What is the oxidation number of chlorine atoms in HCl, C{{l}_{2}}and HClO?

The oxidation number of an element in a compound or molecule is a measure of the charge that element would have if the compound or molecule were ionic. Here are the oxidation numbers for chlorine atoms in the compounds you mentioned:

In HCl (hydrochloric acid), chlorine (Cl) has an oxidation number of -1. This is because hydrogen (H) is assigned an oxidation number of +1 in most cases, and the sum of the oxidation numbers in a neutral compound is equal to zero. Therefore, the oxidation number of chlorine must be -1 to balance the equation: +1 (H) + (-1) (Cl) = 0.

In Cl2 (chlorine gas), each chlorine atom is in its elemental form, and its oxidation number is 0. Elements in their elemental form have an oxidation number of 0.

In HClO (hypochlorous acid), chlorine (Cl) has an oxidation number of +1. This is because hydrogen (H) is assigned an oxidation number of +1, and oxygen (O) is usually assigned an oxidation number of -2. The sum of the oxidation numbers in HClO must equal zero, so the chlorine atom is assigned an oxidation number of +1 to balance the equation: +1 (H) + (-2) (O) + (+1) (Cl) = 0.

So, in summary:

In HCl, the oxidation number of chlorine is -1.
In Cl2, the oxidation number of chlorine is 0.
In HClO, the oxidation number of chlorine is +1.