Which species is the dichromate ion?

Which species is the dichromate ion?

• A. CrO4^2-
• B. Cr2O7^2-
• C. CrO3
• D. Cr2O3

Answer: (B)

Dichromate is the Cr2O7^2− species. It consists of two chromium atoms (both in the +6 oxidation state) and seven oxygens, giving an overall 2− charge. In acidic solution, two chromate units combine by adding protons and releasing water to form dichromate: 2 CrO4^2− + 2 H+ → Cr2O7^2− + H2O. This coupling of two chromate units is what distinguishes dichromate from chromate, which has a single chromium and four oxygens (CrO4^2−). The other options, CrO3 and Cr2O3, are neutral oxides (not ions) and contain different compositions and oxidation states, so they are not dichromate.

Dichromate is the Cr2O7^2− species. It consists of two chromium atoms (both in the +6 oxidation state) and seven oxygens, giving an overall 2− charge. In acidic solution, two chromate units combine by adding protons and releasing water to form dichromate: 2 CrO4^2− + 2 H+ → Cr2O7^2− + H2O. This coupling of two chromate units is what distinguishes dichromate from chromate, which has a single chromium and four oxygens (CrO4^2−). The other options, CrO3 and Cr2O3, are neutral oxides (not ions) and contain different compositions and oxidation states, so they are not dichromate.