the complete equation for the reaction is Cr₂O₇²⁻ + 6Cl⁻ + 14H⁺ ---> 2Cr³⁺ + 3Cl₂ + 7H₂O in the oxidation half reaction the oxidation number of the species increases. in the reduction half reaction the oxidation number of the species decreases. the oxidation number of Cr reduces from +6 in Cr₂O₇²⁻ to +3 in Cr³⁺ the oxidation number of Cl increases from -1 in Cl⁻ to Cl₂ therefore oxidation takes place oxidation half reaction is as follows 2Cl⁻ --> Cl₂ + 2e
