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7.1 Atoms and Radiation

GCSE Physics · AQA · Chapter 7: Radioactivity

A free GCSE Physics lesson for AQA (8463) from OBA School: inside the atom, unstable nuclei, the radiation they give out and how a Geiger–Müller tube detects it. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the structure of an atom and give the relative charge and mass of protons, neutrons and electrons
  • Explain what a radioactive substance is and why its nuclei decay
  • Name the types of radiation an unstable nucleus can give out: alpha, beta, gamma and neutrons
  • Explain what is meant by saying radioactive decay is random
  • Use a Geiger–Müller tube and counter to measure a count rate and correct it for background radiation

Key words

  • Nucleus: The tiny centre of an atom. It contains protons and neutrons and nearly all of the mass.
  • Isotopes: Atoms of the same element (same number of protons) with different numbers of neutrons.
  • Ion: An atom that has lost or gained electrons, so it has an overall charge.
  • Radioactive decay: An unstable nucleus giving out radiation to become more stable.
  • Random: You cannot predict when a particular nucleus will decay, and nothing you do to it changes that.
  • Count rate: The number of decays recorded each second (or minute) by a detector such as a Geiger–Müller tube.
  • Background radiation: Radiation that is around us all the time from natural and man-made sources.

Inside the atom

Every atom has a tiny, positively charged nucleus made of protons and neutrons. Electrons move around the nucleus at different distances, called energy levels (or shells). The radius of an atom is about 1 × 10−¹⁰ m, so atoms are far too small to see with a light microscope.

  • proton; Where it is nucleus; Relative mass 1; Relative charge +1
  • neutron; Where it is nucleus; Relative mass 1; Relative charge 0
  • electron; Where it is energy levels around the nucleus; Relative mass about 1/2000 (very small); Relative charge −1

An atom has the same number of electrons as protons, so the charges cancel and the atom is neutral overall. All atoms of one element have the same number of protons. Atoms of the same element with different numbers of neutrons are called isotopes.

Key fact: If an atom loses one or more outer electrons it becomes a positive ion, because it now has more protons than electrons. Radiation that does this is called ionising radiation.

Radioactive substances and unstable nuclei

In most atoms the nucleus is stable and never changes. In a radioactive substance some of the nuclei are unstable. An unstable nucleus becomes more stable by giving out radiation. This is called radioactive decay. The radiation comes from the nucleus, not from the electrons.

  • alpha; Symbol α; What it is two protons + two neutrons (a helium nucleus); Stopped by a sheet of paper or a few cm of air
  • beta; Symbol β; What it is a high-speed electron made and thrown out by the nucleus; Stopped by a few mm of aluminium
  • gamma; Symbol γ; What it is electromagnetic radiation (a wave) from the nucleus; Stopped by thick lead or concrete reduces it
  • neutron; Symbol n; What it is a neutron thrown out of the nucleus; Stopped by thick concrete or water

Radioactive decay is random. You cannot predict which nucleus will decay next or when a particular nucleus will decay. Heating, cooling, squashing or reacting the substance chemically does not change this. With billions of nuclei, though, the average number decaying each second is very predictable (see section 7.5).

Exam tip: Radioactive decay is a property of the nucleus. A chemical reaction only changes the electrons, so a radioactive element is still radioactive when it is part of a compound.

Detecting radiation: the Geiger–Müller tube

A Geiger–Müller tube (GM tube) is connected to a counter. Each time radiation enters the tube it produces a pulse of current and the counter adds one. The number of counts in a set time gives the count rate, in counts per second or counts per minute.

The counter clicks even with no source nearby. This is background radiation from rocks, the air (radon gas), food, cosmic rays and medical uses. To find the count rate from the source alone:

  • measure the background count over several minutes with the source well away, and work out the mean count rate
  • measure the count rate with the source in place
  • subtract: corrected count rate = measured count rate − background count rate

Worked example: correcting for background

With no source, a counter records 230 counts in 10 minutes. With the source 3 cm from the tube it records 1750 counts in 5 minutes.
background count rate = 230 ÷ 10 = 23 counts per minute
measured count rate = 1750 ÷ 5 = 350 counts per minute
corrected count rate = 350 − 23 = 327 counts per minute
in counts per second: 327 ÷ 60 = 5.45 counts per second

Answer: 327 counts per minute (about 5.5 counts per second)

Exam tip: Because decay is random, repeat readings of a count are never exactly the same. Count for a longer time, or take several readings and find the mean, to get a more reliable value.

Working safely

Radiation from radioactive sources can damage living cells. In a school laboratory sources are kept in a lead-lined box, handled only with long tongs and held at arm’s length, pointed away from people, and used for as short a time as possible. You should never touch a source or eat and drink in the laboratory.

Key fact: Photographic film darkens when radiation reaches it. This is how radioactivity was first noticed, and it is still used in some badges that workers wear to check how much radiation they have received.

Common mistakes

  • Saying radiation comes from the electrons. Alpha, beta, gamma and neutron radiation all come from the nucleus.
  • Thinking heating or a chemical reaction can speed up decay. Decay is random and is not affected by physical or chemical conditions.
  • Forgetting to subtract the background count rate before using a count rate.
  • Mixing up count rate units: divide counts per minute by 60 to get counts per second.
  • Calling a beta particle an electron “from the shells”. It is made inside the nucleus when it decays.

Revision questions

Try these, then check your answers below each one.

Question 1. Which particles are found in the nucleus of an atom? (Choose from: Protons and neutrons, Protons and electrons, Neutrons and electrons, Protons only.)

Answer: Protons and neutrons

Working: The nucleus contains protons (+1) and neutrons (0). Electrons (−1) are in energy levels around the nucleus.

Question 2. A Geiger–Müller tube near a source records 1500 counts in 5.0 minutes. The background count rate is 25 counts per minute.<br/>Calculate the count rate due to the source alone, in counts per minute.

Answer: 275 counts per minute

Working: measured count rate = 1500 ÷ 5.0 = 300 counts per minute; corrected count rate = 300 − 25 = 275 counts per minute.

Question 3. True or false? Heating a radioactive source makes its nuclei decay faster.

Answer: False

Working: False. Radioactive decay is random and is not affected by temperature, pressure or chemical reactions.

Related lessons

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This lesson is section 7.1 (AGCPH0701) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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