IB MYP 4-5 Chemistry -Radioactivity- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Radioactivity- Study Notes
Key Concepts
- Radioactivity
Radioactivity
Radioactivity
Radioactivity is the spontaneous process by which unstable atomic nuclei emit radiation to become more stable. This radiation can include alpha particles (α), beta particles (β), or gamma rays (γ). It occurs naturally in certain isotopes known as radioisotopes.
Origin of Radioactivity
Atoms with an unstable nucleus have an imbalance between the number of protons and neutrons. To reach stability, these nuclei release energy in the form of radiation — a process called nuclear decay.
Example of unstable isotopes: \( \mathrm{U^{238},\ C^{14},\ Ra^{226}} \)
Radioactivity is a nuclear phenomenon, not a chemical reaction — it changes the nucleus of an atom, often forming a new element.
Types of Radiation
| Type of Radiation | Symbol | Nature | Penetrating Power | Stopped By |
|---|---|---|---|---|
| Alpha (α) | \( \mathrm{^4_2He^{2+}} \) | Helium nucleus (2 protons + 2 neutrons) | Low | Paper or skin |
| Beta (β⁻) | \( \mathrm{^0_{-1}e} \) | High-speed electron | Medium | Thin aluminum sheet |
| Gamma (γ) | \( \mathrm{\gamma} \) | Electromagnetic wave | Very high | Thick lead or concrete |
Nuclear Decay Reactions
(a) Alpha Decay (α-decay):
The nucleus emits an alpha particle, reducing both mass and atomic numbers.
\( \mathrm{^{238}_{92}U \rightarrow ^{234}_{90}Th + ^4_2He} \)
Mass number decreases by 4; atomic number decreases by 2.
(b) Beta Decay (β-decay):
A neutron changes into a proton, releasing an electron (beta particle).
\( \mathrm{^{14}_6C \rightarrow ^{14}_7N + ^0_{-1}e} \)
Mass number stays the same; atomic number increases by 1.
(c) Gamma Emission (γ-emission):
After α or β decay, the nucleus may release extra energy as gamma radiation.
\( \mathrm{^{60}_{27}Co^* \rightarrow ^{60}_{27}Co + \gamma} \)
No change in atomic or mass number — only energy is released.
Applications of Radioactivity
- Medicine: Radioisotopes like \( \mathrm{Co^{60}} \) are used in cancer treatment (radiotherapy).
- Industry: Detecting pipeline leaks and measuring thickness of materials.
- Carbon Dating: \( \mathrm{C^{14}} \) used to determine the age of fossils.
- Energy Production: Controlled nuclear fission generates electricity in power plants.
Equation (Example — Carbon Dating):
\( \mathrm{^{14}_6C \rightarrow ^{14}_7N + ^0_{-1}e} \)
Dangers and Safety Precautions
Hazards:
- Ionizing radiation can damage living cells and DNA.
- Prolonged exposure causes radiation burns, cancer, or genetic mutations.
Safety Precautions:
- Use protective clothing, gloves, and lead shielding.
- Limit exposure time and maintain safe distance from radioactive sources.
- Store radioactive materials in lead containers.
- Dispose of radioactive waste safely under strict regulations.
Characteristics of Radiation
| Type | Charge | Mass | Penetration | Stopped By |
|---|---|---|---|---|
| Alpha (α) | +2 | 4 u | Low | Paper |
| Beta (β⁻) | −1 | 1/1836 u | Medium | Aluminum |
| Gamma (γ) | 0 | 0 | Very High | Lead or Concrete |
Example
Write a balanced nuclear equation for the alpha decay of uranium-238.
▶️ Answer / Explanation
Step 1: In α-decay, the nucleus emits a helium nucleus (\( \mathrm{^4_2He} \)).
Step 2: \( \mathrm{^{238}_{92}U \rightarrow ^{234}_{90}Th + ^4_2He} \)
Final Answer: Uranium-238 decays to thorium-234 with emission of an alpha particle.
Example
A sample of a radioisotope has a half-life of 10 hours. If 40 g of the isotope is present initially, how much will remain after 30 hours?
▶️ Answer / Explanation
Step 1: \( \mathrm{t = 30\ h,\ t_{1/2} = 10\ h} \)
Step 2: Number of half-lives = \( \mathrm{30 / 10 = 3} \)
Step 3: \( \mathrm{N = N_0 \left(\dfrac{1}{2}\right)^3 = 40 \times \dfrac{1}{8} = 5\ g} \)
Final Answer: 5 g of the isotope remains after 30 hours.
Example
Compare the properties and effects of alpha, beta, and gamma radiations in terms of penetration, ionization, and safety precautions.
▶️ Answer / Explanation
Step 1: Alpha particles have high ionizing power but low penetration — stopped by paper or skin.
Step 2: Beta particles penetrate further but are blocked by thin metal sheets.
Step 3: Gamma rays are highly penetrating, requiring thick lead shielding.
Step 4: All radiations are harmful; exposure should be minimized by using proper protective measures.
Final Answer: Alpha → low penetration, high ionization; Beta → medium both; Gamma → high penetration, low ionization; all need controlled handling for safety.