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Atomic physics

Describe atomic structure, isotopes, nuclear radiation and simple ideas of half-life and safety.

12 free questions · Notes · Worked solutions

Practise this topic

What you will practise

  • Describe protons, neutrons and electrons in an atom.
  • Use atomic and mass numbers to identify particles and isotopes.
  • Compare alpha, beta and gamma radiation.
  • Interpret half-life and describe sensible radiation precautions.

Revision notes

Atomic structure

The nucleus contains protons and neutrons; electrons occupy the surrounding region. Atomic number Z is proton number. Mass number A is proton plus neutron number, so neutrons=A−Z.

Isotopes

Isotopes are atoms of the same element with equal proton number but different neutron numbers. Neutral atoms have equal proton and electron numbers.

Radiation

Alpha particles are helium nuclei: strongly ionising and weakly penetrating. Beta-minus radiation is an electron: moderately ionising and penetrating. Gamma is electromagnetic radiation: weakly ionising and highly penetrating.

Half-life and safety

Half-life is the time for half the undecayed nuclei in a large sample to decay. It is statistical; individual decay is unpredictable. Reduce exposure by limiting time, increasing distance and using suitable shielding.

A worked example

An isotope has atomic number 17 and mass number 37. Find its numbers of protons, neutrons and electrons if the atom is neutral.

  1. Protons=atomic number=17.
  2. Neutrons=mass number−atomic number=37−17=20.
  3. Neutral atom electrons=protons=17.

17 protons, 20 neutrons, 17 electrons

Try it yourself

These are original practice questions, not official past-paper questions. Practice points count correct final answers; working is not automatically assessed.

Choose a question to begin.
Question 1 · Standard1 practice point

How many neutrons are in an atom with mass number 23 and atomic number 11?

Need a hint?

Neutrons=mass number−atomic number.

Worked solution
  1. 23−11=12 neutrons.
Question 2 · Standard1 practice point

Which radiation has the greatest penetrating ability?

Need a hint?

Compare the three common nuclear emissions.

Worked solution
  1. Gamma is the most penetrating of alpha, beta and gamma.
Question 3 · Standard1 practice point

What is an isotope?

Need a hint?

The element identity depends on proton number.

Worked solution
  1. Isotopes share proton number and differ in neutron number.
Question 4 · Standard1 practice point

A radioactive sample has a half-life of 6 hours. What fraction remains after 12 hours?

Need a hint?

Twelve hours is two half-lives.

Worked solution
  1. After one half-life: 1/2 remains.
  2. After two: (1/2)²=1/4.
Question 5 · Foundation1 practice point

An atom has 15 protons and 16 neutrons. Find its mass number.

Need a hint?

Mass number is protons plus neutrons.

Worked solution
  1. A=15+16.
  2. A=31.
Question 6 · Foundation1 practice point

A neutral atom has atomic number 8. How many electrons does it have?

Need a hint?

A neutral atom has equal protons and electrons.

Worked solution
  1. Atomic number gives 8 protons.
  2. Neutrality means 8 electrons.
Question 7 · Standard2 practice points

A sample starts with 800 undecayed nuclei. How many are expected to remain after three half-lives?

Need a hint?

Halve the amount three times.

Worked solution
  1. 800→400 after one half-life.
  2. 400→200 after two.
  3. 200→100 after three.
Question 8 · Foundation1 practice point

A sample has half-life 4 days. How many days pass in 3 half-lives?

Need a hint?

Multiply the half-life by the number of half-lives.

Worked solution
  1. 3×4=12 days.
Question 9 · Standard2 practice points

A sample has 640 nuclei. How many are expected to remain after 2 half-lives?

Need a hint?

Take half twice.

Worked solution
  1. After one half-life: 320.
  2. After two: 160.
Question 10 · Standard2 practice points

An isotope has mass number 40 and 18 neutrons. Find its atomic number.

Need a hint?

Atomic number=mass number−neutrons.

Worked solution
  1. Z=40−18.
  2. Z=22.
Question 11 · Standard2 practice points

A detector records 1200 counts/min from a radioactive source. Ignoring background radiation, what count rate is expected after 2 half-lives?

Need a hint?

The count rate halves twice.

Worked solution
  1. 1200→600 counts/min after one half-life.
  2. 600→300 counts/min after two.
Question 12 · Standard2 practice points

An isotope has atomic number 6 and mass number 14. How many neutrons does its nucleus contain?

Need a hint?

Subtract proton number from nucleon number.

Worked solution
  1. Neutrons=A−Z=14−6.
  2. There are 8 neutrons.

Coverage and sources

Selected IGCSE concepts, with a mix of introductory and more demanding questions. Check your own specification. AI drafted; independent teacher review is pending. Editorial details.

Cambridge IGCSE Physics 0625 syllabus

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