An oil painting of a chemist (by Henrika Šantel in 1932).
Chemistry is the scientific discipline involved with elements and compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during a reaction with other substances.
In the scope of its subject, chemistry occupies an intermediate position between physics and biology. It is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry (botany), the formation of igneous rocks (geology), how atmospheric ozone is formed and how environmental pollutants are degraded (ecology), the properties of the soil on the moon (astrophysics), how medications work (pharmacology), and how to collect DNA evidence at a crime scene (forensics).
Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron(s); ionic bonds, in which a compound donates one or more electrons to another compound to produce ions (cations and anions); hydrogen bonds; and Van der Waals force bonds.
is a chemical element
; its atomic symbol is He
. It is a colorless, odorless, tasteless, non-toxic, and nearly inert monatomic that heads the noble gas
series in the periodic table
. Its atomic number
is 2 and its boiling
points are the lowest among the elements. It exists only as a gas
except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds
, which are all unstable at standard temperature and pressure
. Its most abundant stable isotope
and it has a rare stable isotope
. The behavior of liquid helium
-4's two different states—helium I and helium II—is important to researchers studying quantum mechanics
(in particular the phenomenon of superfluidity
) and those looking at the effects that near absolute zero
temperatures have on matter
(such as superconductivity
Helium is the second most abundant element in the known Universe and second lightest element in the periodic table. In the modern Universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium-4 nuclei produced by alpha-decay). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation.
In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. (The word helium comes from ancient Greek ἥλιος which is, surprisingly, cognate with the English sun.) Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. Helium is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). Inhaling a small volume of the gas temporarily changes the quality of one's voice.
History and Philosophy of Chemistry
(1893-1981) was an American physical chemist
, who won the 1934 Nobel Prize in Chemistry
for his work on isotopes
, specifically the discovery of deuterium
, a hydrogen
isotope, and the production of heavy water
. He also performed pioneering research in cosmochemistry
, which studies the origin and development of elements
and their isotopes, primarily within the solar system. Urey, along with his student Stanley Miller
, may be best remembered for the renowned Miller-Urey experiment
, which shows that a mixture of ammonia
, when exposed to ultraviolet radiation
, can interact to form amino acids
, the "building blocks" of terrestrial life. This experiment followed on from Urey's work on the oxygen
, and is considered to be pioneering work in the field of paleoclimatology
, as it attempts to explain the composition of the early Earth's atmosphere.
Techniques used by chemists
Equipment used by chemists
Chemistry in society
Chemistry in industry
Background color shows subcategory in the metal–metalloid–nonmetal trend: