Welcome to Atomic and Nuclear Physics
Welcome to one of the most exciting topics in your CCEA GCSE Physics course! Everything around you—from your phone screen to the stars in the night sky—is built from tiny particles called atoms. In this chapter, we will look deep inside the atom to discover its hidden structure, learn why some atoms are radioactive, explore how radiation can be used safely to save lives, and investigate the immense power of nuclear fission and fusion.
Don't worry if nuclear physics sounds intimidating at first! We will break every concept down into clear, bite-sized steps with everyday analogies, simple calculations, and tips to help you score full marks in your Unit 1 exam.
---1. Atomic Structure and the Rutherford-Bohr Model
The Rutherford Alpha Particle Scattering Experiment
Over a century ago, scientists believed in J.J. Thomson's plum pudding model, which pictured the atom as a ball of positive charge with negative electrons scattered throughout it like plums in a pudding.
Ernest Rutherford tested this idea by firing positively charged alpha particles at an extremely thin sheet of gold foil. The results completely changed our understanding of the universe.
Here are the three key observations and the conclusions Rutherford drew from them:
1. Observation: Most alpha particles passed straight through the gold foil without any deflection.
Conclusion: The atom is mostly empty space.
2. Observation: Some alpha particles were deflected through small angles.
Conclusion: There is a concentrated positive charge inside the atom (since like charges repel, the positive alpha particles were repelled).
3. Observation: A very small fraction of alpha particles bounced straight back (deflected by more than \(90^\circ\)).
Conclusion: The positive charge and almost all the mass of the atom are concentrated in a tiny central region called the nucleus.
Subatomic Particles
The modern model of the atom contains three main subatomic particles:
• Proton: Relative mass = \(1\), Relative charge = \(+1\), Location = In the nucleus.
• Neutron: Relative mass = \(1\), Relative charge = \(0\) (neutral), Location = In the nucleus.
• Electron: Relative mass = \(\frac{1}{1836}\) (negligible / \(\approx 0\)), Relative charge = \(-1\), Location = Orbiting the nucleus in shells.
Nuclide Notation
In physics, we write an atom of any element \(\text{X}\) using nuclide notation:
\({}^{A}_{Z}\text{X}\)
• \(A\) = Mass number (Nucleon number): The total number of protons plus neutrons in the nucleus.
• \(Z\) = Atomic number (Proton number): The number of protons in the nucleus (this identifies which element it is).
• To find the number of neutrons, simply subtract the atomic number from the mass number: \(\text{Number of neutrons} = A - Z\).
What is an Isotope?
Isotopes are atoms of the same element that have the same number of protons (same atomic number) but a different number of neutrons (different mass number).
Analogy: Think of isotopes like two bicycles of the exact same model and frame, but one has a basket on the front. They do the exact same job, but one is slightly heavier than the other!
Key Takeaway for Section 1
The atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by orbiting electrons in mostly empty space. Isotopes have the same number of protons but different numbers of neutrons.
---2. Radioactivity and the Three Types of Radiation
What is Radioactive Decay?
Some atomic nuclei have an unstable balance of protons and neutrons. To become stable, the unstable nucleus spontaneously disintegrates and emits ionising radiation. This process is called radioactive decay.
Radioactive decay has two essential features:
• It is random: We cannot predict which individual nucleus will decay next, or exactly when it will decay.
• It is spontaneous: The process is completely unaffected by external physical conditions such as changes in temperature or pressure.
The Three Types of Radiation
There are three main types of nuclear radiation, each with different properties:
1. Alpha Radiation (\(\alpha\) or \({}^{4}_{2}\alpha\) or \({}^{4}_{2}\text{He}\)):
• Nature: A helium nucleus made of \(2\) protons and \(2\) neutrons.
• Charge: \(+2\).
• Relative Mass: \(4\text{ u}\).
• Ionising Power: Strongly ionising (because of its large mass and double positive charge, it easily knocks electrons off atoms).
• Penetrating Power: Low (stopped by a single sheet of paper or \(3\text{--}5\text{ cm}\) of air).
