As per the given question, first of all we must know about the valency.
Valency of an element is when an element donates or accepts electrons to complete its octet.
Chlorine can either accept one electron or donate 7 electrons to complete its octet. But as we know chlorine is electronegative in nature, it can only accept one electron to complete its octet.
Even the most electropositive can not donate 7 electrons.
Hence, The valency of Cl is 1 and not 7.
Final answer :
Valency of an element is when an element donates or accepts electrons to complete its octet.
Chlorine can either accept one electron or donate 7 electrons to complete its octet. But as we know chlorine is electronegative in nature, it can only accept one electron to complete its octet.
Even the most electropositive can not donate 7 electrons.
Hence, The valency of Cl is 1 and not 7.