The octet rule is a fundamental principle in chemistry that describes how atoms bond to achieve a stable electron configuration of eight electrons in their outermost shell.

According to the octet rule, atoms tend to form bonds in order to attain a full outer electron shell, which is generally composed of eight electrons. This stable configuration is comparable to that of noble gases, which are among the most stable elements in the periodic table. The underlying reason for this stability is that a complete outer shell minimizes an atom’s energy and maximizes its stability.

Atoms can achieve a full outer shell through bonding with other atoms. There are two primary types of chemical bonds: covalent bonds and ionic bonds.

In covalent bonding, atoms share electrons to fill their outer shells. For instance, a hydrogen atom has one electron in its outer shell but requires two electrons to reach a filled state. By sharing an electron with another hydrogen atom, both hydrogen atoms can achieve a full outer shell.

In contrast, ionic bonding involves the transfer of one or more electrons from one atom to another. This transfer results in both atoms achieving a full outer shell, but it also creates ions—atoms that carry either a positive or negative charge. For example, a sodium atom can transfer one electron to a chlorine atom. This results in the formation of a sodium ion with a positive charge ($\text{Na}^+$) and a chloride ion with a negative charge ($\text{Cl}^-$). The electrostatic attraction between these oppositely charged ions leads to the formation of an ionic bond.

It is important to note that the octet rule is not universally applicable. There are numerous exceptions, particularly among elements in the third period of the periodic table and beyond. These elements can accommodate more than eight electrons in their outer shells due to the availability of d-orbitals. Despite these exceptions, the octet rule remains a valuable guideline for predicting and understanding chemical bonding.