What is radioactive decay in the context of nuclear chemistry?

Radioactive decay is a process by which an unstable atomic nucleus loses energy by emitting radiation. This process is fundamental in nuclear chemistry and involves the transformation of an element into another element or a different isotope. It is a spontaneous process that can result in the emission of alpha particles, beta particles, or gamma rays.

How does radioactive decay relate to the concept of half-life?

The half-life of a radioactive substance is the time required for half of the radioactive atoms in a sample to decay. It is a key concept in understanding radioactive decay, as it helps predict how long a radioactive isotope will remain active. In the IB MYP Science curriculum, students learn to calculate half-life and understand its implications in various scientific and practical applications.

What are the different types of radioactive decay?

There are three main types of radioactive decay: alpha decay, beta decay, and gamma decay. Alpha decay involves the emission of an alpha particle, which consists of two protons and two neutrons. Beta decay involves the transformation of a neutron into a proton with the emission of a beta particle (electron or positron). Gamma decay involves the emission of gamma rays, which are high-energy photons, and usually occurs after alpha or beta decay to release excess energy from the nucleus.

Why is understanding radioactive decay important in science?

Understanding radioactive decay is crucial in science because it has significant implications in fields such as medicine, archaeology, and energy production. For instance, radioactive isotopes are used in medical imaging and cancer treatment, while carbon dating relies on radioactive decay to determine the age of archaeological finds. Additionally, nuclear power plants use controlled radioactive decay to generate energy.

How is radioactive decay measured and detected?

Radioactive decay is measured and detected using instruments such as Geiger-Müller counters, scintillation counters, and cloud chambers. These devices detect the radiation emitted during decay processes, allowing scientists to measure the rate of decay and the type of radiation. Understanding these measurements is essential for safely handling radioactive materials and for applications in nuclear chemistry and other scientific fields.

Why is "Saying beta decay decreases the mass number." a common mistake in Radioactive Decay?

Why it happens: An electron is emitted — feels like mass loss. How to avoid it: Mass number A counts NUCLEONS (protons + neutrons). In beta decay, a neutron becomes a proton — total nucleon count UNCHANGED. Z + 1 because we have one more proton.

Why is "Believing half-life changes with temperature or chemical state." a common mistake in Radioactive Decay?

Why it happens: Lots of chemistry does. How to avoid it: Nuclear decay is independent of chemical and physical conditions. Half-life is fixed per isotope.

Why is "Saying a radioactive sample becomes 'inactive' after a fixed time." a common mistake in Radioactive Decay?

Why it happens: Half-life sounds like a stopwatch. How to avoid it: Each half-life HALVES the activity — it never quite reaches zero. After many half-lives, activity is just very small.

Nuclear chemistryIB MYP Sciences (Years 1-5)

Radioactive Decay

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Radioactive Decay — IB MYP Sciences (Nuclear Chemistry): nuclear equations, half-life and uses

Unstable nuclei decay by emitting alpha, beta or gamma radiation. This MYP Sciences note covers how to write balanced nuclear equations for each decay type, why half-life is predictable, and how radioactive isotopes are used in medicine and dating.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Identify alpha, beta and gamma decay from a nuclear equation.
MYP Sciences A — Write balanced nuclear equations for each decay type.
MYP Sciences A — Define half-life and use it to find activity at a later time.
MYP Sciences D — Evaluate uses of radioactive isotopes against their risks.

Nuclear equations for alpha, beta, gamma decay

Each decay type changes A and Z in a predictable way.

Every nuclear equation must BALANCE on both sides:

Mass number (A) on left = sum on right.
Atomic number (Z) on left = sum on right.

Alpha decay — emits an alpha particle (a helium nucleus). General form:

$^{A}_{Z}\!X \rightarrow {^{A-4}_{Z-2}\!Y} + {^{4}_{2}\!\alpha}$

Example: $^{238}_{92}\text{U} \rightarrow {^{234}_{90}\text{Th}} + {^{4}_{2}\!\alpha}$ . Uranium-238 alpha-decays to thorium-234.

