Electronic spectra of diatomic molecules pdf molecules are molecules composed of only two atoms, of the same or different chemical elements. The prefix di- is of Greek origin, meaning “two”. A periodic table showing the elements that exist as homonuclear diatomic molecules under typical laboratory conditions. 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. All halogens have been observed as diatomic molecules, except for astatine, which is uncertain. Other elements form diatomic molecules when evaporated, but these diatomic species repolymerize when cooled. The bond in a homonuclear diatomic molecule is non-polar.
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. Many 1:1 binary compounds are not normally considered diatomic because they are polymeric at room temperature, but they form diatomic molecules when evaporated, for example gaseous MgO, SiO, and many others.
Hundreds of diatomic molecules have been identified in the environment of the Earth, in the laboratory, and in interstellar space. 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, indeed, dominated by hydrogen atoms. Diatomic elements played an important role in the elucidation of the concepts of element, atom, and molecule in the 19th century, because some of the most common elements, such as hydrogen, oxygen, 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 normally 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, and 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 mostly ignored until 1860, partly due to the belief that atoms of one element would have no chemical affinity toward atoms of the same element, and partly due to apparent exceptions to Avogadro’s law that were not explained until later 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 mostly agree with modern values.