• How to read atomic notation (A, Z) and work out protons/neutrons/electrons in any atom or ion.
• Rutherford's alpha scattering experiment — the evidence that gave us the nuclear model, and how our model of the atom evolved from Dalton → Thomson → Rutherford → Bohr → Quantum Mechanical.
• Thermionic emission — how heating a metal lets electrons escape, and how they're then accelerated across a p.d.
• Two types of particle accelerator (linear accelerators and cyclotrons) and how particle detectors count particles via ionisation.
• Why a charged particle moving through a magnetic field travels in a circle, and the formula linking radius, momentum, charge and field strength.
• How to "read" a bubble/cloud chamber photo — using track curvature and radius to work out charge, momentum, and spot particle creation/annihilation.
• Using high-energy electrons and the de Broglie wavelength to measure nucleon size — and even peer inside nucleons at quarks.
• Annihilation (matter + antimatter → energy) and pair production (energy → matter + antimatter), tied together by E = c²Δm.
• Converting between joules, eV, MeV, GeV — and mass units like MeV/c² and GeV/c².
• Relativistic effects (time dilation & length contraction) that let fast-moving unstable particles like muons survive long enough to be detected.

Every atom gets written in a special shorthand that packs in a huge amount of information — once you know how to read it. It looks like this:

Think of it like a label on a box: Z tells you WHICH element it is (because the number of protons defines the element — change Z and you've made a totally different element). A tells you how HEAVY that particular atom is (because neutrons add mass but don't change what element it is).

iIsotopes

Isotopes are atoms of the same element (same Z, same number of protons) but with a different number of neutrons (different A). Hydrogen is the classic example — every hydrogen atom has exactly 1 proton, but:

• Protium (ordinary hydrogen): 0 neutrons → A = 1
• Deuterium: 1 neutron → A = 2
• Tritium: 2 neutrons → A = 3

They behave almost identically chemically (same number of electrons = same chemistry), but they have different masses and some are radioactive while others aren't.

1Rutherford's Alpha Scattering Experiment

Before 1909, the accepted picture of the atom was Thomson's "plum pudding" model: a blob of positive charge with negative electrons stuck in it like currants — spread out evenly, nothing dense or concentrated anywhere.

To test this, Geiger and Marsden (working under Rutherford) fired a beam of alpha particles (helium nuclei — positively charged, quite massive) at extremely thin gold foil, with a detector able to swing around to catch particles deflected at any angle.

If the plum-pudding model were correct, the alpha particles should have sailed straight through every time — there's nothing dense enough in that model to stop something as heavy and fast as an alpha particle. What actually happened shocked Rutherford:

The atom is genuinely mostly empty space — the nucleus is about 100,000 times smaller in diameter than the atom itself (nucleus ≈ 10⁻¹⁵ m, atom ≈ 10⁻¹⁰ m). If the atom were the size of a football stadium, the nucleus would be about the size of a pea on the centre spot.

2How the Model of the Atom Evolved

Note: James Chadwick discovered the neutron in 1932 — that's what finally completed the picture of protons + neutrons + electrons we use today.

Metals are full of "free" conduction electrons buzzing around inside them (that's what makes metals conduct electricity). Normally these electrons can't escape the metal's surface — there's an energy barrier holding them in.

But if you heat the metal, those electrons gain kinetic (thermal) energy. Heat it enough, and some electrons gain enough energy to break free of the surface entirely. This escape process is called thermionic emission.

Once electrons are free, they can be accelerated using an electric field — typically by placing a positively charged anode nearby, which attracts the newly-freed electrons away from the heated cathode. This is exactly how an electron gun works (used in older CRT TVs, oscilloscopes, and particle accelerators).

1Linear Accelerators (LINACs)

A LINAC accelerates charged particles in a straight line through a series of hollow cylindrical "drift tubes," using only electric fields (no magnetic fields involved).

Here's the clever trick: an AC power supply connects across the tubes so that just as an ion arrives at the gap between two tubes, the polarity flips — repelling it out of the tube it's leaving and attracting it into the next one. Since the AC frequency is fixed, but the ion is getting faster each time, each successive tube has to be built longer so the ion still spends the same amount of time inside it before the field flips again.

2Cyclotrons

A cyclotron accelerates particles along a spiral path instead, using both electric AND magnetic fields together. It's built from two hollow semicircular electrodes called "dees" (named for their D shape).

• A uniform magnetic field (perpendicular to the dees) forces the ion to travel in a circular arc inside each dee.
• An AC electric field in the gap between the dees accelerates the ion each time it crosses.
• As the ion speeds up, its circular radius increases (remember r = mv/Bq — bigger v means bigger r), so it spirals steadily outward until it's fast enough to exit.

Cyclotrons are used in medicine — for producing medical tracer isotopes and generating high-energy beams for radiotherapy.

