Scientists have confirmed the existence of 118 different elements. Some, like gold, silver, copper and carbon, have been known for thousands of years. Others, such as moscovium, nihonium, and tennessine have only recently been created by scientists.

Mendeleev is best remembered for formulating the Periodic Law and creating a farsighted version of the periodic table of elements. He used the Periodic Law not only to correct the then-accepted properties of some known elements, such as the valence and atomic weight of uranium, but also to predict the properties of eight elements that were yet to be discovered.

The first 94 elements occur naturally; the remaining 24, americium to oganesson (95-118), occur only when synthesized in laboratories.

Technetium was isolated by Carlo Perrier and Emilio Segrè in 1937. It was created by bombarding molybdenum atoms with deuterons that had been accelerated by a cyclotron.

Carbon's abundance, its unique diversity of organic compounds, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables this element to serve as a common element of all known life.

Radon is formed by the natural radioactive decay of uranium. Unlike other elements in the decay chain, radon is, under normal conditions, gaseous and easily inhaled. As a result, it is often the single largest contributor to an exposed individual's background radiation dose.

Elements within a group share several common properties and often have the same outer electron arrangement.

Hydrogen has a standard atomic weight of 1.008. It is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass.

Almost 80 percent of the periodic table is composed of metals (shiny elements that conduct heat and electricity well). 15 percent of the elements are nonmetals (poor conductors of heat and electricity). The remaining elements are metalloids, which share properties of both metals and nonmetals.

Science fiction has long imagined alien worlds inhabited by silicon-based life, such as the rock-eating Horta from the original Star Trek series. But according to scientists, the chemistries needed to create a silicon-based lifeform are simply not there. The complex dance of life requires interlocking chains of reactions, and with silicon, these reactions can only take place within a narrow range of temperatures and pH levels.