Atomic Structure & Radioactivity ☢️

From the nucleus to nuclear equations — interactive, animated, and with just enough humour to keep you from decaying.

AQA / Edexcel · GCSE Physics · Exam ready

⚛️ Inside the Atom

Protons, neutrons, and electrons — the original squad. Very small. Very important.

💬 "The atom is 99.9999% empty space. You are mostly nothing. Philosophical? Yes. Useful for exams? Also yes."

Proton number Z: 6

Neutron number N: 6

Element

Carbon

Mass Number A

Z + N

Charge

neutral atom

Colour key:
Protons positive charge +1 each
Neutrons no charge
Electrons negative charge −1 each

Where are they?
Protons + neutrons → nucleus (tiny, central, dense)
Electrons → shells/orbits around nucleus
Most of the atom: empty space

🔬 Isotopes & Ions

Same element, different story. Isotopes change the neutrons. Ions change the electrons.

Element (Z): 6 — Carbon

Neutrons (right atom): 8

Nuclear notation: ᴬ_ZX — where A = mass number (top), Z = proton number (bottom), X = element symbol.
Example: ¹²₆C = Carbon-12 has 6 protons and 6 neutrons. ¹⁴₆C = Carbon-14 has 6 protons but 8 neutrons.

🔴 Alpha Radiation (α)

A helium nucleus that decided to leave. Fast, heavy, easily stopped.

💬 "Alpha radiation is basically a helium nucleus that got angry and left. Stopped by a sheet of paper. Dangerous if inhaled — NOT from outside. Exam favourite."

Properties:
Composition: 2 protons + 2 neutrons (helium-4 nucleus)
Charge: +2
Mass: 4 u (heaviest radiation)
Ionising power: HIGH
Penetration: LOW — stopped by paper or skin

Nuclear equation rule:
A decreases by 4 (loses 2p + 2n)
Z decreases by 2 (loses 2 protons)

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Uranium-238 decays to Thorium-234

Deflection in fields: Alpha (+2 charge) deflects toward the negative plate in an electric field, and curves in a magnetic field according to the left-hand rule. Deflects more than beta (heavier).

🟠 Beta Radiation (β)

A neutron splitting into a proton, an electron, and an antineutrino. Messy but important.

Properties:
Composition: fast-moving electron (from nucleus)
Charge: −1
Mass: very small (~1/1840 of proton)
Ionising power: MEDIUM
Penetration: MEDIUM — stopped by ~5mm aluminium

Nuclear equation rule:
A stays the same (no change in mass number)
Z increases by 1 (neutron → proton)

¹⁴₆C → ¹⁴₇N + ⁰_-1e
Carbon-14 decays to Nitrogen-14

Origin: A neutron in the nucleus transforms: n → p + e⁻ + antineutrino. The electron (beta particle) shoots out at high speed. Deflects opposite direction to alpha in electric fields (negative charge).

💜 Gamma Radiation (γ)

Pure electromagnetic energy. No mass, no charge, and it will go through most things like they aren't there.

Properties:
Composition: electromagnetic wave (photon)
Charge: 0 (not deflected by fields)
Mass: 0
Ionising power: LOW
Penetration: HIGH — needs thick lead or concrete

When does it happen?
Gamma is released AFTER alpha or beta decay. The daughter nucleus is in an excited state — it releases the excess energy as a gamma ray photon.

No change in Z or A — it's just energy loss.

Summary Comparison Table

Property	Alpha α	Beta β	Gamma γ
Composition	2p + 2n (He-4)	Electron (e⁻)	EM wave
Charge	+2	−1	0
Ionising	High	Medium	Low
Penetration	Paper / skin	~5mm Al	Thick Pb/concrete
Deflected?	Yes (+)	Yes (−)	No
Speed	~5% c	up to 90% c	c (speed of light)

🛡️ Penetrating Power — Visual Comparison

The most common exam topic in this whole unit. Know it cold.

Exam tip: Barriers in order — paper, then aluminium (few mm), then thick lead (several cm), then thick concrete. Alpha stops at paper. Beta stops at aluminium. Gamma is only reduced (never truly zero) by thick lead/concrete.

⏱️ Half-Life

The time it takes for half the unstable nuclei in a sample to decay. Not all of them. Half. This distinction costs marks.

💬 "Half-life: the time for half the atoms to decay. NOT the time for ALL of them to be gone. Common exam mistake. Don't make it."

Animation speed: medium

How to calculate half-life problems:
1. Find number of half-lives = total time ÷ half-life
2. Halve the starting amount that many times
Example: 800g, half-life = 10 years. After 30 years: 3 half-lives → 800 → 400 → 200 → 100g

Try a calculation

Starting activity (Bq): 800

Half-life (years): 10

Time elapsed (years): 30

🔢 Nuclear Equations Builder

Both top numbers must balance. Both bottom numbers must balance. Every time.

Parent mass number A: 238

Parent proton number Z: 92

Balancing rule: Mass numbers (top) on left = sum of mass numbers on right. Proton numbers (bottom) on left = sum of proton numbers on right. Check BOTH rows.

🌍 Background Radiation & Uses

Radiation is everywhere, all the time. Most of it is completely natural. Your granite worktop is mildly radioactive. This is fine.

Biggest source: Radon gas (from rocks and soil) accounts for ~50% of background radiation in the UK. It seeps into buildings from granite bedrock. Cornwall gets a lot of it.

Uses of Radioactivity

Use	Type	Why that type?
Medical tracers (PET scan)	Gamma γ	Penetrates body, detected externally
Cancer radiotherapy	Gamma γ	Kills cancer cells (targeted beam)
Sterilising medical equipment	Gamma γ	Kills bacteria without heat
Smoke detectors	Alpha α	Ionises air in chamber; smoke blocks it
Thickness gauging (paper mills)	Beta β	Right penetration for paper thickness
Carbon dating	Beta β	C-14 half-life ~5700 years

✅ Quiz — Test Yourself

5 questions. No pressure. Well. A little pressure. It's an exam topic.

1. Uranium-238 undergoes alpha decay. What are the mass number and proton number of the daughter nucleus?

2. A sample has an initial activity of 3200 Bq and a half-life of 8 years. What is the activity after 24 years?

3. Which type of radiation is stopped by a few millimetres of aluminium but passes through paper?

4. Two atoms are isotopes of each other. Which statement is correct?

5. What is the largest single source of background radiation in the UK?

☢️ Key Equations

A = Z + NMass number = protons + neutrons

N₀ → N₀/2 → N₀/4 → ...Activity halves every half-life

n = t / t½Number of half-lives = time ÷ half-life

α: A→A−4, Z→Z−2Alpha decay rule

β: A unchanged, Z→Z+1Beta decay rule

γ: no change to A or ZGamma = energy release only

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂HeAlpha decay example

¹⁴₆C → ¹⁴₇N + ⁰₋₁eBeta decay example