Can the moment at which one particular unstable nucleus will decay be predicted?

No, decay is random, so only the average behaviour of a large number of nuclei can be predicted.

What is meant by the half-life of a radioactive isotope, in terms of the number of nuclei?

It is the time taken for the number of nuclei of that isotope in a sample to fall to half.

State the meaning of half-life in terms of count-rate.

It is the time taken for the count-rate, or activity, from the sample to fall to half its initial level.

Explain how half-life follows from the random nature of radioactive decay.

Every undecayed nucleus has the same chance of decaying in a given time, so however many are left the number remaining always falls by half over the same interval.

AQA GCSE Physics 8463

Atomic Structure

Topic overview The structure of an atom Mass number, atomic number and isotopes From the plum pudding model to the nuclear model Chadwick, the neutron, and comparing the models Radioactive decay and nuclear radiation Nuclear equations Half-lives and the random nature of radioactive decay Radioactive contamination Background radiation Uses of nuclear radiation, and how half-life affects hazard Nuclear fission and nuclear fusion

Electricity

Topic overview Circuit symbols, electrical charge and current Current, resistance and potential difference Ohmic conductors, filament lamps, diodes and thermistors Light-dependent resistors, and investigating resistance Current and potential difference in series and parallel Constructing circuits, and calculating in series circuits Direct and alternating potential difference, and mains electricity Power and efficiency Appliances, and the energy transferred by electrical work Power, and the power ratings of domestic appliances The National Grid Static charge Electric fields

Energy

Topic overview Energy stores and systems Kinetic energy Elastic and gravitational potential energy Energy changes in systems Power Energy transfers in a system The main energy resources, renewable and non-renewable Environmental impact and trends in energy use

Forces

Topic overview Scalars, vectors, forces and the resultant force Gravity Work done and energy transfer Stretching, compressing, and the spring constant Linear and non-linear extension, and work done on a spring Moments, levers and gears Pressure at a surface, and the equation p = F over A Pressure in a column of liquid, upthrust and floating Atmospheric pressure Resolving forces and free body diagrams Distance, displacement and speed The speed of sound, and the distance-speed-time equation Velocity and the distance-time relationship Acceleration, deceleration and velocity-time graphs Area under a velocity-time graph, and the uniform acceleration equation Newton's First Law Newton's Second and Third Laws Stopping distance and reaction time Factors affecting braking distance Braking force, deceleration and the dangers of stopping quickly Momentum and its conservation Changes in momentum

Magnetism and Electromagnetism

Topic overview Poles of a magnet Magnetic fields around a magnet The compass, and plotting a magnetic field Electromagnetism Fleming's left-hand rule, motors and loudspeakers Induced potential The generator effect, alternators, dynamos and microphones The structure of a transformer and the turns ratio Transformer power, and using the transformer equations

Particle Model of Matter

Topic overview Density of materials Changes of state and internal energy Temperature changes in a system and specific heat capacity Changes of state and specific latent heat Particle motion in gases Pressure in gases, and increasing the pressure of a gas

Space Physics

Topic overview Our solar system, and the orbits of satellites The life cycle of a star Red-shift

Waves

Topic overview Transverse and longitudinal waves, and the wave equation Wave properties, and refraction at a boundary Reflection of waves Sound waves Ultrasound, echo sounding and seismic waves Types and uses of electromagnetic waves Radio waves, and the hazards of ultraviolet, X-rays and gamma rays Lenses Visible light Emission and absorption of infrared radiation, and black bodies

Atomic Structure

Half-lives and the random nature of radioactive decay

3 things to know
Paper 1

Overview

The random nature of radioactive decay, half-life in terms of nuclei and of count-rate, why activity falls over time, finding a half-life from data and from a graph of count-rate against time, subtracting the background count, and expressing the net decline after several half-lives as a ratio.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why the behaviour of a radioactive sample can be predicted, and why its activity falls as time goes on.

Key term

Explain why the decay of a whole sample can be described accurately even though each individual decay is random.

Numbers

A sample contains 8000 undecayed nuclei of an isotope. How many are left after one half-life?

Questions from this subtopic

Each one with the model answer underneath.

1
Can the moment at which one particular unstable nucleus will decay be predicted?
Model answer
No, decay is random, so only the average behaviour of a large number of nuclei can be predicted.
2
Explain why the decay of a whole sample can be described accurately even though each individual decay is random.
Model answer
A sample contains an enormous number of nuclei, so the average behaviour of the sample is very predictable even though no single decay is.
3
What is meant by the half-life of a radioactive isotope, in terms of the number of nuclei?
Model answer
It is the time taken for the number of nuclei of that isotope in a sample to fall to half.
4
State the meaning of half-life in terms of count-rate.
Model answer
It is the time taken for the count-rate, or activity, from the sample to fall to half its initial level.
5
Explain how half-life follows from the random nature of radioactive decay.
Model answer
Every undecayed nucleus has the same chance of decaying in a given time, so however many are left the number remaining always falls by half over the same interval.
6
A sample contains 8000 undecayed nuclei of an isotope. How many are left after one half-life?
Model answer
4000 undecayed nuclei.

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