What you'll learn
This revision guide covers the essential concepts of atomic structure and radioactive decay tested in Pearson Edexcel International IGCSE Physics. You will learn about the structure of atoms, isotopes, the three main types of nuclear radiation, and how to work with decay equations and half-life calculations. Understanding these fundamental principles is crucial for succeeding in both Paper 1 and Paper 2 examinations.
Key terms and definitions
Isotopes — atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.
Alpha particle (α) — a type of ionising radiation consisting of two protons and two neutrons (a helium nucleus), represented as ⁴₂He or ⁴₂α.
Beta particle (β) — a high-speed electron emitted from the nucleus when a neutron transforms into a proton, represented as ⁰₋₁e or ⁰₋₁β.
Gamma ray (γ) — electromagnetic radiation of very short wavelength emitted from unstable nuclei, with no mass and no charge.
Half-life — the time taken for half the nuclei in a sample of a radioactive isotope to decay, or the time for the activity to fall to half its initial value.
Background radiation — ionising radiation present in the environment from natural and artificial sources, which must be subtracted when measuring radioactivity.
Ionisation — the process by which atoms lose or gain electrons to become charged ions, caused by radiation removing electrons from atoms.
Becquerel (Bq) — the SI unit of activity, where 1 Bq equals one nuclear decay per second.
Core concepts
Atomic structure and notation
The atom consists of a small, dense nucleus containing protons and neutrons, surrounded by electrons in shells or energy levels. The nucleus accounts for nearly all the atom's mass but occupies a tiny fraction of its volume.
Standard notation:
The notation ᴬ𝗭X represents an atom where:
- X is the chemical symbol
- A is the mass number (nucleon number) = protons + neutrons
- Z is the atomic number (proton number) = number of protons
For example, ²³⁸₉₂U represents uranium-238 with 92 protons and 146 neutrons.
Key points:
- Protons have relative charge +1 and relative mass 1
- Neutrons have no charge and relative mass 1
- Electrons have relative charge -1 and negligible mass (1/1836)
- In a neutral atom, number of protons = number of electrons
- Atoms of the same element always have the same number of protons
Isotopes:
Carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are isotopes. Both have 6 protons (making them carbon), but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Carbon-14 is unstable and radioactive, while carbon-12 is stable. Isotopes have identical chemical properties because they have the same electron configuration, but different physical properties due to different masses.
Types of nuclear radiation
Unstable nuclei emit radiation to become more stable. The three main types you must know are alpha, beta, and gamma radiation.
Alpha radiation (α):
- Consists of 2 protons and 2 neutrons (helium nucleus)
- Represented as ⁴₂He or ⁴₂α
- Charge: +2
- Most ionising (causes most ionisation along its path)
- Least penetrating — stopped by paper or a few centimetres of air
- Range in air: approximately 5-10 cm
- Deflected by electric and magnetic fields (towards negative plate)
Beta radiation (β):
- High-speed electron emitted from the nucleus
- Represented as ⁰₋₁e or ⁰₋₁β
- Charge: -1
- Moderately ionising
- Moderately penetrating — stopped by a few millimetres of aluminium
- Range in air: approximately 1 metre
- Deflected by electric and magnetic fields (towards positive plate, more than alpha due to lower mass)
Gamma radiation (γ):
- Electromagnetic wave of very short wavelength
- Represented as ⁰₀γ
- No charge, no mass
- Least ionising
- Most penetrating — reduced by several centimetres of lead or metres of concrete
- Unlimited range in air (follows inverse square law)
- Not deflected by electric or magnetic fields
Nuclear decay equations
Alpha decay:
When a nucleus emits an alpha particle, the mass number decreases by 4 and the atomic number decreases by 2.
Example: Radium-226 decaying to radon-222 ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α
Beta decay:
When a nucleus emits a beta particle, a neutron converts to a proton. The mass number stays the same, but the atomic number increases by 1.
Example: Carbon-14 decaying to nitrogen-14 ¹⁴₆C → ¹⁴₇N + ⁰₋₁β
Balancing equations:
The total mass number and total atomic number must be equal on both sides of the equation.
For the equation: ²³⁴₉₀Th → ²³⁴₉₁Pa + X
The unknown particle X must have:
- Mass number: 234 = 234 + A, so A = 0
- Atomic number: 90 = 91 + Z, so Z = -1
Therefore X is ⁰₋₁β (beta particle).
Half-life and radioactive decay
Radioactive decay is random and spontaneous:
- Random: impossible to predict which nucleus will decay next or when
- Spontaneous: not affected by external conditions (temperature, pressure, chemical bonding)
The activity of a source is the rate at which nuclei decay, measured in becquerels (Bq).
Half-life calculations:
The half-life is constant for a particular isotope. After each half-life:
- Number of undecayed nuclei halves
- Activity halves
- Count rate halves
Method for half-life problems:
- Divide the initial value by 2 for each half-life
- Count how many half-lives have passed
- Multiply the number of half-lives by the half-life time period
Example: A sample has activity 800 Bq and half-life 3 hours. What is the activity after 9 hours?
