Discovery

• The Russian scientist Dimitri Mendeleev first put together the 65 elements that were known at the time into the periodic table of elements in 1869. These elements were listed according to their atomic weight and chemical valency. Mendeleev also predicted that there were unknown elements that later came to be known as the lanthanides.
  
  

Synthetic Elements

• Out of the 118 elements on the period table, only 92 are found naturally, according to the website Science Clarified, some of which can only be reached by radioactive decay from other natural elements. The remaining elements are synthetic elements that are produced with the help of humans, most of them in particle accelerators. The first element to be produced synthetically was technetium (which occurred rarely in nature) in 1937 by Italian American physicist Emilio Segrè and his colleague C. Perrier.
  
  

Hydrogen

• Hydrogen, the first element on the table, has only a single electron orbiting one proton. There are no neutrons in the nucleus. It constitutes 75 percent of the elemental mass and 90 percent of the visible matter in the universe. In stars, hydrogen nuclei are fused into helium in the process known as nuclear fusion.
  
  

Ununoctium

• The last element on the periodic table, ununoctium, wasn't created until 2006. It has a half-life of 0.00089 seconds, meaning that half of it decays into another element in that period of time. It is not known which state ununoctium appears in at room temperature, but it is believed to be a gas. The instability of higher elements means that there is a limit to the number of elements that can exist.
  
  

Isotopes

• Many elements are unstable and eventually decay, particularly isotopes. Isotopes are elements with a varying level of neutrons. An isotope of potassium-40 has a half-life of about 1.25 billion years. Carbon-14, on the other hand, has a half-life of about 5,730 years. The reason carbon-14 doesn't run out is because it is constantly replenished.