What is the oxidation half reaction in the chemical reaction cr2o72-(aq) + 6 cl-(aq) + 14 h+(aq) → 2 cr3+(aq) + 3 cl2(aq) + 7 h2o(l)?

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

Copper, silver, aluminum, cobalt, chromium, mercury