Radioactivity is a central topic in IB Chemistry Topic 12 (Atomic Structure). It explains why some nuclei are unstable, how they break down over time, and why they emit radiation. Radioactivity connects directly to nuclear reactions, half-life, decay equations, mass defect, and binding energy. Understanding this concept helps IB students make sense of nuclear stability, medical imaging, radiometric dating, and energy production.
What Is Radioactivity?
Radioactivity is the spontaneous emission of particles or energy from an unstable atomic nucleus.
This happens because:
- Some nuclei contain too many protons or neutrons
- The strong nuclear force cannot hold them together efficiently
- They release energy to achieve greater stability
The emitted energy or particles is called radiation.
Radioactivity is completely natural and occurs in rocks, soils, stars, and even inside the human body.
Why Some Nuclei Are Unstable
A nucleus becomes unstable when:
- Proton-to-neutron ratio is incorrect
- There are too many nucleons overall
- Binding energy per nucleon is too low
Large nuclei (like uranium or radium) are especially unstable because:
- Protons repel each other strongly
- The strong nuclear force cannot overcome repulsion across a large distance
To reach stability, they undergo radioactive decay.
Types of Radioactive Emissions
IB Chemistry focuses on three main types: alpha, beta, and gamma radiation.
1. Alpha radiation (α)
- Consists of helium nuclei (²⁴He)
- Positively charged
- Low penetrating power
- Stopped by paper or skin
Occurs in heavy nuclei like uranium.
2. Beta radiation (β⁻ or β⁺)
Beta-minus (β⁻):
- A neutron turns into a proton
- Emits an electron
Beta-plus (β⁺):
- A proton turns into a neutron
- Emits a positron
Beta particles have medium penetrating power.
3. Gamma radiation (γ)
- High-energy electromagnetic radiation
- No mass, no charge
- Extremely penetrating
- Often released after alpha or beta decay
Gamma emission reduces energy, not particle number.
Radioactive Decay Equations (IB Level)
A decay equation shows how a nucleus emits radiation and changes into a new element.
Examples:
Alpha decay:
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Beta-minus decay:
¹⁴₆C → ¹⁴₇N + β⁻
Beta-plus decay:
¹¹₆C → ¹¹₅B + β⁺
Gamma decay:
⁶⁰₂₇Co* → ⁶⁰₂₇Co + γ
These appear in IB Paper 1 and Paper 2.
Why Radioactive Decay Occurs
Radioactive decay is driven by the search for greater nuclear stability.
Unstable nuclei:
- Have low binding energy
- Have incorrect proton–neutron ratios
- Are too large to hold together
Decay allows them to transform into more stable isotopes.
Radioactive decay is:
- Spontaneous (not controlled by external conditions)
- Random (cannot predict when a specific atom will decay)
- Exponential (described by half-life)
Radioactivity and Half-Life
Half-life is the time required for half the radioactive nuclei in a sample to decay.
Radioactive decay follows an exponential pattern:
- After 1 half-life → 50% remains
- After 2 half-lives → 25% remains
- After 3 half-lives → 12.5% remains
Half-life does not change with temperature, pressure, or chemical state.
How Radioactivity Is Detected
Common instruments include:
1. Geiger–Müller tube
Detects individual radiation events.
2. Cloud chambers
Visualize tracks of charged particles.
3. Scintillation counters
Measure gamma rays.
These tools help identify the type and strength of radiation emitted.
Uses of Radioactivity
Radioactivity has many beneficial applications:
1. Medical imaging
- PET scans
- Cancer treatment (radiotherapy)
2. Carbon dating
Using C-14 to determine age of organic materials.
3. Industrial applications
- Leak detection
- Sterilization
- Thickness monitoring
4. Energy production
Nuclear fission reactors use radioactive fuel.
Dangers of Radioactivity
Although useful, radiation can be harmful in large doses:
- Causes ionization
- Damages DNA
- Increases cancer risk
- Requires shielding (lead, concrete)
Understanding these risks is part of IB safety considerations.
Common IB Misunderstandings
“Radioactivity can be sped up or slowed down.”
Decay rate is constant and unaffected by conditions.
“Gamma radiation has mass.”
Gamma rays are pure energy with no mass.
“All radioactive materials are dangerous at small doses.”
Risk depends on intensity, exposure time, and type of radiation.
“Stable isotopes can suddenly become radioactive.”
Stability is intrinsic to the nucleus.
FAQs
Why do large nuclei tend to be unstable?
Electrostatic repulsion between protons overwhelms nuclear forces at long distances.
Can radioactive decay produce heat?
Yes—radioactive decay releases energy that warms Earth’s interior.
Is radioactivity always harmful?
Low, controlled doses are used safely in medicine.
Conclusion
Radioactivity is the spontaneous emission of energy or particles from an unstable nucleus. It occurs because nuclei seek greater stability and is characterized by alpha, beta, and gamma emissions. Understanding radioactivity helps IB Chemistry students grasp nuclear stability, half-life, decay mechanisms, and the enormous energy involved in nuclear processes.