2. Beta Radiation (\(\beta\) or \({}^{0}_{-1}\beta\) or \({}^{0}_{-1}\text{e}\)):
• Nature: A fast-moving, high-energy electron ejected directly from the nucleus.
• Origin: Formed inside the nucleus when a neutron changes into a proton and an electron.
• Charge: \(-1\).
• Relative Mass: Negligible (\(\approx 0\)).
• Ionising Power: Moderately ionising.
• Penetrating Power: Moderate (passes through paper and air for \(\approx 1\text{ m}\), but stopped by a few millimetres of aluminium).
3. Gamma Radiation (\(\gamma\) or \({}^{0}_{0}\gamma\)):
• Nature: A high-energy electromagnetic wave (photon).
• Charge: \(0\) (neutral).
• Relative Mass: \(0\).
• Ionising Power: Weakly ionising.
• Penetrating Power: High (has a very long range in air; its intensity is only reduced or absorbed by thick lead or several metres of concrete).
Key Takeaway for Section 2
Alpha particles (\({}^{4}_{2}\alpha\)) are heavy, strongly ionising, and easily stopped by paper. Beta particles (\({}^{0}_{-1}\beta\)) are fast electrons stopped by aluminium. Gamma rays (\(\gamma\)) are uncharged electromagnetic waves with very high penetrating power, stopped only by thick lead or concrete.
---3. Nuclear Decay Equations
When an unstable nucleus decays by emitting an alpha or beta particle, it turns into a new element. We write these transformations as nuclear equations. In every nuclear equation, two golden rules must be satisfied:
1. The total mass number (\(A\)) must be equal on both sides.
2. The total atomic number (\(Z\)) must be equal on both sides.
Alpha (\(\alpha\)) Decay Equations
When an unstable nucleus emits an alpha particle (\({}^{4}_{2}\alpha\)), it loses \(2\) protons and \(2\) neutrons. Therefore, its mass number decreases by 4 and its atomic number decreases by 2:
\({}^{A}_{Z}\text{X} \longrightarrow {}^{A-4}_{Z-2}\text{Y} + {}^{4}_{2}\alpha\)
Example: The alpha decay of Uranium-238 into Thorium (\(\text{Th}\)):
\({}^{238}_{92}\text{U} \longrightarrow {}^{234}_{90}\text{Th} + {}^{4}_{2}\alpha\)
• Top numbers balance: \(238 = 234 + 4\)
• Bottom numbers balance: \(92 = 90 + 2\)
Beta (\(\beta\)) Decay Equations
Inside the nucleus, a neutron turns into a proton (which stays in the nucleus) and a high-speed electron (which is emitted as a beta particle, \({}^{0}_{-1}\beta\)). Because a neutron becomes a proton, the mass number stays unchanged and the atomic number increases by 1:
\({}^{A}_{Z}\text{X} \longrightarrow {}^{A}_{Z+1}\text{Y} + {}^{0}_{-1}\beta\)
Example: The beta decay of Carbon-14 into Nitrogen (\(\text{N}\)):
\({}^{14}_{6}\text{C} \longrightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}\beta\)
• Top numbers balance: \(14 = 14 + 0\)
• Bottom numbers balance: \(6 = 7 + (-1)\)
Gamma (\(\gamma\)) Emission
Gamma emission occurs when an excited nucleus sheds excess energy. Because gamma rays have no mass and no charge, the mass number and atomic number remain completely unchanged:
\({}^{A}_{Z}\text{X}^* \longrightarrow {}^{A}_{Z}\text{X} + \gamma\)
Key Takeaway for Section 3
Alpha decay reduces \(A\) by \(4\) and reduces \(Z\) by \(2\). Beta decay keeps \(A\) the same and increases \(Z\) by \(1\). Always check that the top numbers on the left equal the top numbers on the right, and bottom numbers on the left equal bottom numbers on the right!
---4. Background Radiation and Half-Life
What is Background Radiation?
Background radiation is low-level ionising radiation that is present around us in the environment at all times.
