The half-life of a radioisotope is an essential concept in nuclear physics, chemistry, and various scientific applications. It helps us understand how radioactive substances decay over time, making it useful in fields like medicine, archaeology, nuclear energy, and environmental science.
If you are preparing for a Quizlet study session or an exam, having a strong grasp of half-life will help you answer questions related to radioactive decay, nuclear reactions, and isotope stability.
This guide provides a detailed explanation of half-life, its calculation, decay processes, real-world applications, and common Quizlet-style questions to test your knowledge.
What Is the Half-Life of a Radioisotope?
The half-life of a radioisotope is the time it takes for half of the radioactive atoms in a sample to decay into a more stable form.
Key Characteristics of Half-Life:
✔ Constant for each isotope – It does not change over time.
✔ Independent of external factors – Temperature, pressure, or chemical state do not affect it.
✔ Varies widely between isotopes – Some decay in seconds, while others take millions of years.
✔ Follows an exponential decay pattern – The amount of the isotope reduces by half after each half-life period.
Examples of Half-Life for Common Radioisotopes
| Radioisotope | Half-Life | Uses |
|---|---|---|
| Carbon-14 | 5,730 years | Dating ancient fossils and artifacts |
| Uranium-238 | 4.5 billion years | Geological dating, nuclear energy |
| Iodine-131 | 8 days | Medical treatment for thyroid disorders |
| Radon-222 | 3.8 days | Radiation exposure monitoring |
| Cobalt-60 | 5.3 years | Cancer treatment, sterilization |
How to Calculate Half-Life?
The decay of radioactive substances follows a predictable pattern, which can be calculated using this formula:
Where:
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N = remaining quantity of the isotope
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N₀ = initial quantity of the isotope
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t = time elapsed
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T = half-life of the isotope
✔ Example Question:
A 40-gram sample of Iodine-131 (which has a half-life of 8 days) undergoes decay for 24 days. How much remains?
After 24 days, only 5 grams of Iodine-131 will remain.
Half-Life Quizlet: Key Definitions
Here are some key terms to know when studying half-life on Quizlet:
✔ Radioactive Decay – The process in which unstable atomic nuclei lose energy by emitting radiation.
✔ Parent Isotope – The original radioactive isotope before decay.
✔ Daughter Isotope – The stable isotope that forms after radioactive decay.
✔ Decay Curve – A graph that shows the decrease of radioactive material over time.
✔ Exponential Decay – A process where the substance decreases by a fixed proportion over equal time periods.
Types of Radioactive Decay
Radioactive isotopes decay in different ways, affecting their half-life and energy release:
1. Alpha Decay (α-decay)
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Releases an alpha ptopic (2 protons and 2 neutrons).
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Occurs in heavy elements like Uranium-238.
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Example:
text{Uranium-238} rightarrow text{Thorium-234} + alpha -
Reduces the atomic number by 2 and the mass number by 4.
2. Beta Decay (β-decay)
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Emits a beta ptopic (electron or positron).
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Converts a neutron into a proton (or vice versa).
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Example:
text{Carbon-14} rightarrow text{Nitrogen-14} + beta -
Changes the atomic number by ±1.
3. Gamma Decay (γ-decay)
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Emits high-energy gamma rays.
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Usually occurs after alpha or beta decay to release excess energy.
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Does not change the element’s identity.
Factors That Do Not Affect Half-Life
The half-life of a radioisotope remains constant regardless of:
✔ Temperature – Heating or cooling does not change decay speed.
✔ Pressure – High or low pressure has no effect.
✔ Chemical State – Being in a compound or isolated does not alter half-life.
Common Quizlet Questions on Half-Life
1. What does the term "half-life" mean?
✔ It is the time required for half of a radioactive substance to decay.
2. If a substance has a half-life of 10 hours, what fraction 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 for radiocarbon dating?
✔ Carbon-14.
4. How does half-life affect nuclear waste management?
✔ Isotopes with long half-lives require secure storage because they remain radioactive for thousands of years.
5. What type of decay occurs when a nucleus emits a helium nucleus?
✔ Alpha decay.
Real-World Applications of Half-Life
1. Carbon Dating in Archaeology
✔ Carbon-14 helps determine the age of ancient artifacts and fossils.
2. Nuclear Medicine
✔ Iodine-131 treats thyroid disorders.
✔ Technetium-99m is used in diagnostic imaging.
3. Nuclear Power and Energy
✔ Uranium-235 is used in nuclear reactors to generate electricity.
4. Environmental Science
✔ Radon-222 is monitored to prevent radiation exposure.
Half-Life in Space Exploration
✔ Plutonium-238 (half-life: 88 years) is used in space probes like Voyager.
✔ Scientists estimate the age of celestial objects using radioactive dating.
✔ Example:
Lunar rocks were dated using Uranium-238, confirming the Moon’s age of 4.5 billion years.
Radiation Safety and Half-Life
✔ Radiation Exposure – Prolonged exposure can cause health issues.
✔ Nuclear Waste Management – Long-lived isotopes require safe storage.
✔ Protection Methods:
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Lead shielding for gamma rays.
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Proper waste disposal of radioactive materials.
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Radiation detectors to monitor exposure.
The half-life of a radioisotope is a fundamental concept in nuclear science, medicine, archaeology, and environmental studies. Understanding half-life allows scientists to:
✔ Date ancient artifacts
✔ Develop medical treatments
✔ Safely manage nuclear energy and waste
By mastering this topic, students can confidently answer Quizlet questions and apply this knowledge in real-world situations.