons, 8 neutrons, and no electrons inside.

Drag the slider or press play. Watch the proton count tick up and see where the electron comes from.

Physics · Radioactive decay

Where does the beta particle come from?

A nucleus decays and a beta-minus particle shoots out. Your gut says it came from…

Which is closest to what you think right now?

It's one of the atom's orbiting electrons that got knocked off the outside.It was already sitting inside the nucleus, waiting to escape.A proton changes into a neutron and an electron.A neutron in the nucleus changes into a proton and an electron, and the electron is thrown out.

How sure are you?

I'm sureI think soI'm guessing

It's one of the atom's orbiting electrons that got knocked off the outside.

This is a common misconception

Tempting, because orbiting electrons are the electrons you already know. But they live outside the nucleus, and the decay happens inside it. Knocking off an outer electron wouldn't touch the nucleus, so the element couldn't change. A beta-minus particle is an electron made in the nucleus. It has charge −1 and negligible mass, just like any electron.

It was already sitting inside the nucleus, waiting to escape.

This is a common misconception

Closer, because it does leave the nucleus. But the nucleus contains protons and neutrons and no electrons. Nothing was waiting. The electron is created at the moment a neutron changes into a proton, then ejected.

A proton changes into a neutron and an electron.

This is a common misconception

Right idea (a nucleon changes and an electron leaves), wrong direction. In beta-minus decay it's a neutron that changes into a proton. That's why the proton count goes up, which you'll count in a second.

A neutron in the nucleus changes into a proton and an electron, and the electron is thrown out.

This matches the evidence

Yes. The proton stays in the nucleus. The electron leaves at high speed as the beta-minus particle, with charge −1 and negligible mass.

The electron comes from the one place with no electrons in it. It's made when a neutron changes into a proton, and then it's ejected.

Exam line: The beta-minus electron is made in the nucleus by a neutron changing into a proton. It is not a shell electron.

Count the nucleus before and after

Before: carbon-14vsAfter: nitrogen-14

Read across each row. Which numbers move, and which one stays put?

Focus

Protons

Before: carbon-14

After: nitrogen-14

The insight

One neutron became a proton, so there is one more proton.

Neutrons

Before: carbon-14

After: nitrogen-14

Protons + neutrons (mass number)

Before: carbon-14

After: nitrogen-14

Atomic number

Before: carbon-14

After: nitrogen-14

Element

Before: carbon-14

Carbon

After: nitrogen-14

Nitrogen

Nuclear equation

Complete the equation yourself

Carbon-14 decays by beta-minus decay into an atom of nitrogen. Finish the nuclear equation: ¹⁴₆C → ? + ⁰₋₁e

Start with the parent nucleus on the left and the beta-minus particle on the right: ¹⁴₆C → ? + ⁰₋₁e. The daughter nucleus is the unknown.Top number = mass number. Bottom number = atomic number.
The electron has mass number 0 and atomic number −1, so it adds nothing on top and takes 1 off the bottom.Charge −1 is the bottom number. Negligible mass is the top number 0.
missing step

Which line is step 3?

Balance the top numbers: 14 = 14 + 0, so the daughter has mass number 14.Balance the top numbers: 14 = 13 + 0, so the daughter has mass number 13.Balance the top numbers: 14 = 15 + 0, so the daughter has mass number 15.

Exam line: Top numbers balance. Bottom numbers balance. The electron is ⁰₋₁e.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

An unstable nucleus can decay by emitting a beta-minus particle, written β⁻.
A beta-minus particle is a fast-moving electron, with charge −1 and negligible mass.
It is not one of the atom's orbiting electrons, because the nucleus itself is where the decay happens.
Inside the nucleus, a neutron changes into a proton and an electron.
Have a goYour mate says: "The nucleus is stuffed with electrons, obviously, and one just pops out." Is that what happens?
No. The nucleus has no electrons stored in it. The electron is made when a neutron changes.
Nothing is waiting in the nucleus: the beta-minus electron is created at the moment a neutron becomes a proton.
The proton stays in the nucleus, and the electron is ejected at high speed.
So the nucleus has one more proton, and its atomic number goes up by 1.
Have a goA nucleus with 15 protons decays by beta-minus decay. How many protons does the new nucleus have?
16.
A neutron became a proton, so the proton count goes up by 1. It doesn't go down because of the electron's minus sign.
It lost a neutron and gained a proton, so the mass number stays the same.
Have a goThat same nucleus had mass number 32 before it decayed. What is its mass number afterwards?
32. It doesn't change.
One neutron became one proton, so the total of protons and neutrons is the same even though the make-up has changed.
A new atomic number means a new element: carbon-14 (6 protons) becomes nitrogen-14 (7 protons).
In a nuclear equation, write the electron as ⁰₋₁e and balance the top numbers and bottom numbers separately.

The big picture

In beta-minus decay a neutron in an unstable nucleus changes into a proton and an electron. The electron is ejected as the beta-minus particle. The atomic number rises by 1, the mass number stays the same, and the atom becomes a new element.

Key points

1Beta-minus decay: a neutron in an unstable nucleus changes into a proton and an electron.

2The beta-minus particle is that electron, ejected at high speed. Its charge is −1 and its mass is negligible.

3The atomic number goes up by 1, the mass number is unchanged, and the element changes.

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Worked example

Problem

Strontium-90 has 38 protons and mass number 90. It decays by beta-minus decay into yttrium, which has atomic number 39. Write the balanced nuclear equation and say how many protons and neutrons the new nucleus has.

⚠ Watch out

Thinking the beta particle is an orbiting electron knocked off the atom. It is made in the nucleus when a neutron changes into a proton. The related slip is letting the −1 pull the atomic number down: it goes up by 1.

🧠

Memory hook

The neutron goes in as a neutron and comes out as a proton, throwing an electron out of the door on the way.

✓

Check yourself

Nucleus X has 27 protons and 33 neutrons, and it decays by beta-minus decay. How many protons, neutrons and nucleons does the new nucleus have, and where did the electron come from?

Flashcards

(8)

What is a beta-minus particle?

A high-speed electron ejected from the nucleus, with charge −1 and negligible mass.

What happens inside the nucleus in beta-minus decay?

A neutron changes into a proton and an electron.

Is the beta-minus electron one of the atom's orbiting electrons?

No. It is created in the nucleus when the neutron changes, then ejected. The nucleus holds no electrons beforehand.

How does the atomic number change in beta-minus decay?

It increases by 1, because the nucleus gains one proton.

How does the mass number change in beta-minus decay?

It doesn't. One neutron is lost and one proton is gained, so the number of nucleons is unchanged.

What happens to the element in beta-minus decay?

It becomes a different element: the one with atomic number 1 higher.

What symbol stands for a beta-minus particle in an equation?

⁰₋₁e, the same symbol as an electron.

How do you check a nuclear equation is balanced?

The top numbers add up to the same total on both sides, and so do the bottom numbers.

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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Follow one neutron inside a carbon-14 nucleus

Where does the beta particle come from?

Complete the equation yourself

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