Electronic configuration of Cl = 2, 8, 7 (atomic number = 17). Cl needs 1 electron to complete its octet in the valence shell. Therefore, the valency of Cl is 1. The valency of Cl could be 7 only. When it loses 7 electrons from its valence shell. But losing 7 electrons is very difficult than gaining a single electron. Therefore, the agency of Cl is not 7. It is 1.
