Diatomic molecule
If a diatomic molecule consists of two atoms of the same element, such as hydrogen (H2) or oxygen (O2), then it is said to be homonuclear.
The only chemical elements that form stable homonuclear diatomic molecules at standard temperature and pressure (STP) (or at typical laboratory conditions of 1 bar and 25 °C) are the gases hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), and chlorine (Cl2), and the liquid bromine (Br2).
[2] At slightly elevated temperatures, the halogens bromine (Br2) and iodine (I2) also form diatomic gases.
[3] All halogens have been observed as diatomic molecules, except for astatine and tennessine, which are uncertain.
Ditungsten (W2) and dimolybdenum (Mo2) form with sextuple bonds in the gas phase.
Many elements can combine to form heteronuclear diatomic molecules, depending on temperature and pressure.
Examples are gases carbon monoxide (CO), nitric oxide (NO), and hydrogen chloride (HCl).
Hundreds of diatomic molecules have been identified[5] in the environment of the Earth, in the laboratory, and in interstellar space.
About 99% of the Earth's atmosphere is composed of two species of diatomic molecules: nitrogen (78%) and oxygen (21%).
All diatomic molecules are linear and characterized by a single parameter which is the bond length or distance between the two atoms.
For example, Dalton assumed water's formula to be HO, giving the atomic weight of oxygen as eight times that of hydrogen,[7] 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 also partly due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules.
These weights were an important prerequisite for the discovery of the periodic law by Dmitri Mendeleev and Lothar Meyer.
In quantum theory, an electronic state of a diatomic molecule is represented by the molecular term symbol where
is the total electronic angular momentum quantum number along the internuclear axis, and
give additional quantum mechanical details about the electronic state.
determines whether reflection in a plane containing the internuclear axis introduces a sign change in the wavefunction.
The aforementioned fluorescence occurs in distinct regions of the electromagnetic spectrum, called "emission bands": each band corresponds to a particular transition from a higher electronic state and vibrational level to a lower electronic state and vibrational level (typically, many vibrational levels are involved in an excited gas of diatomic molecules).
Vegard-Kaplan bands) are present in the spectral range from 0.14 to 1.45 μm (micrometres).
[9] A given band can be spread out over several nanometers in electromagnetic wavelength space, owing to the various transitions that occur in the molecule's rotational quantum number,
Bands are spread out even further by the limited spectral resolution of the spectrometer that is used to measure the spectrum.
The molecular term symbol is a shorthand expression of the angular momenta that characterize the electronic quantum states of a diatomic molecule, which are also eigenstates of the electronic molecular Hamiltonian.
It is also convenient, and common, to represent a diatomic molecule as two-point masses connected by a massless spring.
[12][13] These potentials give more accurate energy levels because they take multiple vibrational effects into account.
Concerning history, the first treatment of diatomic molecules with quantum mechanics was made by Lucy Mensing in 1926.
Classically, the kinetic energy of rotation is where For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by where
Another method, for English-speakers, is the sentence: "Never Have Fear of Ice Cold Beer" as a representation of Nitrogen, Hydrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine.