9 hours ÷ 3 hours = 3 half-lives
After 1 half-life: 800 → 400 Bq After 2 half-lives: 400 → 200 Bq After 3 half-lives: 200 → 100 Bq
Activity = 100 Bq
Reading half-life from graphs:
On a graph of activity versus time:
- Choose any activity value on the y-axis
- Find the time on the x-axis
- Halve the activity value
- Find the new time on the x-axis
- Subtract: half-life = time₂ - time₁
Background radiation
Background radiation is always present in the environment. When measuring radioactivity, you must account for it.
Sources of background radiation:
- Natural sources:
- Cosmic rays from space
- Rocks and soil (especially granite containing uranium)
- Radon gas from rocks (largest contributor in UK)
- Food and drink (carbon-14, potassium-40)
- Artificial sources:
- Medical procedures (X-rays, radiotherapy)
- Nuclear weapons testing (historical)
- Nuclear power stations
- Nuclear accidents (Chernobyl, Fukushima)
Correcting for background:
When measuring a radioactive source, take a background count first with the source removed. Subtract this from all subsequent readings.
Corrected count rate = Measured count rate - Background count rate
Uses and hazards of radiation
Medical uses:
- Gamma rays: sterilising medical equipment, treating cancer (radiotherapy)
- Tracers: technetium-99m (gamma emitter, 6-hour half-life) used to detect blockages
- Beta radiation: treating thyroid conditions with iodine-131
Industrial uses:
- Thickness monitoring: beta sources control paper/metal sheet thickness
- Smoke detectors: alpha sources (americium-241) ionise air
- Carbon dating: measuring carbon-14 in organic materials (archaeology)
Safety precautions:
- Store radioactive sources in lead-lined containers
- Handle with tongs or robotic arms to maximise distance
- Minimise exposure time
- Use shielding appropriate to radiation type
- Never point sources at people
- Wear protective clothing and dosimeter badges
- Ensure proper ventilation (radon gas risk)
Worked examples
Example 1: Nuclear equation balancing
Plutonium-239 undergoes alpha decay. Write the balanced nuclear equation for this decay. (3 marks)
Solution: ²³⁹₉₄Pu → ²³⁵₉₂U + ⁴₂α (or ⁴₂He)
Mark scheme:
- Mass number correctly balanced (239 = 235 + 4) — 1 mark
- Atomic number correctly balanced (94 = 92 + 2) — 1 mark
- Correct notation and symbols — 1 mark
Example 2: Half-life calculation
A radioactive isotope has a half-life of 8 days and initial activity 960 Bq. Calculate the activity after 24 days. (3 marks)
Solution: Number of half-lives = 24 ÷ 8 = 3 half-lives (1 mark)
After 1 half-life: 960 ÷ 2 = 480 Bq After 2 half-lives: 480 ÷ 2 = 240 Bq After 3 half-lives: 240 ÷ 2 = 120 Bq (1 mark)
Activity after 24 days = 120 Bq (1 mark)
Example 3: Identifying radiation type
A student investigates an unknown radiation source. The radiation passes through paper but is stopped by 3 mm of aluminium. It is deflected by a magnetic field. Identify the radiation type and explain your answer. (3 marks)
Solution: The radiation is beta (β) radiation (1 mark).
Beta radiation passes through paper (unlike alpha) (1 mark) but is stopped by a few millimetres of aluminium (unlike gamma). Beta particles are charged so are deflected by magnetic fields (1 mark).
Common mistakes and how to avoid them
Confusing mass number with atomic number — Remember: mass number is always the larger number (top), atomic number is the smaller number (bottom). The atomic number tells you which element it is.
Incorrect balancing of decay equations — Always check both the mass numbers and atomic numbers balance separately on both sides of the equation. Mass number and atomic number are conserved.
Treating half-life as a linear process — Half-life is exponential, not linear. After two half-lives, you have ¼ remaining, not zero. The activity never reaches zero.
Forgetting to subtract background radiation — In practical questions and calculations, always subtract the background count from your measurements to get the true count rate from the source.
Mixing up penetration and ionisation — Alpha is most ionising but least penetrating. Gamma is least ionising but most penetrating. These properties are inversely related.
Stating that half-life is affected by external conditions — Radioactive decay is spontaneous and random; half-life cannot be changed by temperature, pressure, or chemical reactions.
Exam technique for "Radioactivity and Particles"
"State" questions require brief answers without explanation. For example, "State what is meant by isotopes" needs only the definition (1-2 marks). "Explain" questions require reasoning and linking ideas (3+ marks).
Nuclear equations must show proper notation with mass and atomic numbers in correct positions. Check both totals balance before moving on. Expect 2-3 marks for a complete balanced equation.
Half-life calculations often carry 3-4 marks. Show your working clearly: state the number of half-lives, show each division step, and give your final answer with units (Bq).
6-mark extended response questions may ask you to compare radiation types or evaluate uses of radioactivity. Structure your answer with clear paragraphs, use correct scientific terminology, and make explicit comparisons (alpha compared to beta, not just describing each separately).
Quick revision summary
Atoms consist of protons and neutrons in the nucleus, with electrons in shells. Isotopes have the same proton number but different neutron numbers. Unstable nuclei emit alpha (⁴₂He), beta (⁰₋₁e), or gamma (⁰₀γ) radiation. Alpha is most ionising but least penetrating; gamma is least ionising but most penetrating. In decay equations, mass and atomic numbers must balance. Half-life is the time for activity to halve; it is constant and unaffected by external conditions. Always subtract background radiation from measurements.