Halogens form diatomic molecules because they require one additional electron to achieve a stable electron configuration.

Halogens are located in Group 7 of the Periodic Table, which includes the elements fluorine ($\text{F}$), chlorine ($\text{Cl}$), bromine ($\text{Br}$), iodine ($\text{I}$), and astatine ($\text{At}$). Each of these elements possesses seven electrons in their outermost energy level. This configuration means that they are one electron short of attaining a stable, full outer energy level of eight electrons, commonly referred to as an octet.

To achieve this stable electron configuration, halogens have a tendency to gain one electron through chemical reactions. This leads to the formation of diatomic molecules, which are composed of two atoms of the same element bonded covalently. Halogens naturally form these diatomic molecules as a straightforward means to acquire the additional electron they need. For instance, when two fluorine atoms bond, they share a pair of electrons to create a molecule of $\text{F}_2$. As a result, each fluorine atom effectively has access to eight electrons, fulfilling the stable configuration requirement.

While the formation of diatomic molecules is not exclusive to halogens, it is particularly significant within this group due to their electron configuration. Other elements that typically form diatomic molecules include hydrogen ($\text{H}$), nitrogen ($\text{N}$), and oxygen ($\text{O}$). However, what sets halogens apart is that all members of this group readily form diatomic molecules under normal conditions. This characteristic is a fundamental aspect of halogens’ chemical behavior.