Diatomic molecule or diatomic elements are the one which contains two chemically bonded atoms. If the two atoms are similar, such as in the oxygen molecule (O2), they form a homonuclear diatomic molecule, while if the atoms are different, such as in the carbon monoxide molecule, they form a heteronuclear diatomic molecule (CO), where they make up the heteronuclear diatomic molecule.
Diatomic molecules like oxygen and polyatomic molecules like water contain additional rotational motions, which also store the thermal energy in their kinetic energy of rotation. Every additional degree of freedom contributes more amount R to cV because the diatomic molecules can rotate about two axes.
All other diatomic molecules are the chemical compounds of two various elements. Several elements combine to produce heteronuclear diatomic molecules based on pressure and temperature.
Examples are gases nitric oxide (NO), hydrogen chloride (HCl), and carbon monoxide (CO).
Several 1:1 binary compounds are not regularly considered diatomic due to the reason they are polymeric at room temperature, but when evaporated, they produce the diatomic molecules—for example, gaseous SiO, MgO, and others.
In the Earth's environment, hundreds of diatomic molecules have been identified in interstellar space and in the laboratory. About 99% of the atmosphere of Earth is composed of 2 species of diatomic molecules: oxygen (21%) and nitrogen (78%). The natural abundance of hydrogen in the atmosphere of Earth is only of the order of parts per million. However, hydrogen is the most universally abundant diatomic molecule. The interstellar medium can be dominated by hydrogen atoms.
All the diatomic molecules are linear and characterized by a single parameter which is the bond length or distance between the two atoms. The diatomic nitrogen atom has a triple bond, the diatomic oxygen atom has a double bond, while the diatomic hydrogen, fluorine, chlorine, iodine, and bromine atoms all have single bonds.
Since a number of the more important elements, such as carbon, are diatomic, they played a key role in the 19th-century elucidation of the principles of atom, molecule, and elements, like oxygen, nitrogen, and hydrogen takes place as diatomic molecules. The original atomic hypothesis of John Dalton assumed that all the elements were monatomic and that the atoms present in compounds would commonly have the simplest atomic ratios with respect to each other. For suppose, the formula of Dalton assumed water to be HO, giving the atomic weight of the oxygen as eight times that of hydrogen, instead of a modern value of up to 16. As a result, for almost half a century, there was uncertainty about molecular formulas and atomic weights.
As early as 1805, von Humboldt and Gay-Lussac exhibited that water is formed of one volume of oxygen and two volumes of hydrogen, and also by 1811, Amedeo Avogadro had arrived at the exact interpretation of the composition of the water, depending on what is currently known as Avogadro's law and the diatomic elemental molecule assumption.
However, until the 1860s, these observations were dismissed, partly due to the assumption that atoms of one element would have no chemical affinity for atoms of a similar element, and partly due to obvious exceptions to Avogadro's rule that were not clarified until later in terms of dissociating molecules.
Cannizzaro revived Avogadro's theories and used them to construct a coherent table of atomic weights that largely agrees with current principles at the Karlsruhe Congress on atomic weights in the 1860s. These weights were said as prerequisites for the discovery of periodic law by Lothar Meyer and Dmitri Mendeleev.
Excited Electronic States
Normally, the diatomic molecules are in their ground or lowest state, which conventionally is also called the 'X' state. When the diatomic molecule's gas can be bombarded by the energetic electrons, a few of the molecules can be excited to the higher electronic states as they take place. For suppose, in the natural aurora, nuclear explosions of high-altitude and the rocket-borne electron gun experiments. Such type of excitation can also take place when the gas absorbs either light or other electromagnetic radiation.
Also, the excited states are unstable and relax back to the ground state naturally. Over different short time scales after excitation (typically fraction of seconds, or at times longer than a second, if there is a metastable excited state), transitions occur from the higher to lower electronic states and finally to the ground state, and in every transition results, there emits a photon. This emission is called fluorescence.
And, successively, higher electronic states can be conventionally named as A, B, C, and so on. The excitation energy should always be either greater than or equal to the electronic state energy in order for the excitation to take place.
In the quantum theory, the diatomic molecule's electronic state can be represented by the molecular term symbol as given below:
2S + 1 Λ(v)
Where S is given as the total electronic spin quantum number, Λ is given as the total electronic angular momentum quantum number along the internuclear axis, and v is given as the vibrational quantum number. Λ takes on the values of 0, 1, 2, and so on, which are represented by the symbols of electronic states such as Σ, π, Δ and so on.