Ustable elements that emit radiation behave chemically the same as any other element. However, radioactive elements will eventually decay into something else. All of the elements above lead in the periodic table are unstable, as are two below lead.
Here are described significant radioactive elements not covered elsewhere.
In the late 1920s, there were only four gaps left in the periodic table, one at number 43. This gap is below manganese and above recently-discovered rhenium. It could be expected to have similar chemical properties. Several false discoveries were announced in the preceding decades.
The invention of the particle accelerator (specifically, the cyclotron) meant that elements could be manufactured. The cyclotron at UC Berkely bombarded a sample of molybdenum with protons in 1936. The sample was examined at the University of Palermo, Sicily, and the new element was found within. It was named technetium after the Greek for "artificial". This is the lowest-numbered element that is always radioactive.
The second element to be manufactured rather than discovered was element 61. This is among the rare earths that all have maddeningly similar chemical properties, but no 61 was ever discovered in a sample of lanthanum or cerium.
This element was possibly created by cyclotron at Ohio State University in 1938, but the claim was not proven. Seven years later, the nuclear pile at Oak Ridge National Laboratory produced many chemical byproducts of uranium fission, and number 61 was definitely one of them.
Prometheum is another low-numbered element that is strictly radioactive. It is named after Prometheus, the god who stole fire from the gods.
Polonium was discovered by the famous scientists Marie Curie and her husband Pierre. She noticed around 1896 that the ore of uranium, which is pitchblende, is more radioactive than uranium itself. The Curies sorted through a large quantity of pitchblende residue from a Bohemian mine and found something new. The new radioactive element was named in honor of Mme. Curie's homeland, Poland.
Polonium is highly radioactive. It emits energetic alpha particles that can, nevertheless, be easily blocked. The element is used in radioisotope thermal generators (RTGs) that power space probes. Russian dissident Alexander Litvinenko was assassinated in 2006 by the Russian secret police by polonium radiation poisoning.
Francium is a highly radioactive metal that is a decay product of actinium and therefore exists in trace amounts in uranium and thorium ores. The most stable isotope has a half-life of only 22 minutes, so it very quickly decays. The element was not found until 1939 by French physicist Marguerite Perey, who named it after her native France.
Francium has no commercial applications. It is even more chemically active than caesium, but visible quantities are never collected in one place.
Actinium is the first element in the separate bottom series of elements in the periodic table. This series is called the actinides, similar to the lanthanides or rare earths. The actinides are more varied chemically, however, and some of them have very brief lifespans, so their chemistry is not of much practical interest.
Actinium was first found in 1899 by chemist Andr&eacut; Debierne in pitchblende when he was removing the actual rare earth elements from the ore.
Actinium is a strongly radioactive silvery metal that glows in the dark. It is only used commercially for its radioactivity. For example, it is used medically for radiation therapy and it has been used in RTGs.
Protactinium is a very minor component of pitchblende ore. Polish-American Kasimir Fajans and German Oswald Helmuth Göhring discovered it in decay products of uranium in 1913. It was later isolated and identified by two different groups of chemists in 1917. This element decays into actinium, hence it is proto-actinium.
Two isotopes of this silvery metal dense metal are produced as decay products of uranium and thorium. Protactinium therefore exists in nature in only small quantities, and it has no commercial applications.
Elements beyond uranium, number 92, are obtained by bombarding a lower-numbered element in a particle accelerator. If the resulting atomic nucleus has more protons, it is a new higher-numbered element. The first of these was created in 1940 at the Berkely Radiation Laboratory. As the planet Neptune is beyond Uranus, so this element neptunium is one beyond uranium. As it happens, though, small amounts of neptunium and the next element, plutonium, are created by radioactive decay in uranium ore. So, a bit of neptunium exists in nature.
Neptunium is a silvery ductile metal of very high density.
Americium and Beyond
Americium is a trans-uranium element (element beyond uranium) first created by Glenn Seaborg's group at UC Berkeley in 1944. Although it must be created syntheticaly, americium is actually used commercially, in smoke detectors.
Several other elements have been created in the last few decades, with atomic numbers up to at least element 118. These all have very short half-lives and exist only for brief periods of time in the laboratory.