The chromate ion, CrO₄²⁻, is a polyatomic ion made of one chromium and four oxygen atoms with a 2- charge, putting chromium in the +6 oxidation state. In AP Chem it shows up in redox chemistry (Topic 4.9), where chromium(VI) species act as oxidizing agents and get reduced to Cr³⁺.
The chromate ion is CrO₄²⁻, a polyatomic ion built from one chromium atom covalently bonded to four oxygens, carrying an overall 2- charge. Run the oxidation number math and chromium comes out at +6 (four oxygens at -2 give -8, and the ion's charge is -2, so Cr must be +6). That high oxidation state is the whole story. Chromium at +6 has a lot of room to gain electrons, which makes chromate and its close cousin dichromate (Cr₂O₇²⁻) strong oxidizing agents.
In AP Chem, chromium(VI) ions are workhorses for redox practice. The classic reduction half-reaction takes dichromate to chromium(III) in acidic solution: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. Notice the bookkeeping. Each chromium drops from +6 to +3, a gain of 3 electrons, and there are two chromiums, so 6 electrons total. Chromate and dichromate also interconvert depending on pH, which is why exam problems set in acidic solution almost always use the dichromate form.
Chromate lives in Unit 4 (Chemical Reactions), specifically Topic 4.9 on oxidation-reduction reactions. It directly supports learning objective 4.9.A, which asks you to represent a balanced redox equation using half-reactions (EK 4.9.A.1). Chromium(VI) is the College Board's favorite vehicle for this skill because it forces you to do everything at once. You assign oxidation numbers, track a 6-electron transfer, balance oxygen with water, balance hydrogen with H⁺, and check conservation of charge at the end. If you can balance a dichromate-to-Cr³⁺ half-reaction cleanly, you've basically mastered the topic. It also crosses into Unit 2, since the 2026 long FRQ asked about the covalent Cr-O bonding inside CrO₄²⁻ before getting to the chemistry.
Keep studying AP® Chemistry Unit 4
Dichromate ion, Cr₂O₇²⁻ (Unit 4)
Dichromate is two chromate units fused together, and it's the form chromium(VI) takes in acidic solution. Every chromium in both ions is +6, so they oxidize other species the same way. Most AP redox problems use the dichromate form because they're set in acid.
Oxidation Numbers (Unit 4)
Chromate is a perfect oxidation-number drill. Oxygen is -2, the ion's charge is -2, so chromium must be +6. That number tells you chromium can be reduced to Cr³⁺, gaining 3 electrons per atom, which is the first step in writing any half-reaction with it.
Conservation of Charge (Unit 4)
Balancing chromium(VI) half-reactions is really a charge-accounting exercise. In Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, the left side sums to +6 and so does the right. Practice questions love asking you to justify the 6 electrons using exactly this logic.
Lewis Structures and Covalent Bonding (Unit 2)
Inside the chromate ion, the Cr-O bonds are covalent even though the ion as a whole pairs ionically with cations. The 2026 long FRQ opened by asking about this covalent bonding in CrO₄²⁻, so be ready to treat chromate as a bonding question, not just a redox one.
Chromium(VI) ions show up in both multiple choice and FRQs. The 2017 short FRQ had a student choose dichromate as a titrant for finding the concentration of H₂O₂, and the 2026 long FRQ asked directly about the chromate and dichromate ions, starting with the covalent Cr-O bonding in CrO₄²⁻. Multiple-choice stems typically give you the reduction of dichromate to Cr³⁺ in acidic solution and ask you to justify electron counts, verify conservation of mass and charge, or do mole-ratio stoichiometry (for example, how many moles of dichromate oxidize 0.0600 mol of ethanol). What you actually have to do: assign oxidation numbers, write and balance half-reactions with H⁺ and H₂O, count electrons transferred (6 per dichromate), and use that ratio in calculations. Memorize the pattern, not just the answer.
Chromate is CrO₄²⁻ (one chromium); dichromate is Cr₂O₇²⁻ (two chromiums). Both carry a 2- charge and both have chromium at +6, so they're chemically siblings, not different oxidation states. The practical difference on the exam is the electron count when reduced to Cr³⁺. One chromate would account for 3 electrons, but one dichromate accounts for 6 because it contains two chromium atoms. In acidic solution, the dichromate form dominates, which is why acidic redox problems use Cr₂O₇²⁻.
The chromate ion is CrO₄²⁻, a polyatomic ion with a 2- charge in which chromium has an oxidation number of +6.
Chromium(VI) species like chromate and dichromate are strong oxidizing agents because chromium can be reduced from +6 down to +3.
In acidic solution, chromium(VI) appears as dichromate, and its reduction half-reaction is Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
Six electrons are transferred per dichromate ion because each of the two chromium atoms gains three electrons going from +6 to +3.
A correctly balanced half-reaction conserves both mass and charge, and the exam frequently asks you to verify or justify that balance.
The Cr-O bonds within the chromate ion are covalent, so chromate can show up in bonding questions (Unit 2) as well as redox questions (Unit 4).
Chromate is CrO₄²⁻, a polyatomic ion with one chromium and four oxygens carrying a 2- charge. Chromium sits at the +6 oxidation state, which makes chromate an oxidizing agent that gets reduced to Cr³⁺ in redox reactions (Topic 4.9).
Chromate is CrO₄²⁻ and dichromate is Cr₂O₇²⁻. Both have chromium at +6, but dichromate contains two chromium atoms, so reducing one dichromate to Cr³⁺ transfers 6 electrons while one chromate would transfer 3. Acidic solutions favor the dichromate form.
No, that's a common trap. The 2- is the charge on the whole ion, not on chromium. Four oxygens at -2 each total -8, so chromium must be +6 to leave an overall charge of -2.
Each chromium goes from +6 in Cr₂O₇²⁻ to +3 in Cr³⁺, gaining 3 electrons, and there are two chromium atoms per dichromate. You can confirm it with conservation of charge: -2 + 14 - 6 = +6 on the left matches 2(+3) = +6 on the right.
Yes. The 2026 long FRQ Q2 asked about both CrO₄²⁻ and Cr₂O₇²⁻, including the covalent Cr-O bonding, and the 2017 short FRQ Q7 used dichromate as a titrant to find the concentration of hydrogen peroxide.
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