Why Do Atoms Combine?

The answer to this question is hidden in the electronic configurations of the noble gases. It was found that noble gases namely helium, neon, argon, krypton, xenon and radon did not react with other elements to form compounds. They were non-reactive.

In the initial stages they were also called inert gases due to their non-reactive nature. Thus it was, thought that these noble gases lacked reactivity because of their specific electronic arrangements which were quite stable. When you write the electronic configurations of the noble gases, you find that except helium all of them have 8 electrons in their outermost shell.

It was concluded that atoms having 8 electrons in their outermost shell are very stable and they did not form compounds. It was also observed that other atoms such as hydrogen, sodium, chlorine which do not have 8 electrons in their outermost shell undergo chemical reactions. They can stabilize by combining with each other and attain the configurations of noble gases i.e. 8 electrons (or 2 electrons in case of helium) in their outermost shells.

Thus, atoms tend to attain a configuration in which they have 8 electrons in their outermost shells. This is the basic cause of chemical bonding. This attainment of eight electrons for stable structure is called the octet rule. The octet rule explains the chemical bonding in many compounds.

Atoms are held together in compounds by the forces of attraction which result in formation of chemical bonds. The formation of chemical bonds results in the lowering of energy which is less than the energy the individual atoms. The resulting compound is lower in energy as compared to sum of energies of the reacting atom/molecule and hence is more stable.

Thus, stability of the compound formed is an important factor in the formation of chemical bonds.