The periodic table of the elements lists 118 elements. However, not all of these elements occur naturally. Those that do not occur in nature in even tiny amounts have only been produced under laboratory or otherwise artificial conditions. Of the 118 elements, 98 have been found in nature, although some of these, those with the highest atomic weights, occur in very small quantities and in very specific conditions.
What is an element?
An element is a substance in which every molecule consists of only one type of atom. All these atoms are of identical structure. If the individual atom were broken down any more, the element would no longer exhibit the same properties and would therefore be another element.
Elements are classified by their atomic number, which represents the number of protons found in the nucleus. By extension, this also represents the number of electrons in each individual atom. An atom in which the number of electrons and protons is not equal carries either a positive or negative electric charge and is considered an ion, or a variation on the individual element. Neutrons, which have no charge, are not calculated in the atomic number, but are included in the element's atomic weight. An element can exist with the same number of protons and electrons, but with different numbers of neutrons; this is known as an isotope. In elements with higher atomic numbers, the number of neutrons often determines the element's stability. Therefore some isotopes of these elements are more common than others and are easier to find in nature or to create in a laboratory.
Naturally occurring elements
The first periodic table of the elements to be widely recognized, formulated in 1869 by Dmitri Mendeleev, featured just over 60 elements and predicted the existence of several more based on the structure of the periodic table. Mendeleev also posited that certain characteristics of elements could be determined by their atomic weights, leading the known elements to be classified according to their chemical properties. His predictions proved to be true, and new elements were eventually discovered to fill the gaps in the periodic table as it existed at that time.
The first periodic tables represented only elements that had been discovered in nature, and scientists continued to populate the table as new elements were discovered. As science progressed, however, nuclear theory enabled physicists to create new elements, either in laboratory conditions or as byproducts of nuclear reactions.
Most of the artificially occurring elements in the periodic table have been produced in the lab or during nuclear reactions. These elements have very high atomic weights and tend to be unstable. At one time, many of the unstable radioactive elements were thought not to exist in nature at all, but some have been found in trace quantities in a mineral called uraninite. This uranium-rich mineral is also referred to as pitchblende and is found in silver mines.
Since the 15th century, miners have been aware of uraninite, but not until recently were scientists able to break down its chemical structure to discover extremely rare elements such as technetium, plutonium, and californium. These elements were first synthesized in laboratory experiments and found in nuclear fallout or nuclear energy byproducts. Until they were found in uraninite, it was thought they did not exist in nature. Californium, with an atomic weight of 98, is the heaviest element found in nature so far.