Beta decay — a neutron turns into a proton + electron. The electron is emitted as a beta particle:

$^{A}_{Z}\!X \rightarrow {^{A}_{Z+1}\!Y} + {^{0}_{-1}\!\beta}$

A stays the same (mass barely changes), but Z increases by 1.

Example: $^{14}_{\;6}\text{C} \rightarrow {^{14}_{\;7}\text{N}} + {^{0}_{-1}\!\beta}$ . Carbon-14 beta-decays to nitrogen-14.

Gamma decay — the nucleus rearranges to a lower-energy state. A and Z are unchanged; only the excess energy is released as a γ photon. Gamma usually follows α or β decay rather than happening alone.

Alpha: A − 4, Z − 2. Beta: A same, Z + 1. Gamma: A and Z unchanged.

Alpha: A − 4, Z − 2 (emits a He nucleus).
Beta: A same, Z + 1 (a neutron becomes a proton + emitted electron).
Gamma: A and Z unchanged (only energy released).
Balance both A and Z in any nuclear equation.

Half-life and decay over time

Half-life is the constant time for half the nuclei to decay.

Radioactive decay is random — you can't predict when a specific nucleus will decay. But statistically, in a large sample:

The TIME for half the nuclei to decay is constant — the half-life (t½).

Half-life is independent of temperature, pressure, chemical environment or how much you started with. Some examples:

Isotope	Half-life	Notes
Polonium-214	0.16 ms	Very fast decay
Iodine-131	8 days	Medical tracer (thyroid)
Carbon-14	5 730 years	Carbon dating
Plutonium-239	24 000 years	Reactor product, weapon material
Uranium-238	4.5 billion years	Oldest natural decay chain

Activity (decays per second, in Bq) halves every half-life:

Start: A₀.
After 1 half-life: A₀ / 2.
After 2 half-lives: A₀ / 4.
After n half-lives: A₀ / 2ⁿ.

A short half-life means lots of decay quickly (high activity but short-lived). A long half-life means slower decay over much longer time scales.

Half-life: time for half the nuclei to decay (or activity to halve).
Constant for each isotope — doesn't change with conditions.
After n half-lives, activity = A₀ / 2ⁿ.

Uses (and risks) of nuclear chemistry

Tracers, dating, sterilising, power — all balanced against radiation hazard.

Radioactive isotopes have many practical uses, chosen by matching their properties (radiation type, half-life) to the job:

Medical tracers (e.g. technetium-99m): short half-life (~6 h), gamma emitter → can image organs without long irradiation.
Cancer therapy (e.g. cobalt-60): gamma source that kills cancer cells from outside.
Sterilising surgical instruments: gamma irradiation kills microbes without heating.
Smoke alarms (americium-241): alpha source disturbs air ionisation when smoke enters.
Carbon dating (¹⁴C): organic archaeological samples up to ~50 000 years.
Nuclear power (uranium-235): controlled fission for electricity generation.

Risks: ionising radiation can damage DNA → cancer or radiation sickness in high doses. Background radiation (~2-3 mSv/year per person) is normally harmless; medical X-rays add a small dose. Containment, shielding (lead, concrete) and short exposure times keep workers safe.

MYP criterion D extension: discuss whether the medical benefits of imaging and therapy justify the radiation risks. Modern protocols (short-half-life tracers, targeted gamma therapy, ALARA — As Low As Reasonably Achievable doses) make most medical uses very safe.

Choose the radiation type and half-life to suit the job.
Short half-life + gamma: medical tracers.
Alpha (highly ionising, short range): smoke alarms.
Always weigh dose against benefit.

How it’s examined

Criterion A tests balancing nuclear equations and identifying decay types. Criterion C: half-life calculations and graph interpretation. Criterion D: choosing or evaluating a use of a radioisotope.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Radioactive Decay

Step-by-step solutions to past-paper-style questions on radioactive decay, written exactly the way a tutor would explain them at the board.

