Over the ages, many ideas were formed about what all matter is made of. The ancient Greeks believed there to be only four elements- air, earth, fire, and water. We now know that elements are the simplest type of a pure substance. Because of the many different properties of elements, they have been organized in lots of different ways. Dmitri Mendeleev devised the periodic table that we use today. He lined up all of the known elements and then put them in increasing order by atomic mass, leaving room for those that he predicted were missing. There were some errors in Mendeleev's periodic table, however. When the elements are arranged in order of increasing atomic mass, several elements appeared out of place. Years later, Henry Moseley, a British scientist, discovered the atomic number of elements, and everything fit properly in the periodic table when they were placed in order by increasing atomic number.
Mendeleev knew that there were gaps in the table because some elements that were next to each other didn't share common properties. He was able to predict the properties of the missing elements by looking at the properties of elements next to them and the elements that were in the same family.
Radon was discovered in 1900. This element was discovered Friedrich Ernst Dorn. He discovered it in Germany while he was studying the radium decay chain. The most abundant isotope of this element is Radon-222, which is formed by the radioactive decay of radium-226.
Since this element is radioactive, there aren't very many uses for it. During the 1950s, radon was used in luminous paint of the numerals of clocks, but when it was found to be radioactive in 1962, those clocks were recalled. Radon is also used in the treatment of cancer. It is currently...