• Natural Sources: Radon gas (released from rocks and soil), cosmic rays (from outer space), food and drink (such as potassium-40 in bananas), and naturally occurring radioactive rocks/building materials.
• Artificial (Man-made) Sources: Medical applications (X-rays, diagnostic tests, radiotherapy) and nuclear power / fallout from weapons testing.
Calculating the Corrected Count Rate
When measuring the activity of a radioactive source in an experiment, a radiation detector will record both the source radiation AND the background radiation. To find the true activity of the source alone, we must calculate the corrected count rate:
\(\text{Corrected Count Rate} = \text{Measured Total Count Rate} - \text{Background Count Rate}\)
Activity and Half-Life
• Activity: The number of radioactive decays occurring per unit time. It is measured in Becquerels (Bq), where \(1\text{ Bq} = 1\text{ decay per second}\).
• Half-life (\(t_{1/2}\)): The time taken for half of the unstable nuclei in a sample to decay, OR the time taken for the activity (count rate) of a source to halve.
Step-by-Step Half-Life Calculations
After each half-life, the fraction of the original radioactive material remaining halves:
• After \(1\) half-life: \(\frac{1}{2}\) remaining (\(50\%\))
• After \(2\) half-lives: \(\frac{1}{4}\) remaining (\(25\%\))
• After \(3\) half-lives: \(\frac{1}{8}\) remaining (\(12.5\%\))
• After \(n\) half-lives: \(\text{Fraction remaining} = \left(\frac{1}{2}\right)^n\)
Worked Example: A sample of Technetium has an initial activity of \(800\text{ Bq}\). Its half-life is \(6\text{ hours}\). What will its activity be after \(18\text{ hours}\)?
Step 1: Find the number of half-lives that have passed: \(\text{Number of half-lives} = \frac{18\text{ hours}}{6\text{ hours}} = 3\text{ half-lives}\).
Step 2: Halve the initial activity \(3\) times:
Start: \(800\text{ Bq}\)
After 1st half-life (6 hrs): \(\frac{800}{2} = 400\text{ Bq}\)
After 2nd half-life (12 hrs): \(\frac{400}{2} = 200\text{ Bq}\)
After 3rd half-life (18 hrs): \(\frac{200}{2} = 100\text{ Bq}\)
Answer: \(100\text{ Bq}\).
Key Takeaway for Section 4
Half-life is the time required for the number of unstable nuclei (or the activity) to drop to half its starting value. In real experiments, always subtract the background count rate first to get the corrected count rate.
---5. Uses and Hazards of Radioactivity
Everyday and Medical Uses
• Smoke Alarms (Alpha radiation): An alpha source (such as Americium-241) ionises the air inside a detector chamber, creating a small electric current. If smoke enters the chamber, smoke particles block the alpha particles, dropping the current and sounding the alarm.
• Thickness Gauging of Paper and Aluminium Foil (Beta radiation): Beta particles are directed through rolling sheets of material into a detector. If the sheet gets too thick, fewer beta particles pass through; if it gets too thin, more pass through. The detector signals the rollers to adjust their pressure automatically.
• Medical Tracers & Cancer Treatment (Gamma radiation): Gamma emitters with short half-lives (like Technetium-99m) are injected into the bloodstream to trace blood flow or check organ function without harming the patient long-term. Targeted beams of gamma radiation are also used to destroy cancerous tumours.
• Sterilisation (Gamma radiation): Gamma rays kill bacteria and microorganisms on packaged surgical instruments and food without damaging the items or making them radioactive.
• Archaeological Dating: Measuring the remaining amount of Carbon-14 in dead organic matter allows scientists to calculate its age.
Hazards and the Difference Between Irradiation and Contamination
Ionising radiation can damage living cells, cause mutations in DNA, trigger cancerous growths, or cause acute radiation sickness at high doses.
It is vital to understand the difference between irradiation and contamination:
• Irradiation: Occurs when an object or person is exposed to radiation from an external source. The object itself does not become radioactive. As soon as the radioactive source is removed, the exposure stops completely.
• Contamination: Occurs when unwanted radioactive material gets on the skin, clothing, or inside the body. The contaminated person or object will continue to emit radiation until the material is washed off or completely decays.
