GCSE · Physics · Edexcel · Spec 1PH0

Ionising radiations

Take one tiny electron away from a neutral atom and something odd happens: the atom is suddenly charged. That one change is the whole story behind the word ionising, and you are about to watch it happen.

Physics · Ionising radiation

One electron goes missing

View: 1 · Neutral atom. Showing 2 layers: Nucleus, Neutral atom

View

Meet a helium atom. Protons are positive and electrons are negative. There are two of each, so the charges cancel and the atom is neutral.

Step through the four views, left to right. Watch the charge change.

Physics · Meet the five

One unstable nucleus, five things it can throw out

Showing 2 layers: Unstable nucleus, Emissions

Explore

Tap a name below the picture to see what that radiation is.

Physics · Can you predict it?

When will this nucleus decay?

A sample holds a huge number of identical unstable nuclei. Each one will decay at some point, and each decay sends out ionising radiation.

Which is closest to what you think about when ONE particular nucleus will decay?

The oldest nuclei go first: each one has a waiting time, and the longest-waiting nucleus is next.Decay is random: nobody can predict when one particular nucleus will decay.If we measured one nucleus carefully enough, we could work out exactly when it will decay.Nuclei decay at regular intervals, like a clock ticking, one after another.

How sure are you?

I'm sureI think soI'm guessing

The oldest nuclei go first: each one has a waiting time, and the longest-waiting nucleus is next.

This is a common misconception

It is a natural thought: queues work like that. But if the oldest nucleus always went first, you could predict the order, and you cannot. Radioactive decay is random: it is not possible to predict when a particular nucleus will decay.

Decay is random: nobody can predict when one particular nucleus will decay.

This matches the evidence

Exactly. The decay of a particular unstable nucleus is a random event. You can know that it is unstable and that it emits ionising radiation when it decays, but not when.

If we measured one nucleus carefully enough, we could work out exactly when it will decay.

This is a common misconception

This is the one that survives longest, because it sounds like "we just do not know yet". But random does not mean "hidden". It is not possible to predict when a particular nucleus will decay, however much we measure.

Nuclei decay at regular intervals, like a clock ticking, one after another.

This is a common misconception

A regular tick would be perfectly predictable, and decay is not. Radioactive decay is random: there is no ticking clock telling a particular nucleus when to go.

Unstable nuclei emit ionising radiation as they decay, but when a particular nucleus will decay is random and cannot be predicted.

Physics · Your turn

Explain the name

Explain why alpha, beta, gamma and neutron radiation are called ionising radiations. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The radiation passes through a material (such as air). (1 mark)
As it passes, it can knock electrons out of the atoms of the material. (1 mark)
This turns the atoms into charged ions (the atoms are ionised). (1 mark)

Model answer

As the radiation passes through a material, it can knock electrons out of the atoms. The atoms are left with an overall charge, so they have become ions. Radiation that does this is called ionising.

Three links in one chain: passes through, knocks electrons out, leaves charged ions.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

What happens to an atom when radiation ionises it, and why it ends up charged
The five radiations an unstable nucleus can emit, and what each one is
Why you cannot predict when one particular nucleus will decay

The big picture

Unstable nuclei decay at random and emit ionising radiation: alpha particles, beta-minus particles, beta-plus particles (positrons), gamma rays or neutrons. These radiations are called ionising because they can knock electrons out of atoms, turning the atoms into charged ions.

Key points

1Unstable nuclei emit ionising radiation as they decay. When one particular nucleus will decay is random and cannot be predicted.

2The five radiations are alpha particles, beta-minus particles, beta-plus particles (positrons), gamma rays and neutrons.

3Alpha is 2 protons and 2 neutrons (a helium nucleus). Beta-minus is a high-speed electron from the nucleus. Beta-plus is a positron from the nucleus. Gamma is high-frequency electromagnetic radiation. Neutron radiation is a stream of neutrons.

4They are called ionising because, passing through a material, they can knock electrons out of atoms and turn the atoms into charged ions.

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Worked example

Problem

Radiation passes through some oxygen gas and knocks two electrons out of one neutral oxygen atom. A neutral oxygen atom has 8 protons and 8 electrons. Describe the atom afterwards.

⚠ Watch out

Treating "ionising radiation" as one single thing. It is five different emissions, and gamma is not a particle at all. Also keep two electrons apart: the electrons knocked out of atoms by ionisation, and a beta-minus particle, which is a high-speed electron emitted from the nucleus.

🧠

Memory hook

Take a minus away and you are left with a plus: knock an electron (−) out of a neutral atom and the atom ends up positive. Ionising means ion-making.

✓

Check yourself

A neutral atom has 6 protons and 6 electrons. Radiation knocks one electron out. How many of each does it have now, and what is its charge?

Flashcards

(9)

What does ionising radiation do to atoms?

It can knock electrons out of atoms, turning the atoms into charged ions.

What is an ion?

An atom with an overall charge, because its protons and electrons no longer balance.

An atom loses an electron. Is it now positive or negative, and why?

Positive. It still has all its protons (+) but one fewer electron (−), so the positives outnumber the negatives.

Can you predict when one particular unstable nucleus will decay?

No. Radioactive decay is a random process.

Name the five radiations that unstable nuclei can emit.

Alpha, beta-minus, beta-plus, gamma and neutron radiation.

What is an alpha particle?

Two protons and two neutrons: the same as a helium nucleus.

What are beta-minus and beta-plus particles?

Beta-minus: a high-speed electron emitted from the nucleus. Beta-plus: a positron (the positively charged antiparticle of the electron) emitted from the nucleus.

What is a gamma ray?

High-frequency electromagnetic radiation emitted from the nucleus.

What is neutron radiation?

A stream of neutrons released from nuclei.

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Ionising radiations

Five radiations, five fingerprints

When will this nucleus decay?

Explain the name

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