3Particle Detectors

Most particle detectors (Geiger-Müller tubes, spark chambers, cloud/gas chambers) work using the same underlying principle: ionisation. A fast-moving charged particle passing through a gas knocks electrons off surrounding atoms, creating ion-electron pairs. These charged fragments are then accelerated by an applied electric field, producing a tiny pulse of current that gets counted electronically.

Particles can also be deflected (scattered) as they pass through material — this can cause multiple scattering, slightly blurring their path.

Here's one of the most important results in this whole chapter. When a charged particle moves through a uniform magnetic field that's perpendicular to its velocity, the magnetic force on it is always perpendicular to its velocity too. A force that's always perpendicular to velocity doesn't speed the particle up or slow it down — it just constantly changes its direction. That's exactly the definition of circular motion.

So: the magnetic force is the centripetal force. We can set the two expressions equal to each other and derive something very useful.

This formula tells us three really useful things about how particles behave in a magnetic field:

A simple counter like a Geiger-Müller tube can tell you a particle passed by — but not much else. Modern detectors (bubble chambers, cloud chambers, spark chambers) show the actual curved paths particles take through a magnetic field, and that curvature is like a fingerprint — physicists can read off charge, momentum, and even spot particles being born or destroyed.

1What the Curvature Tells You

• Radius of the track → tells you the particle's momentum (r ∝ p, from the formula above)
• Direction the track curves → tells you the particle's charge sign (use Fleming's Left Hand Rule — the force/thumb direction points toward the centre of the circle)
• Radius shrinking as the track spirals inward → the particle is losing momentum (and therefore kinetic energy) as it repeatedly ionises the material it travels through

2Spotting Creation and Annihilation

Sometimes a photo shows two tracks appearing to start from a single point out of "nowhere" (i.e., not connected to any other visible track). This is the signature of pair production — an uncharged photon converting into a particle-antiparticle pair.

Key clues that this is a genuine particle-antiparticle pair:

• Both tracks curve in opposite directions (they must have opposite charge — that's what "particle + antiparticle" means)
• Both tracks have the same radius at the point of creation (same mass → same momentum, since they were created together from the same photon energy)

Charge, energy, and momentum must always be conserved in any interaction — that consistency is exactly how physicists can confidently identify what's happening in these photos.

1Measuring the Size of a Nucleon

To "see" something incredibly small, you need a probe with an incredibly small wavelength — this comes straight from the de Broglie wavelength idea. If you want to resolve detail the size of a nucleon (~10⁻¹⁵ m), you need particles whose de Broglie wavelength is roughly that small too.

Why use electrons for this rather than, say, alpha particles? Because electrons don't experience the strong nuclear force — they can get extremely close to a nucleon purely via the electromagnetic force, without any messy additional interaction distorting the picture. This makes electron scattering a much cleaner probe of nuclear size than alpha-particle scattering.

2Peering Inside the Nucleon

Since λ ∝ 1/v, accelerating electrons to even higher energies (higher v) shrinks their de Broglie wavelength even further. Push it small enough, and the electron beam becomes sensitive to structure inside the nucleon — this is exactly how physicists discovered that protons and neutrons aren't fundamental at all, but are made of even smaller particles: quarks.

1Annihilation

When a particle meets its exact antiparticle partner (e.g. an electron meeting a positron), they don't just bounce off each other — they completely destroy each other, and all of their combined mass converts directly into energy, released as two gamma-ray photons travelling in opposite directions.

Why two photons and not one? Because momentum has to be conserved. If the electron and positron collide with roughly equal and opposite momenta (net momentum ≈ 0), a single photon flying off in one direction would carry momentum — breaking conservation. Two photons flying off in opposite directions can have their momenta cancel out, keeping everything balanced.

2Pair Production

Pair production is annihilation running in reverse: a high-energy photon interacting with a nucleus converts its energy directly into a particle-antiparticle pair (e.g. an electron and a positron).

3The Mass-Energy Link

Both of these processes are governed by Einstein's iconic mass-energy relation:

Joules are usually far too big and clunky for particle-scale energies, so physicists use the electronvolt (eV) instead — a much more "human-sized" unit for this world.

Since E = c²Δm links energy and mass directly, particle physicists also use "mass units" that are secretly just energy units divided by c²:

When particles get accelerated close to the speed of light, ordinary Newtonian physics starts to break down, and two "relativistic effects" become important.

1Time Dilation

A moving clock runs slower than a stationary one, as measured by a stationary observer. For an unstable particle, this means: the faster it travels, the longer its "lifetime" appears to be to us in the lab, even though from the particle's own point of view nothing has changed.

2Length Contraction

A moving object appears shorter (contracted) along its direction of travel, as measured by a stationary observer. For particles, this means a fast-moving particle can travel a much greater apparent distance through a detector than you'd expect from its short lifetime alone — this is really the flip side of the same underlying relativistic story as time dilation, just viewed from a different reference frame.

Tick each one off only once you could explain it out loud to someone else, without looking at your notes.