Elements are a form of matter that can not be broken down by ordinary chemical or physical means. Each element is represented on the periodic table of elements by its own symbol. Currently, 112 elements are known to exist although only 92 of these occur in nature. The other 20 are known as transuranium elements and have only been produced artificially. While elements were once thought to be irreducible, they have since been found to be composed of subatomic particles.
The idea that matter was made up of elements was first proposed by ancient Greek philosophers. Many of these early scholars believed that nature was not fundamentally complex, but was instead made up of a small number of basic materials. These ideas eventually led to a proposal by Empedocles (c. 490-430 B.C.) in the fifth century B.C. that all matter was based on four elements: earth, air, fire, and water. He imagined that every substance was formed by blending various proportions of two or more of these elements. His concept was adopted and further developed by a variety of other scholars including Aristotle.
Over the next 2,000 years, our concept of the composition of matter evolved slowly. Most of the study of chemistry was dominated by pseudoscientific alchemists who focused their efforts on turning cheap metals into gold. However, some new discoveries were made during this time. For example, in the eighth century A.D. the Arabic philospher Jabir ibn Haiyan (c. 721-815) discovered mercury and sulfur. He suggested that these substances were the only true elements.
As more and more "fundamental" materials were discovered, the concept of elements became confused. Some scholars tried to maintain the Greek ideas that matter was composed of only a small number of elements. However, these new discoveries made this notion impossible. In 1661, Robert Boyle proposed the modern definition for chemical elements. In his theory, any substance that could not be broken down into simpler substances was an element. As this theory was accepted, the Greek system of four elements was discarded.
With Boyle's definition of elements in hand, scientists began to compile lists of elements. In 1789, Antoine-Laurent Lavoisier (1743-1794) published the first modern textbook of chemistry, Elementary Treatise on Chemistry. This book contained a list of 33 known elements. Many of these substances, such as sulfur and phosphorous, continue to be considered elements. Others such as chaux and silice were found to be compounds. Two of the materials on Lavoisier's list, lumiere (light) and calorique (heat) were shown to be forms of energy.
Over time it became apparent to scientists that certain elements had similar properties and may be organized in groups. The first chemist to formally recognize that the properties of elements followed certain patterns was Johann Dobereiner. He developed a system that grouped elements in triads, but applications of this approach were severely limited. Other scientists proposed alternate systems to describe the periodic nature of the properties of elements. Eventually two scientists, Dmitri Mendeleev and Julius Lothar Meyer, independently developed the modern periodic table of elements. It was first published in 1872.
The periodic table of elements lists substances by their atomic symbol. Each element is made up of a single type of atom that has a distinct atomic weight and atomic number. The atomic number is the number of protons in the atom's nucleus. All elements can be classified as either metals or nonmetals, although sometimes a separate class called metalloids is used. Elements that have similar chemical and physical properties are grouped together on the periodic table. These groups represent families of elements such as noble gases, halogens, alkaline earth metals, and alkali metals.
It is estimated that 25 of the known elements are critical to the existence of life. In fact, four of these (oxygen, carbon, nitrogen, and hydrogen) make up 96% of all living matter. Oxygen is a primary component of all types of biomolecules, including polysaccharides, nucleic acids, and proteins. It makes up about 65% of human body weight. Carbon is also found in biomolecules, constituting 18.5% of human body weight. Nitrogen is an important component in proteins. Other elements such as phosphorous, calcium, potassium, sulfur, sodium, chlorine,and magnesium make up a large portion of the remaining 4%.
Trace elements are substances that organisms require in tiny amounts. Iron is one trace element that is required by nearly all forms of life. Certain trace elements are required only by certain organisms. For example, iodine is a required element in vertebrates. Other significant trace elements include boron, chromium, cobalt, copper, fluorine, iodine, manganese, molybdenum, selenium, silicon, tin, vanadium, and zinc.
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