Which ion contains chromium in a +6 oxidation state?

Which ion contains chromium in a +6 oxidation state?

• A. CN-
• B. Cr2O7^2-
• C. SiO3^2-
• D. MnO4-

Answer: (B)

Determining oxidation states in polyatomic ions involves assigning typical charges to each element so that the total matches the ion’s overall charge. For Cr2O7^2-, there are two chromium atoms and seven oxygens. Oxygen is almost always −2, giving −14 from the oxygens. Let the oxidation state of chromium be x; with two chromiums that’s 2x. The sum must equal the overall charge: 2x + (−14) = −2. Solving gives 2x = 12, so x = +6. Therefore chromium is in +6 oxidation state in Cr2O7^2−. The other ions don’t contain chromium in that oxidation state: CN− has no chromium, SiO3^2− has silicon (not chromium) in its common +4 state, and MnO4− has manganese in +7.

Determining oxidation states in polyatomic ions involves assigning typical charges to each element so that the total matches the ion’s overall charge. For Cr2O7^2-, there are two chromium atoms and seven oxygens. Oxygen is almost always −2, giving −14 from the oxygens. Let the oxidation state of chromium be x; with two chromiums that’s 2x. The sum must equal the overall charge: 2x + (−14) = −2. Solving gives 2x = 12, so x = +6. Therefore chromium is in +6 oxidation state in Cr2O7^2−. The other ions don’t contain chromium in that oxidation state: CN− has no chromium, SiO3^2− has silicon (not chromium) in its common +4 state, and MnO4− has manganese in +7.