Chlorine belongs to Group 17 (Group 7A) of the periodic table, also known as the halogens. Elements in this group have seven valence electrons because they have one electron less than a stable octet (eight valence electrons) configuration.
Chlorine has an electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁵. In its neutral state, chlorine has seven valence electrons in the 3p orbital. To achieve a stable octet configuration, chlorine tends to gain one electron to complete its outermost energy level. By gaining one electron, chlorine achieves a stable configuration similar to the noble gas argon (1s² 2s² 2p⁶), which has a complete octet.
As a result, chlorine typically has a valency of -1 when it forms ionic compounds by gaining an electron. By gaining one electron, chlorine completes its valence shell and achieves a stable configuration. This allows chlorine to form a single covalent or ionic bond with other elements, resulting in a valency of 1.
It's important to note that the concept of valency can vary depending on the specific context. Valency refers to the combining capacity of an element, and in the case of chlorine, it commonly exhibits a valency of -1. However, in certain compounds or reactions, chlorine can also exhibit different valencies, such as +1, +3, +5, or +7, depending on the oxidation state and the specific chemical environment.