Final answer:
In the given reaction, chlorine undergoes disproportionation, where it is both oxidized and reduced, changing from an oxidation state of 0 to -1 and +5 in the products.
Step-by-step explanation:
In the reaction 3Cl₂ (g) + 6NaOH (aq) → 5NaCl (aq) + NaClO₃ (Aq) + 3H₂O, the element that undergoes disproportionation is chlorine.
Disproportionation is a redox reaction in which the same element is both oxidized and reduced. To solve this type of reaction, we look at the oxidation states of the element in question before and after the reaction.
In its molecular form, Cl₂, chlorine has an oxidation state of 0. In the product side, chlorine exists in two different compounds: NaCl, where it has an oxidation state of -1, and NaClO₃, where it has an oxidation state of +5.
Thus, chlorine is simultaneously reduced to chloride ions (Cl-) and oxidized to chlorate ions (ClO₃⁻), which is classic behavior for a disproportionation reaction.