Safety Precautions
To reduce radiation dose and risk, scientists and medical workers follow strict safety rules:
1. Minimise time: Keep exposure times as short as possible.
2. Maximise distance: Use long-handled tongs or robotic arms to handle sources.
3. Use shielding: Store and handle sources behind thick lead screens, wear lead aprons, or use lead-lined containers.
4. Protective clothing & monitoring: Wear gloves, lab coats, and radiation dose badges (dosimeters) to track personal exposure.
Key Takeaway for Section 5
Radioactivity has vital uses in smoke alarms (alpha), thickness control (beta), and medicine/sterilisation (gamma). Irradiation is exposure from the outside (does not make you radioactive), while contamination is having radioactive material on or inside you.
---6. Nuclear Reactions: Fission and Fusion
Nuclear Fission
Nuclear fission is the splitting of a large, unstable nucleus into two smaller, lighter nuclei (called daughter nuclei), releasing \(2\text{--}3\) neutrons and a large amount of energy.
• Induced Fission: In a nuclear reactor, fission is triggered when a heavy nucleus (such as Uranium-235) absorbs a slow-moving (thermal) neutron.
• Chain Reaction: The \(2\text{--}3\) neutrons released during the fission of one Uranium-235 nucleus can go on to be absorbed by other nearby Uranium-235 nuclei, triggering further fission events. This self-sustaining sequence is called a chain reaction.
Inside a Nuclear Reactor Core
A nuclear power station safely harnesses fission energy using four key components:
• Fuel rods: Contain the fissionable nuclear fuel (e.g., Uranium-235).
• Moderator (graphite or water): Slows down fast-moving neutrons so they can be easily captured by Uranium-235 nuclei to cause further fission.
• Control rods (made of boron or cadmium): Absorb excess neutrons. Raising or lowering the control rods regulates the rate of the chain reaction or shuts it down completely.
• Coolant and Shielding: The coolant carries thermal energy away from the core to produce steam that drives turbines; thick concrete shielding prevents harmful radiation from escaping into the environment.
Nuclear Fusion
Nuclear fusion is the joining (fusing) of two small, light nuclei (such as hydrogen isotopes: Deuterium \({}^{2}_{1}\text{H}\) and Tritium \({}^{3}_{1}\text{H}\)) to form a heavier nucleus (Helium \({}^{4}_{2}\text{He}\)), releasing huge amounts of energy.
• Where it occurs: Fusion is the process that naturally powers the Sun and all other stars.
• Conditions required: Fusion requires extremely high temperatures and high pressures. These extreme conditions are needed to provide the positively charged nuclei with enough kinetic energy to overcome their strong electrostatic repulsion so they can collide and fuse together.
Key Takeaway for Section 6
Fission = Splitting heavy nuclei (e.g., U-235 in nuclear power stations).
Fusion = Joining light nuclei (e.g., Hydrogen isotopes in the Sun).
Nuclear reactors control fission chain reactions using a moderator to slow neutrons down and control rods to absorb excess neutrons.
7. Quick Review: Common Pitfalls and Exam Tips
Avoid these common exam mistakes to ensure maximum marks in your Unit 1 paper:
• Forgetting Background Radiation in Calculations: If a question gives you a background count rate alongside experimental data, ALWAYS subtract background radiation before finding the half-life or halving the count rate!
• Defining Half-Life Incorrectly: Never say "half the time it takes an atom to decay". Always write: "The time taken for half of the unstable nuclei in a sample to decay" OR "the time taken for the activity/count rate to halve."
• Misunderstanding Beta Particle Origin: Remember that a beta particle does NOT come from electron shells. It is an electron ejected from the nucleus when a neutron transforms into a proton.
• Beta Decay Atomic Number Math: In a beta decay equation (\({}^{0}_{-1}\beta\)), the atomic number \(Z\) on the right increases by 1 (because \(Z = (Z + 1) + (-1)\)).
• Confusing Fission and Fusion: Remember: Fission = Splits; Fusion = Joins.