1Balancing an alpha decay equation
Getting started• nuclear equation
Question
Uranium-238 undergoes alpha decay. Write the balanced nuclear equation.
Step-by-step solution
1. Step 1
  Alpha = ²₄He: A decreases by 4, Z by 2.
2. Step 2
  ²³⁸₉₂U → ²³⁴₉₀X + ²₄α. Z = 90 is thorium (Th).
Answer
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α.
2Balancing a beta decay equation
Building confidence• nuclear equation
Question
Carbon-14 undergoes beta decay. Write the balanced nuclear equation.
Step-by-step solution
1. Step 1
  Beta = ⁰₋₁e: A unchanged, Z + 1.
2. Step 2
  ¹⁴₆C → ¹⁴₇X + ⁰₋₁β. Z = 7 is nitrogen (N).
Answer
¹⁴₆C → ¹⁴₇N + ⁰₋₁β.
3Half-life calculation
Building confidence• half-life
Question
A radioisotope has half-life 4 days. Initial activity = 800 Bq. Find the activity after 12 days.
Step-by-step solution
1. Step 1
  Number of half-lives: 12 / 4 = 3.
2. Step 2
  Activity halves 3 times: 800 → 400 → 200 → 100.
Answer
100 Bq.
4Choosing an isotope for a medical scan
Stretch• applications
Question
A doctor needs an isotope to image a patient's thyroid. Choose between (a) plutonium-239 (alpha, 24 000-yr half-life) or (b) iodine-131 (beta/gamma, 8-day half-life). Justify.
Step-by-step solution
1. Step 1
  Alpha is too ionising once inside the body; long half-life means the patient stays irradiated for years. Plutonium is out.
2. Step 2
  Iodine-131 has a short half-life (decays away in weeks) and emits gamma (detectable outside the body). Beta + gamma are less ionising than alpha.
3. Step 3
  Plus: the thyroid naturally absorbs iodine, so I-131 collects there. Perfect targeting.
Answer
Iodine-131. Short half-life, gamma-detectable outside the body, naturally taken up by the thyroid. Plutonium-239 would be wrong on every count.

Key Formulae — Radioactive Decay

The formulae you need to memorise for radioactive decay on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Activity after n half-lives
$Aₙ = A₀ / 2ⁿ$
$A₀$
starting activity
$n$
number of half-lives elapsed
When to use
Find remaining activity when an exact number of half-lives have passed.

Key Definitions and Keywords — Radioactive Decay

Definitions to memorise and the exact keywords mark schemes credit for radioactive decay answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Radioactive decay
Examiner keyword
The spontaneous emission of α, β or γ radiation from an unstable atomic nucleus.
Alpha decay
Examiner keyword
Emission of a He nucleus. A − 4, Z − 2.
Beta decay
Examiner keyword
A neutron in the nucleus becomes a proton; the electron is emitted. A unchanged, Z + 1.
Gamma decay
Emission of a high-energy photon. A and Z unchanged.
Half-life
Examiner keyword
Constant time for half the radioactive nuclei (or activity) to decay.

Common Mistakes and Misconceptions — Radioactive Decay

The traps other students keep falling into on radioactive decay questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying beta decay decreases the mass number.
Why it happens
An electron is emitted — feels like mass loss.
How to avoid it
Mass number A counts NUCLEONS (protons + neutrons). In beta decay, a neutron becomes a proton — total nucleon count UNCHANGED. Z + 1 because we have one more proton.
✕Believing half-life changes with temperature or chemical state.
Why it happens
Lots of chemistry does.
How to avoid it
Nuclear decay is independent of chemical and physical conditions. Half-life is fixed per isotope.
✕Saying a radioactive sample becomes 'inactive' after a fixed time.
Why it happens
Half-life sounds like a stopwatch.
How to avoid it
Each half-life HALVES the activity — it never quite reaches zero. After many half-lives, activity is just very small.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Radioactive Decay — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Radioactive Decay

Detailed Notes

What you’ll learn

How it’s examined

1Balancing an alpha decay equation

2Balancing a beta decay equation

3Half-life calculation

4Choosing an isotope for a medical scan

Activity after n half-lives

Radioactive decay

Alpha decay

Beta decay

Gamma decay

Half-life

✕Saying beta decay decreases the mass number.

✕Believing half-life changes with temperature or chemical state.

✕Saying a radioactive sample becomes 'inactive' after a fixed time.

Practice questions

Past paper style quiz

Assess your understanding

Radioactive Decay — frequently asked questions