We’re coming to the end of the rare elements on the periodic table. The last section, known as actinides, rests on the very bottom of the table and includes all the elements with atomic numbers between 90 and 109. These aren’t elements that you’ll find in your kitchen though — or even in a typical science lab unless you happen to live near a particle accelerator.
Let’s take a look at the actinides group, how abundant they are and what sets them apart from the other elements on the periodic table.
Properties of Actinides
In spite of the fact that 15 elements fall under the actinides classification, but you can only five of them in nature. For decades, the periodic table ended with uranium, with an atomic number of 92. The radioactive element was the heaviest element ever discovered, and scientists didn’t think that they would ever find anything heavier — until the discovery of neptunium in 1940.
Fun fact: Every element in the actinides group that comes after uranium is classified as transuranium, or heavier than uranium.
The elements in the actinides group include:
Two of these are found in the Earth’s crust — thorium and uranium. Plutonium and neptunium are found in uranium ore veins. Actinium and protactinium both appear during the decay of thorium and uranium.
All of the rest of these elements are considered synthetic and are produced in a lab or a particle accelerator.
Fun fact: There exist elements that are heavier than Lawrencium, the last actinide in the family, but they belong to the lanthanide, or rare earth element, group.
Chemical & Physical Traits
Every element in the actinides family is highly radioactive. There have, to this day, been no stable isotopes discovered for any actinides. The metals, when exposed to air, tarnish quickly and can even ignite. They also tend to be very malleable and ductile — easy to shape and form — though you don’t want to handle any of these elements with your bare hands. They are all solid at room temperature, and when placed in boiling water or diluted acid, they produce hydrogen gas.
All of these elements are electropositive. When they react with other elements, they give up their electrons fairly easily to form positive ions. Each element, except actinium, has multiple allotropes, different physicals forms, that can be found or created. Plutonium has at least six allotropes!
All of the members of this elemental family are also paramagnetic. This means that while they can demonstrate some weak attraction to magnetic poles, they are not inherently magnetic and don’t retain any magnetism once you remove the magnetic field.
You won’t encounter most of these elements in your everyday life, but that doesn’t mean that they don’t have any real-life applications.
The most commonly used element in this family is uranium and its various isotopes. The inherent radioactivity of actinides makes them useful in a variety of useful, although potentially dangerous, ways. Uranium-238 is the most abundant isotope of this element and has an unusually long half-life. “Half-life” refers to the amount of time that it takes half of a given sample to decay. The half-life of uranium-238 is so long that half of the uranium that exists today has been around since the birth of our planet roughly 4.5 billion years ago.
Uranium-235, on the other hand, is fissionable, meaning the atoms can be split apart when bombarded by neutrons. This capability allows it to be used in power plants to generate energy and in the creation of nuclear weaponry.
Thorium might be one of the most abundant actinides, but that doesn’t mean there’s a lot of it as compared to materials outside the actinide family. Annually, industry mines only a few hundred tons of this element and use most of that in the production of gas mantles. Thorium’s high melting and boiling points make thorium an ideal insulator. It melts at 3,100F and boils at 8,100F, so there’s plenty of room for error.
Plutonium is another actinide that is used in the production of nuclear weapons and can be a source of power for nuclear plants. It has also been used to power spacecraft, as the energy source for a radioisotope thermoelectric generator. Curiosity, the rover that is currently exploring Mars, has one of these generators and uses roughly 4.8 kilograms of plutonium-238 as its primary power source. Plutonium was even used in heart pacemakers for decades because it enabled the pacemaker battery to last longer. While it isn’t used as a pacemaker power source anymore, there are probably still people with plutonium-powered pacemakers in their chests. Say that five times fast.
Californium is useful for creating neutrons. In a detection device, this can be used to identify gold or silver on the spot without having to send samples to a lab. You can also find it in smoke detectors and moisture gauges.
Curium, named for Marie Curie and her husband Pierre, is primarily used for scientific research but it has some emerging uses as well. It produces more energy than plutonium, enabling it to be used as a power source for spacecraft and in medicine.
Americium is another element that is under consideration for use as spacecraft fuel. It’s also a component in smoke detectors. One isotope, americium-241, is used in brachytherapy, a treatment for cancer.
The majority of the other actinides on this list are used solely for research purposes. Scientists must create them in a lab, and many have only been produced a couple of atoms at a time. This allows scientists to observe them but makes them essentially useless for anything other than lab work and research.
You don’t want to include actinides in your next lab experiment unless you’ve got lots of protective equipment and the proper expertise. The inherent radioactivity of these elements makes them dangerous. Marie Curie’s research notebooks are still radioactive nearly a century after her death and probably will be for another 1,500 years. Even her body is buried in a coffin lined with lead because she absorbed so much radioactivity during her life.
While Marie Curie might not have worked with the actinide that bears her name, she discovered polonium, another radioactive element that she named after her home country of Poland. Polonium is 100 times more radioactive than uranium. She would probably be fascinated to see how many new radioactive elements we’ve discovered since her death.
You might not see these elements in your everyday life, but they’ve got plenty of applications that help make modern life and space exploration possible.