A.1 Atomic Structure & Radioactivity
Exploring the constituents of the atom, isotopes, nuclear decay, and the safety precautions for handling radioactive sources.
Questions to explore
- ? What is everything made of, and how do we know?
- ? Why are some atoms unstable, and what happens when they fall apart?
💡 Key ideas, explained simply
⚛️ Inside the atom
An atom has a tiny central nucleus of positive protons and neutral neutrons, surrounded by negative electrons. The atomic number () is the number of protons; the mass number () is protons + neutrons.
Isotopes are atoms of the same element (same protons) with different numbers of neutrons.
☢️ Three types of radiation
Unstable nuclei decay and emit radiation:
Alpha (α) — a helium nucleus; highly ionising but stopped by paper.
Beta (β) — a fast electron; stopped by thin aluminium.
Gamma (γ) — a high-energy wave; only reduced by thick lead or concrete.
Alpha, beta and gamma radiation
| Type | What it is | Charge | Stopped by | Ionising power |
|---|---|---|---|---|
| Alpha α | Helium nucleus | +2 | Paper / skin | Very high |
| Beta β | Fast electron | −1 | ~3 mm aluminium | Medium |
| Gamma γ | EM wave | 0 | Thick lead/concrete | Low |
📖 Key terms
- Proton
- Positive particle in the nucleus; its count sets the element.
- Neutron
- Neutral particle in the nucleus; changes the isotope.
- Isotope
- Same element, different number of neutrons (e.g. C-12 and C-14).
- Atomic number
- Number of protons in the nucleus.
- Mass number
- Protons + neutrons, .
- Half-life
- Time for half the radioactive nuclei in a sample to decay.
1 Key Formulas
✏️ Worked example
Half-life
A source has a half-life of . Starting with , how much remains after ?
- 1
Count the half-lives
half-lives.
- 2
Halve each time
12.5 g remains — after 3 half-lives, one-eighth of the original is left.
🗿 Physics around you
Carbon-14 dating uses half-life to age ancient bones and artefacts. Living things absorb carbon-14; once they die it decays at a known rate, so the amount left is a clock reaching back tens of thousands of years.
🎯 Nail it in the exam
Alpha, Beta, Gamma – Properties and Equations
MYP frequently asks you to compare the three types of radiation in a table. Know:
| Radiation | Nature | Charge | Penetration | Ionising ability |
|-----------|--------|--------|-------------|------------------|
| Alpha () | Helium nucleus | +2 | Stopped by paper/skin | Highly ionising |
| Beta () | Fast electron | -1 | Stopped by ~3 mm aluminium | Medium |
| Gamma () | EM wave | 0 | Reduced by thick lead/concrete | Low |
Nuclear equations must balance mass number (top) and atomic number (bottom). e.g., .
-
Always write the full equation, not just the daughter nucleus.
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Beta decay: a neutron turns into a proton, so atomic number increases by 1, mass number stays the same.
-
Gamma emission often accompanies alpha or beta decay – does not change the element.
Half‑life Calculations and Graphs
Half‑life appears in both graphical and numerical form. From a graph showing activity (or count rate) vs time:
Find the time for the activity to halve (e.g., from 800 to 400 counts/s).
Repeat for a second halving to confirm constancy.
Step‑by‑step half‑life problems:
- After 1 half‑life: amount = initial / 2
- After 2 half‑lives: amount = initial / 4, etc.
- Number of half‑lives = total time / half‑life.
Typical question: “A sample has a half‑life of 5 years. How much of a 100 g sample remains after 15 years?” → 15/5 = 3 half‑lives, remaining mass = 100 / 2³ = 12.5 g.
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When reading a graph, choose a clear starting point on the curve and show working lines.
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The background count rate must sometimes be subtracted before halving.
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If asked to ‘explain why a radioactive source is stored in a lead container’, mention penetrating power and ionising risk.
🧠 Check your understanding
Tap an answer to see if you're right — and why.
Q1. Which type of radioactive decay emits a helium nucleus?
Q2. Carbon-12 and Carbon-14 are isotopes. What do they have in common?
📝 Exam-style questions
Try each one, then reveal the model answer.