Why do halogens form diatomic molecules?

Halogens form diatomic molecules because they need one more electron to achieve a stable electron configuration.

Halogens are elements found in Group 7 of the Periodic Table. This group includes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements have seven electrons in their outermost energy level, which means they are one electron short of the stable, full outer energy level of eight electrons, also known as an octet.

In order to achieve this stable configuration, halogens tend to gain one electron through chemical reactions. This is where diatomic molecules come into play. A diatomic molecule is a molecule composed of two atoms of the same element in a covalent bond. Halogens form these diatomic molecules because it's an easy way for them to gain the one electron they need. For example, two fluorine atoms can share a pair of electrons, forming a molecule of F2. Each fluorine atom now has access to eight electrons, achieving the desired stable configuration.

This tendency to form diatomic molecules is not unique to halogens, but it is particularly pronounced in this group due to their electron configuration. Other elements that commonly form diatomic molecules include hydrogen, nitrogen, and oxygen. However, the halogens are unique in that all members of the group form diatomic molecules under normal conditions. This is a key characteristic of halogens and is a fundamental aspect of their chemical behaviour.