Isotopes are forms of an element that have different numbers of neutrons. All isotopes of an element have the same atomic number and number of protons, but they have different atomic masses from each other. Isotopes of an element share similar chemical properties, but have different nuclear properties.
Every element has isotopes. The 81 stable elements have 275 isotopes. But, elements with stable isotopes also have radioactive isotopes or radioisotopes. The radioactive elements, on the other hand, have no stable isotopes. Over 800 radioactive isotopes have been identified. Some of the radioactive isotopes are natural, while others have only been produced in the laboratory.
Key Takeaways: Isotopes
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
- All isotopes of an element have the same atomic number but different mass numbers.
- Isotopes share the same chemical properties but may have different nuclear stability and physical properties.
- Some isotopes are stable, while others are radioactive and undergo decay.
- Scientists use isotopes in medicine, archaeology, power generation, and scientific research.
Isotope Definition
An isotope is one of two or more forms of an element that have the same number of protons but different numbers of neutrons in the nucleus. Because they have the same number of protons, isotopes occupy the same place on the periodic table and exhibit nearly identical chemical behavior. However, differences in neutron count affect their mass and nuclear stability.
Word Origin
The term isotopes was coined by Scottish doctor Margaret Todd in 1913. She suggested the word to chemist Frederick Soddy. He adopted it for use in chemistry and physics. The word comes from the Greek words isos (equal) and topos (place). Isotopes of an element occupy the same position on the periodic table. The atomic mass of an element on the periodic table is an average of the masses of the naturally occurring isotopes of that element.
Isotope Notation
There are two common methods of writing isotopes:
(1) List the element name or symbol first, followed by a dash and then the mass number of the isotope. For example, hydrogen-3 or H-3 refers to the hydrogen isotope with 1 proton and 2 neutrons, which add together to give a mass number of 3. Carbon-12 or C-12 refers to the stable isotope of carbon with 6 protons and 6 neutrons.
(2) Cite the mass number or both the mass number and atomic number on the upper lefthand side of an element symbol. For example, the isotope of carbon with 6 protons and 6 neutrons is 126C . Ideally, the mass number is positioned directly over the atomic number, but this is not always possible for typed notation.
Isotope Examples
The isotopes of hydrogen are hydrogen-1 (protium, which is a stable isotope), hydrogen-2 (deuterium, which is another stable isotope), and hydrogen-3 (tritium, which is a radioisotope).
Uranium-235 and uranium-238 are two isotopes of uranium. Both are natural isotopes that occur in the Earth’s crust.
Carbon-12 and carbon-14 are two carbon isotopes. Carbon-12 is stable, while carbon-14 is radioactive.
Isotope Examples Table
| Element | Isotope | Mass Number | Type | Notes |
|---|---|---|---|---|
| Hydrogen | ¹H (protium) | 1 | Stable | 0 neutrons |
| Hydrogen | ²H (deuterium) | 2 | Stable | 1 neutron |
| Hydrogen | ³H (tritium) | 3 | Radioactive | Half-life ≈ 12.3 years |
| Carbon | ¹²C | 12 | Stable | Most common carbon isotope |
| Carbon | ¹⁴C | 14 | Radioactive | Useful in radiocarbon dating |
| Uranium | ²³⁵U | 235 | Radioactive | Fissile isotope |
| Uranium | ²³⁸U | 238 | Radioactive | Most abundant uranium form |
Parent and Daughter Isotopes
When a radioisotope undergoes radioactive decay, the starting isotope is called the parent isotope. Decay produces one or more daughter isotopes. For example, uranium-238 is the parent isotope that decays into the daughter isotope thorium-234.
Stable vs Radioactive Isotopes
Most elements have both stable and radioactive isotopes, also called radioisotopes. The key difference between them lies in nuclear stability:
Stable Isotopes
- Do not undergo radioactive decay.
- Occur naturally and remain unchanged indefinitely.
- Example: Carbon-12 (¹²C), Oxygen-16 (¹⁶O), and Iron-56 (⁵⁶Fe).
Radioactive Isotopes
- Unstable nuclei that emit radiation as they decay into other elements.
- Are natural (e.g., uranium-238) or synthetic (e.g., technetium-99m).
- Some have short half-lives and decay quickly, while others persist for billions of years.
There are 275 known stable isotopes and over 800 radioactive isotopes.
Isotope vs Nuclide
An isotope refers to a sample of atoms. When the number of protons and neutrons of an individual atom is studied, it is called a nuclide of the element. Nuclear scientists prefer the term nuclide over the term isotope. Nuclides with the same mass number as each other are called isobars. For example, argon-40, potassium-40, and calcium-40 are isobars.
Uses of Isotopes
Isotopes play essential roles in medicine, science, industry, and energy production:
Medicine
- Diagnostic imaging: Technetium-99m and iodine-123 find use in scans.
- Cancer treatment: Cobalt-60 and iodine-131 emit radiation that targets tumors.
Science and Research
- Radiocarbon dating: Carbon-14 helps estimate the age of organic materials.
- Tracer studies: Deuterium and other stable isotopes track chemical pathways.
Industry
- Quality control and inspection: Gamma-emitting isotopes like cobalt-60 reveal defects in materials.
- Smoke detectors: Americium-241 helps detect smoke particles.
Energy
- Nuclear fuel: Uranium-235 and plutonium-239 are important in fission reactors and weapons.
How Isotopes Are Identified
Scientists identify isotopes using techniques that distinguish between atoms based on their mass or nuclear properties:
1. Mass Spectrometry
- Separates ions by mass-to-charge ratio.
- Accurately determines isotopic composition and atomic weights.
2. Nuclear Magnetic Resonance (NMR)
- Detects specific isotopes (e.g., ¹H, ¹³C, ³¹P) based on how their nuclei respond to magnetic fields.
3. Radiation Detection
- Uses Geiger counters, scintillation detectors, or cloud chambers for measuring emissions from radioactive isotopes.
4. Neutron Activation Analysis
- Identifies elements and isotopes by irradiating a sample with neutrons and detecting the resulting gamma rays.
These tools are essential in environmental science, archeology, biochemistry, nuclear physics, and geology.
References
- Nagel, Miriam C. (1982). “Frederick Soddy: From Alchemy to Isotopes.” Journal of Chemical Education. 59 (9): 739–740. doi:10.1021/ed059p739
- Soddy, Frederick (1913). “Intra-atomic charge.” Nature 92 (2301), Springer Nature Publishing AG. doi:10.1038/092399c0
- Strömholm, Daniel; Svedberg, Theodor (1909). “Untersuchungen über die Chemie der radioactiven Grundstoffe II.” (Investigations into the chemistry of the radioactive elements, part 2). Zeitschrift für anorganischen Chemie. 63: 197–206.