What is Molecule? 

Molecule, is a collection of two or more atoms together (which constitute the smallest observable unit) into a pure material. There are some ways to represent the structure of a molecule. Factor symbols represent atoms in Lewis structures, and dots represent electrons that accompany them. A pair of mutual electrons (covalent bond) may be represented by a single dash. The ball-and-stick model depicts the spatial structure of the atoms better. 

The Kekulé structure is commonly used for aromatic compounds, in which each bond is described by a dash, carbon atoms are inferred when two or more lines intersect, and hydrogen atoms are normally omitted. Bond-line formulas, including the Kekulé configuration, are often used for complex non-aromatic organic compounds. Sugars are sometimes depicted as Fischer projections, with the carbon "backbone" drawn as a straight vertical line and carbon atoms inferred where horizontal lines overlap the vertical one. 

History of Diatomic Molecules 

In the 19th century, to elucidate the conceptualization of molecules, atoms and elements people relied upon the study of diatomic elements. This is due to the most common elements occurring are diatomic molecules. The original atomic hypothesis proposed by John Dalton states that every element found is monatomic and that normally the atomic ratios found in atoms of the compounds would be simple with respect to one another. Dalton assumed the formula of water to be HO. So, for almost 50 years the chemists were confused about the atomic weights and molecular formula. 

In the early 1800s, the formulation of water to be two parts of hydrogen with one part of oxygen was shown by Gay-Lussac and von Humboldt. The water’s composition was also correctly interpreted by Amedeo Avogadro. This is the basis for the Avogadro's law proposed followed by the diatomic elemental molecules assumptions that are in use now. The ignorance towards these findings are due to what was believed about the chemical affinity of an element would generally not be towards the same element. Avogadro's law contained unexplained exceptions that added to this disbelief. 

The resurrection of the ideas of Avogadro's by Cannizzaro with the help of Karlsruhe Congress on atomic weights enabled the production of a table of atomic weights that was consistent and in accord with the modern values. This proved essential for the periodic law discovered by Dmitri Mendeleev and Lothar Meyer. 

Concept 

Kinetic theory of gases 

The kinetic theory of gases is a basic, culturally interesting model of gas that founded several fundamental thermodynamic concepts. A gas is defined by the model as a large number of identical submicroscopic particles (atoms or molecules) that are all in continuous, fast, random motion. Their size is thought to be much smaller than the normal particle diameter. Random elastic collisions occur between the particles and with the container's enclosing walls. The simplest version of the model represents the ideal gas and takes into account no other particle interactions. 

The kinetic theory of gases describes macroscopic properties of gases like volume, friction, and temperature, and even some transport properties like viscosity, thermal conductivity, including mass diffusivity. The model also takes into account similar phenomena like Brownian motion. 

Assumptions made 

In kinetic theory of gases some assumptions are as it is based on ideal gas 

• The particles have the same mass. 
• The number of particles is so large that a statistical treatment of the problem is well justified. 
• The rapidly moving particles constantly collide among themselves and with the walls of the container. These particles are hard spheres since the collisions being perfectly elastic.   
• Except during collisions, the interactions among molecules are negligible. 
• Gas is made up of very small particles. Because of their limited scale, the amount of the number of individual gas molecules is marginal in comparison to the volume of the gas bottle. This is analogous to saying that the average distance between gas particles is high in comparison to their size and that the elapsed time of collision between particles and container wall is negligible in comparison to time between collisions. 

Diatomic 

Diatomic molecules are made up of just two atoms, which may be of the same or different chemical elements. The prefix di- comes from Greek and means two. A diatomic molecule is considered to be homonuclear if it contains two atoms of the same compound, such as hydrogen (H₂) or oxygen (O₂). Anything other than that, a diatomic molecule is said to be heteronuclear if it contains two separate atoms, including such carbon monoxide (CO) or nitric oxide (NO). A non-polar bond exists in a homonuclear diatomic molecule. 

Only the gases hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), and chlorine (Cl₂) form stable homonuclear diatomic molecules at normal temperature and pressure (STP) (or common laboratory pressures of 1 bar and 25 °C) (Cl₂).  The noble gases (helium, neon, argon, krypton, xenon, and radon) all seem to be monatomic gases at STP. 

The halogens, bromine (Br₂) and iodine (I₂) also form diatomic gases at slightly elevated temperatures. With the exception of astatine and tennessine, both halogens have indeed been detected as diatomic molecules. 

Some binary compounds exist as polymers but when they are evaporated, they become diatomic compounds, however these diatomic species repolymerize until cooled. As elemental phosphorus is heated ("cracked"), it produces diphosphorus, P₂. The majority of sulphur vapour is disulfur (S₂). In the gas phase, dilithium (Li₂) and disodium (Na₂) are identified. Throughout the gas phase, ditungsten (W₂) and dimolybdenum (Mo₂) form sextuple bonds. Dirubidium (Rb₂) is a diatomic element. 

A diatomic molecule is linear and is described by a single parameter, the bond length or gap between the two atoms. Diatomic nitrogen has three bonds, diatomic oxygen has two, and diatomic hydrogen, fluorine, chlorine, iodine, and bromine both have one. 

Other Heteronuclear Molecules 

Some other diatomic molecules are organic structures made up of two distinct elements. Based on the temperature and strain, several elements can merge to produce heteronuclear diatomic molecules. For example, hydrogen chloride, nitric oxide 

Note: Various 1:1 binary compounds are not usually called diatomic since they are polymeric at room temperature, but when evaporated, they form diatomic molecules, such as gaseous MgO, SiO, and many others. 

Many of the diatomic molecules have already been identified in the Earth's atmosphere, laboratories, and outer space. Approximately 99 percent of the Earth's atmosphere is made up of two types of diatomic molecules such as nitrogen (78 percent) and oxygen (21 percent). 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for         

• Bachelors in Science Chemistry 
• Masters in Science Chemistry 
• Bachelors in Science Physics