Atomic Number and Mass Number
Every atom is identified by two key numbers:
- Atomic number (Z): Number of protons in the nucleus. This defines which element it is (e.g., all atoms with 1 proton are hydrogen).
- Mass number (A): Total number of protons and neutrons in the nucleus.
Formula:
Number of neutrons = Mass number – Atomic number
Example:
Carbon-12 has 6 protons and 6 neutrons → A = 12, Z = 6.
Isotopes:
Atoms of the same element (same number of protons) that have different numbers of neutrons. For example, Carbon-12 and Carbon-14 are both carbon but have different mass numbers.
The Structure of an Atom
Atoms are made up of three types of particles:
| Particle | Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | Very small | Orbitals (shells) |
- The nucleus is tiny compared to the overall atom but contains almost all of its mass.
- Electrons move around the nucleus in energy levels or shells.
- The atom is neutral overall because the number of electrons = number of protons.
Key idea: Most of an atom is empty space — this was a big discovery that changed our understanding of matter.
Electronic Arrangement
Electrons are arranged in shells (energy levels) around the nucleus.
- The first shell holds up to 2 electrons.
- The second shell holds up to 8 electrons.
- The third shell also holds up to 8 electrons (for the first 20 elements).
Example:
Sodium (atomic number 11) → Electron arrangement = 2,8,1
Atoms are most stable when their outer shell is full — this explains chemical bonding.
When energy is absorbed: electrons jump to higher shells.
When energy is released: electrons fall back down and emit light — this is how neon signs and fireworks produce colours.
The History of the Atom
The atomic model developed over hundreds of years:
- Democritus (400 BC): Suggested matter was made of tiny indivisible particles called “atoms”.
- John Dalton (1803): Proposed atoms were solid spheres and that each element was made of identical atoms.
- J.J. Thomson (1897): Discovered the electron using the cathode ray tube. Proposed the plum pudding model — a positive “pudding” with negative electrons inside.
- Ernest Rutherford (1909): Carried out the gold foil experiment, proving atoms have a small, dense, positively charged nucleus.
- Niels Bohr (1913): Suggested electrons orbit the nucleus in fixed shells (energy levels).
- James Chadwick (1932): Discovered the neutron, explaining why atomic masses didn’t match proton counts.
Each stage made the model more accurate — from solid spheres → plum pudding → nuclear → Bohr model → modern quantum model.
The Alpha Scattering Experiment
Rutherford, Geiger, and Marsden fired alpha particles (positively charged helium nuclei) at thin gold foil.
Results:
- Most alpha particles passed straight through → atom mostly empty space.
- Some deflected → small positive centre (nucleus).
- A few bounced back → nucleus is very dense and massive.
Conclusion:
- The atom has a tiny, dense, positive nucleus surrounded by mostly empty space.
- This led to the nuclear model of the atom, replacing Thomson’s plum pudding idea.
Radiation
Radioactive decay happens when an unstable nucleus releases energy or particles to become more stable.
There are three main types of nuclear radiation:
| Type | What it is | Penetration | Ionising Power | Stopped By |
|---|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons (helium nucleus) | Weak | Very strong | Paper or skin |
| Beta (β) | Fast-moving electron | Medium | Moderate | Aluminium |
| Gamma (γ) | Electromagnetic wave | Very strong | Weak | Thick lead or concrete |
Key point:
Alpha is heavy and ionises strongly, but can’t travel far.
Gamma travels far and through most materials, but ionises weakly.
Properties of Ionising Radiation
Ionisation means knocking electrons out of atoms, turning them into charged particles (ions).
- Alpha: Heavily ionising → causes the most damage if inside the body.
- Beta: Medium ionising → can penetrate skin but not deep organs.
- Gamma: Least ionising → can pass through the body and damage cells at a distance.
Health risks:
Ionising radiation can damage DNA → mutations, cancer, or cell death.
Safety:
- Limit exposure time.
- Keep distance.
- Use shielding (lead or concrete).
Nuclear Equations
Radioactive decay changes one element into another. Nuclear equations show these changes and must balance for mass number (A) and atomic number (Z).
Examples:
- Alpha decay:
Uranium-238 → Thorium-234 + Alpha particle
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He - Beta decay:
Carbon-14 → Nitrogen-14 + Beta particle
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Rules:
- In alpha decay, A decreases by 4 and Z decreases by 2.
- In beta decay, A stays the same and Z increases by 1.
Half-Life
The half-life of a radioactive isotope is the time it takes for half of the radioactive nuclei to decay or for the count rate to halve.
- Measured using a Geiger-Müller tube connected to a counter.
- Half-life is different for each isotope — could be seconds or millions of years.
Example:
If a sample starts at 800 counts per minute and has a half-life of 10 minutes:
After 10 minutes → 400 cpm
After 20 minutes → 200 cpm
After 30 minutes → 100 cpm
Uses:
- Carbon dating (using Carbon-14).
- Medical tracers (short half-life isotopes).
- Nuclear waste management (long half-life isotopes are more dangerous).
Irradiation and Contamination
- Irradiation: When an object is exposed to radiation, but does not become radioactive. Used to sterilise food or medical equipment.
- Contamination: When radioactive material gets onto or into an object. This is more dangerous because radiation continues to be emitted.
Prevention:
- Use gloves and tongs.
- Store materials in lead-lined boxes.
- Avoid ingestion or inhalation of radioactive dust.
Remember: Irradiation = exposure. Contamination = contact.
Background Radiation
We are all exposed to a small amount of background radiation every day.
Natural sources:
- Cosmic rays from space
- Radioactive rocks (granite)
- Radon gas from the ground
Artificial sources:
- Nuclear power
- Medical procedures (X-rays, radiotherapy)
- Fallout from nuclear testing
Typical background dose: Around 2.7 millisieverts per year in the UK (higher in areas like Cornwall due to granite).
Nuclear Radiation in Medicine
Medical uses of radiation include:
- Tracers: Radioactive isotopes injected into the body to track processes (e.g., Technetium-99m). Gamma rays pass through the body and can be detected externally.
- PET scans: Detect gamma rays from positron-emitting isotopes, producing 3D images of organs.
- Radiotherapy: Focused beams of gamma or beta radiation kill cancer cells.
Key requirements:
- Tracers should have a short half-life.
- Emit gamma (not alpha or beta).
- Be non-toxic and quickly leave the body.
Nuclear Fission
Fission is the splitting of a large, unstable nucleus (such as Uranium-235 or Plutonium-239) into two smaller nuclei.
Process:
- A neutron is absorbed by the nucleus.
- The nucleus splits into two smaller nuclei.
- Energy, two or three neutrons, and gamma rays are released.
The neutrons can go on to cause chain reactions — this is how nuclear power stations work.
Controlled reaction: Used in reactors with control rods (absorb excess neutrons).
Uncontrolled reaction: Causes nuclear explosions.
Nuclear Fusion
Fusion is when two light nuclei combine to form a heavier nucleus (like hydrogen forming helium).
Example:
Hydrogen + Hydrogen → Helium + Energy
Key facts:
- Releases far more energy than fission.
- Requires extremely high temperatures and pressures to overcome electrostatic repulsion.
- Occurs naturally in the Sun and stars.
- Scientists are trying to develop fusion reactors (like ITER) to provide clean, limitless energy.
