Radioactivity

Slide 1 · Learning Outcomes

What you will be able to do

Some nuclei are unstable. They break up on their own, giving out radiation and energy. This is radioactivity, discovered by Henri Becquerel in 1896 and studied by Marie and Pierre Curie.

Describe the nucleus and explain the stability of isotopes.
Compare alpha, beta and gamma radiation: nature, charge, penetration and ionising power.
Write and balance nuclear equations for alpha and beta decay.
Define half-life and use it in calculations and decay curves.
Distinguish between nuclear fission and nuclear fusion.
State uses of radioisotopes in medicine, agriculture, industry and dating.
Explain the dangers of radiation and safety precautions.

Notation: A
ZX where A = mass number (protons + neutrons) and Z = atomic number (protons). Neutrons = A − Z.

Slide 2 · Isotopes and stability

Why are some nuclei unstable?

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Protons repel each other; neutrons help hold the nucleus together through the strong nuclear force. If the neutron : proton ratio is wrong, or the nucleus is very large (Z > 83), the nucleus is unstable and decays.

Activity: build a nucleus

Protons6

Neutrons6

For light elements, stable nuclei have about equal numbers of protons and neutrons (n/p ≈ 1). Heavier stable nuclei need more neutrons (n/p up to about 1.5). Too many neutrons → β⁻ decay; too heavy → α decay.

Slide 3 · Types of radiation

Alpha, beta and gamma

	Alpha (α)	Beta (β)	Gamma (γ)
Nature	Helium nucleus 4 2He	Fast electron 0 −1e	Electromagnetic wave
Charge	+2	−1	0
Penetration	Stopped by paper / few cm of air	Stopped by a few mm of aluminium	Reduced by thick lead / concrete
Ionising power	Very high	Moderate	Low

Activity: fire the radiation

Choose a radiation.

Slide 4 · Behaviour in an electric field

Which way do they bend?

Passing radiation between charged plates shows its charge. α bends towards the negative plate, β towards the positive plate, and γ goes straight.

α β γ

Why does β bend more than α? A β particle is about 7000 times lighter than an α particle, so it is deflected much more strongly even though its charge is smaller.

Slide 5 · Nuclear equations

Balancing nuclear equations

In every nuclear equation, the mass numbers (top) and the atomic numbers (bottom) must add up to the same total on both sides.

α decay: A falls by 4, Z falls by 2.

β decay: A unchanged, Z rises by 1 (a neutron changes into a proton + electron).

Activity: decay chain builder

Start nucleus

Slide 6 · Half-life

Half-life

Decay is random: we cannot say when one nucleus will decay. But for a large number, the half-life (t½) is the time taken for half of the radioactive nuclei in a sample to decay. It is constant for a given isotope.

Activity: decay simulator

Half-life calculator

Starting mass (g)

Half-life (days)

Time elapsed (days)

Slide 7 · Fission and fusion

Splitting and joining nuclei

Nuclear fission

A heavy nucleus (e.g. uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing 2–3 neutrons and a large amount of energy.

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + energy

Used in nuclear power stations and atomic bombs.

Nuclear fusion

Two light nuclei join to form a heavier nucleus, releasing even more energy per gram. Needs extremely high temperatures (millions of °C).

²₁H + ³₁H → ⁴₂He + ¹₀n + energy

Powers the Sun and stars; hydrogen bomb.

Activity: chain reaction

Slide 8 · Uses and dangers

Radioisotopes at work

Activity: match the isotope to its use

Tap a card, then tap the box it belongs to.

Medicine

Industry and agriculture

Dating

Dangers: radiation ionises molecules in living cells. It can cause burns, radiation sickness, cancers (e.g. leukaemia), sterility and genetic mutations passed to offspring. Nuclear accidents (Chernobyl 1986, Fukushima 2011) and radioactive waste contaminate land and water for many years.

Safety: handle sources with long tongs; store in thick lead containers; wear protective clothing and film badges; keep exposure time short and distance large; never eat or drink near sources; bury waste deep in sealed containers.

Slide 9 · Practice

Worked example and quick checks

Worked example: mass remaining

A radioisotope has a half-life of 5 days. What mass of an 80 g sample remains after 20 days?

Step 1: Number of half-lives = 20 ÷ 5 = 4

Step 2: 80 g → 40 g → 20 g → 10 g → 5 g

Step 3: Or use: remaining = 80 × (½)⁴ = 80 ÷ 16

Answer: 5 g remains (75 g has decayed)

Your turn

1. ²¹⁰₈₄Po emits an α particle. The mass number of the new nucleus is .

2. ²³⁴₉₀Th emits a β particle. The atomic number of the new nucleus is .

3. 64 g of an isotope decays to 8 g in 24 hours. Its half-life is hours.

4. The radiation that is not deflected in an electric field is .

Slide 10 · Quiz

Check your understanding

Part A: Fill in the blanks (auto-marked, 1 mark each)

1. Atoms of the same element with different numbers of neutrons are called .

2. An alpha particle is a nucleus.

3. The radiation stopped by a sheet of paper is .

4. In beta decay, a neutron changes into a proton and an .

5. When a nucleus emits an α particle, its atomic number decreases by .

6. The time taken for half of the radioactive nuclei in a sample to decay is the .

7. Splitting a heavy nucleus into two lighter nuclei is nuclear .

8. The Sun's energy comes from nuclear .

9. The isotope used to estimate the age of wood and bones is carbon- .

10. Radioactive sources are stored in containers made of .

Part B: Short answers (smart-marked against key points)

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