The half-life of a radioisotope is one of the most important concepts in nuclear physics and chemistry. It helps scientists determine how quickly a radioactive substance decays and is crucial in fields like medicine, archaeology, nuclear energy, and environmental science.
If you are studying for a quizlet or an exam, understanding the half-life of a radioisotope will help you answer questions about radioactive decay, nuclear reactions, and real-world applications.
This topic will provide a detailed explanation of half-life, how it is calculated, and common questions that appear in quizzes and exams.
What Is the Half-Life of a Radioisotope?
The half-life of a radioisotope is the time required for half of the radioactive atoms in a sample to decay into a more stable form. This decay follows an exponential pattern, meaning that with each half-life period, half of the remaining radioactive material is lost.
✔ Key Points to Remember:
- The half-life is constant for each isotope.
- It does not change due to external factors like temperature or pressure.
- Different isotopes have different half-lives, ranging from fractions of a second to billions of years.
- The decay process follows an exponential function.
Examples of Half-Lives for Common Radioisotopes
| Radioisotope | Half-Life | Common Uses |
|---|---|---|
| Carbon-14 | 5,730 years | Archaeological dating |
| Uranium-238 | 4.5 billion years | Geological dating, nuclear power |
| Iodine-131 | 8 days | Medical treatments (thyroid diseases) |
| Radon-222 | 3.8 days | Radiation exposure monitoring |
| Cobalt-60 | 5.3 years | Cancer treatment, sterilization |
How to Calculate Half-Life?
The formula to calculate half-life is:
Where:
- N = remaining quantity of the isotope
- N₀ = initial quantity
- t = time elapsed
- T = half-life of the isotope
✔ Example Question:
A sample of Iodine-131 (T = 8 days) starts with 40 grams. How much will remain after 24 days?
Thus, after 24 days, only 5 grams of Iodine-131 will remain.
Half-Life Quizlet: Key Terms and Definitions
To prepare for a quizlet or exam, it’s helpful to memorize key terms related to half-life:
✔ Radioactive Decay: The process by which an unstable atomic nucleus loses energy by emitting radiation.
✔ Parent Isotope: The original radioactive isotope before decay.
✔ Daughter Isotope: The stable isotope formed after radioactive decay.
✔ Decay Curve: A graph that shows the decrease of a radioactive substance over time.
✔ Exponential Decay: A process where the amount of radioactive material decreases by a constant fraction over equal time periods.
Types of Radioactive Decay
Radioisotopes decay through different processes, which affect how their half-lives behave:
1. Alpha Decay (α-decay)
- Emits an alpha ptopic (2 protons and 2 neutrons).
- Example: Uranium-238 → Thorium-234.
- Reduces atomic number by 2 and mass number by 4.
2. Beta Decay (β-decay)
- Emits a beta ptopic (electron or positron).
- Example: Carbon-14 → Nitrogen-14.
- Increases or decreases the atomic number by 1.
3. Gamma Decay (γ-decay)
- Emits gamma rays (high-energy photons).
- Does not change the element but lowers its energy.
- Often occurs alongside alpha or beta decay.
Factors Affecting Half-Life
Half-life is a fixed property of an isotope and does not change due to:
✔ Temperature
✔ Pressure
✔ Chemical reactions
However, half-life can appear different based on how the isotope interacts with other substances.
Common Half-Life Quiz Questions and Answers
1. What is the definition of half-life?
✔ The time required for half of a radioactive substance to decay.
2. If an isotope has a half-life of 10 hours, how much remains after 30 hours?
✔ After 10 hours: 50% remains.
✔ After 20 hours: 25% remains.
✔ After 30 hours: 12.5% remains.
3. Which isotope is commonly used in carbon dating?
✔ Carbon-14 (C-14).
4. How does half-life affect radioactive waste disposal?
✔ Isotopes with long half-lives require secure storage because they remain hazardous for thousands of years.
5. What type of decay occurs when an isotope emits a helium nucleus?
✔ Alpha decay.
Real-World Applications of Half-Life
1. Carbon Dating in Archaeology
✔ Used to determine the age of fossils and ancient artifacts.
✔ Based on the 5,730-year half-life of Carbon-14.
2. Nuclear Medicine
✔ Iodine-131 is used to treat thyroid conditions.
✔ Technetium-99m is used in medical imaging.
3. Nuclear Power
✔ Uranium-235 is used as fuel in nuclear reactors.
✔ Half-life calculations help control nuclear reactions safely.
4. Environmental Science
✔ Radon-222 is monitored in homes to prevent radiation exposure.
✔ Cesium-137 is tracked in ecosystems after nuclear disasters.
Half-Life in Space and Cosmic Research
✔ Plutonium-238 (half-life: 88 years) is used in space probes like the Voyager mission.
✔ Half-life calculations help scientists determine the age of planets and meteorites.
✔ Example:
The Moon’s age was estimated using Uranium-238 dating, proving it is 4.5 billion years old.
Half-Life and Safety Concerns
✔ Radiation Exposure: Can cause cell damage and mutations.
✔ Nuclear Waste Disposal: Long-lived isotopes require secure storage.
✔ Protective Measures:
- Lead shielding for gamma radiation.
- Proper disposal of radioactive materials.
- Radiation monitoring for workers and the public.
The half-life of a radioisotope is a key principle in nuclear physics, medicine, archaeology, and environmental science. Understanding half-life helps scientists:
✔ Date ancient artifacts
✔ Develop life-saving medical treatments
✔ Safely manage nuclear energy and waste
By mastering this topic, students can confidently answer quiz questions and apply this knowledge to real-world scenarios.