The elements of Group  are known as the halogens.  Chlorine is the second member of this group.  The elemental form of all of the halogens is a diatomic molecule.

Like most other non-metals, chlorine has both positive and negative oxidation states, the maximum being equal to the last digit of the group number (+7), the minimum being the Group number - (-1). Similar to other p-block elements, chlorine has additional positive oxidation states at intervals of two (+5, +3, +1).  Some species in each of these oxidation states are shown below.

+7	ClO4–, HClO4
+5	ClO3–, HClO3
+3	ClO2– (less important)
+1	ClO–, HClO
0	Cl2
-1	Cl–, HCl (naturally occurring)

The naturally occurring oxidation state (-1) is the most stable state for chlorine.  This means that elemental chorine and compounds containing chlorine in positive oxidation states are good oxidants.
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Elemental chlorine acts as an oxidant toward most other elements to form products that are referred to as chlorides.

Excess chlorine oxidises most elements to their maximum oxidation state (equal to the last digit of their Group number).
 
The trends in the composition, physical and chemical properties of these halides is one of the sets of facts upon which the periodic table is based.