Diatomic elements are defined as pure elements which form the molecules contain the two atoms for bonding together and the atoms can be the same or different elements that were bonding together.  The term di- is coming from Greek origin, referring to the word two.  Many researchers suggested that these chemical elements are unique in natures.   There are only a few chemical elements that form stable homonuclear diatomic molecules in normal temperature and pressure which include mainly gases such as hydrogen (H2),  nitrogen  (N2), oxygen (O2 ), chlorine (Cl2 )fluorine. The researchers suggested that two same atoms who are making bonds are referred to as the homonuclear molecules.  On the other hand, when a diatomic molecule contains two different atoms, for example, carbon monoxide (CO) or nitric oxide (NO), then the molecule is said to be heteronuclear.  It was observed that in high temperature or slightly elevated temperatures, the halogens bromine (Br2) and iodine (I2) also form diatomic gases.

 While many researchers debated regarding the state of the diatomic molecule, a significant number of researchers suggested that other elements tend to form diatomic molecules when they are in evaporated states. That is the reason common of the diatomic molecules are in gaseous in nature.   There are other diatomic elements such as  BrINClHOF which is pronounced as  "Brinkleho and HONClBrI which is pronounced as pronounced "Honkelbrif” and HOFBrINCl have been added to the list of diatomic elements. Interestingly, these unique diatomic species depolymerize when they are cooled in nature, unlike other diatomic molecules which are formed when evaporates.  In the heating state,  phosphorus gives rise to diphosphorus and sulfur vapor is mostly give rise to di-sulfur.   On the other hand, out of all diatomic molecules Nitrogen stands out as diatomic molecules because its atoms share a strong triple bond, making it a very stable substance and it is impossible to break this stable bond.

  In the 19th century, Diatomic elements played a significant role in the illumination of the concepts of element, atom, and molecule, due to some of the most common elements, such as hydrogen, oxygen, and nitrogen, occur as diatomic molecules.    The original hypothesis of john Dalton suggested that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios concerning one another.  In this context, the example can be given such as Dalton assumed that the formula of water is HO, providing the idea that giving the atomic weight of oxygen as eight times that of hydrogen.  

 Considering the electronic states, diatomic molecules are the molecules are normally present in their lowest or ground state, which predictably is also known as the ground state. When a gas of diatomic molecules is bombarded with energetic electrons, certain of the molecules may be excited to reach higher electronic states, as occurs.  The excited states of the diatomic molecules are unstable in nature and naturally relax back to the ground state. Hence, in order to be excited, diatomic molecules must require excitation energy equal to or more than the electronic state.

In this case,    there are certain crucial key points are required to remember. The first point is that in the case of heteronuclear diatomic molecules, atomic orbitals only mix with each other when the electronegativity values of both the atom are similar.

The second crucial point to remember is that orbital interactions in order to produce bonding or anti-bonding orbitals in heteronuclear diatomics are more likely to occur only if there is sufficient overlapping has been done in between atomic orbitals, which can be determined by their symmetries as well as similarity in orbital energies.