A Guide to the Actinides
Explore the radioactive f-block elements that define the nuclear age.
What Are Actinides?
The actinides (or actinoids) are a series of 15 metallic chemical elements with atomic numbers 89 through 103, from Actinium (Ac) to Lawrencium (Lr). They are the second row of the f-block elements and are located below the lanthanides in standard periodic table layouts.
The defining characteristic of the actinides is that all of them are radioactive. This means their atomic nuclei are unstable and spontaneously decay, releasing energy and particles. Only Thorium and Uranium occur naturally on Earth in significant quantities; the rest are produced synthetically in nuclear reactors or particle accelerators.
Key Characteristic: Radioactivity
Every actinide element is radioactive. Their large, unstable nuclei undergo radioactive decay, transforming into other elements over time.
- Unstable Nuclei: The large number of protons and neutrons in their nuclei creates immense repulsive forces, leading to instability.
- Transuranic Elements: All actinides beyond Uranium (atomic number 92) are called "transuranic" and are man-made. They are highly unstable with very short half-lives.
This radioactivity is the source of their power in nuclear reactors and weapons, but also makes them extremely hazardous to handle.
Location on the Periodic Table
The actinides are placed in the bottom row of the two rows shown below the main body of the periodic table. They belong in period 7.
Uses of Actinides
Due to their radioactivity, the uses of actinides are highly specialized and controlled.
| Element | Symbol | Key Use(s) |
|---|---|---|
| Thorium | Th | Potential fuel for a safer type of nuclear reactor, gas mantles. |
| Uranium | U | The primary fuel for conventional nuclear power reactors and nuclear weapons. |
| Plutonium | Pu | Used as fuel in some nuclear reactors and is a key component in modern nuclear weapons. Also used as a power source for deep-space probes (e.g., Voyager, Curiosity rover). |
| Americium | Am | Tiny amounts are used in most common household smoke detectors as a source of ionizing radiation. |
Key Takeaways
- Actinides are the second row of the f-block, located below the lanthanides.
- Their single most important property is that all of them are radioactive.
- Only Thorium and Uranium are found naturally in significant amounts; the rest are synthetic.
- They are central to all nuclear applications, including power generation, weapons, and specialized power sources.