Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. The prefix di- is of Greek origin, meaning "two". If a diatomic molecule consists of two atoms of the same element, such as hydrogen or oxygen it is said to be homonuclear. Otherwise, if a diatomic molecule consists of two different atoms, such as carbon monoxide or nitric oxide, the molecule is said to be heteronuclear; the bond in a homonuclear diatomic molecule is non-polar. The only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure are the gases hydrogen, oxygen and chlorine; the noble gases are gases at STP, but they are monatomic. The homonuclear diatomic gases and noble gases together are called "elemental gases" or "molecular gases", to distinguish them from other gases that are chemical compounds. At elevated temperatures, the halogens bromine and iodine form diatomic gases. All halogens have been observed as diatomic molecules, except for astatine, uncertain.
Other elements form diatomic molecules when evaporated, but these diatomic species repolymerize when cooled. Heating elemental phosphorus gives diphosphorus, P2. Sulfur vapor is disulfur. Dilithium and disodium are known in the gas phase. Ditungsten and dimolybdenum form with sextuple bonds in the gas phase. Dirubidium is diatomic. All other diatomic molecules are chemical compounds of two different elements. Many elements can combine to form heteronuclear diatomic molecules, depending on temperature and pressure. Examples are gases carbon monoxide, nitric oxide, hydrogen chloride. Many 1:1 binary compounds are not considered diatomic because they are polymeric at room temperature, but they form diatomic molecules when evaporated, for example gaseous MgO, SiO, many others. Hundreds of diatomic molecules have been identified in the environment of the Earth, in the laboratory, in interstellar space. About 99% of the Earth's atmosphere is composed of two species of diatomic molecules: nitrogen and oxygen.
The natural abundance of hydrogen in the Earth's atmosphere is only of the order of parts per million, but H2 is the most abundant diatomic molecule in the universe. The interstellar medium is, dominated by hydrogen atoms. All diatomic molecules are linear and characterized by a single parameter, the bond length or distance between the two atoms. Diatomic nitrogen has a triple bond, diatomic oxygen has a double bond, diatomic hydrogen, fluorine and bromine all have single bonds. Diatomic elements played an important role in the elucidation of the concepts of element and molecule in the 19th century, because some of the most common elements, such as hydrogen and nitrogen, occur as diatomic molecules. John Dalton's original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would have the simplest atomic ratios with respect to one another. For example, Dalton assumed water's formula to be HO, giving the atomic weight of oxygen as eight times that of hydrogen, instead of the modern value of about 16.
As a consequence, confusion existed regarding atomic weights and molecular formulas for about half a century. As early as 1805, Gay-Lussac and von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen, by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules. However, these results were ignored until 1860 due to the belief that atoms of one element would have no chemical affinity toward atoms of the same element, partly due to apparent exceptions to Avogadro's law that were not explained until in terms of dissociating molecules. At the 1860 Karlsruhe Congress on atomic weights, Cannizzaro resurrected Avogadro's ideas and used them to produce a consistent table of atomic weights, which agree with modern values; these weights were an important prerequisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer. Diatomic molecules are in their lowest or ground state, which conventionally is known as the X state.
When a gas of diatomic molecules is bombarded by energetic electrons, some of the molecules may be excited to higher electronic states, as occurs, for example, in the natural aurora. Such excitation can occur when the gas absorbs light or other electromagnetic radiation; the excited states are unstable and relax back to the ground state. Over various short time scales after the excitation, transitions occur from higher to lower electronic states and to the ground state, in each transition results a photon is emitted; this emission is known as fluorescence. Successively higher electronic states are conventionally named A, B, C, etc.. The excitation energy must be greater than or equal to the energy of the electronic state in order for the excitation to occur. In quantum theory, an electronic state of a diatomic molecule is repr
The following highways are numbered 400: Murray Valley Highway Ontario Highway 400 D400 road R400 road Japan National Route 400 Capital Region Second Ring Expressway R400 road in the Eastern Cape N-400 road D.400, a west-east state road in Turkey running from Datça, Muğla Province to Esendere at the Iranian border. A400 road in London B400 road U. S. Route 400 Arkansas Highway 400 Florida State Road 400 Georgia State Route 400 Kentucky Route 400 Louisiana Highway 400 Maryland Route 400, a former state highway Nevada State Route 400 New York: New York State Route 400 County Route 400 North Carolina Highway 400 Pennsylvania Route 400 Puerto Rico Highway 400 South Carolina Highway 400 Tennessee State Route 400 Texas: Texas State Highway Spur 400 Farm to Market Road 400 Utah: Road 400 Virginia State Route 400 Virginia State Route 400
Edward Robeson Taylor was the 28th Mayor of San Francisco serving from July 16, 1907 to January 7, 1910. Edward Robeson Taylor was born on September 24, 1838 in Springfield, the only son of Henry West Taylor and the former Mary Thaw of Philadelphia, Pennsylvania He was a lawyer and a poet in California before he became mayor, publishing an 1898 book of sonnets based on the paintings of William Keith. Taylor was appointed mayor after his eight-day term; when he was sworn in, he became the oldest mayor of San Francisco to be sworn in at 68 years old and still holds the record today. He died in San Francisco on July 5, 1923, his remains are housed at the San Francisco Columbarium. The political economist Henry George credits Taylor for influencing his work on Progress and Poverty, one of the most popular and influential books in American history. Exhibition spotlights career... An Extraordinary Mayor for an Extraordinary City Edward Robeson Taylor, Sonnets of Edward Robeson Taylor on some pictures painted by William Keith, San Francisco: Printed by the E.
D. Taylor Co. Edward Robeson Taylor at Find a Grave