--- title: "Chromate Ion — AP Chem Definition & Redox Guide" description: "Chromate ion (CrO₄²⁻) is a polyatomic ion with chromium in the +6 oxidation state. Learn how it powers redox half-reactions and how AP Chem tests it vs dichromate." canonical: "https://fiveable.me/ap-chem/key-terms/chromate-ion" type: "key-term" subject: "AP Chemistry" unit: "Unit 4" --- # Chromate Ion — AP Chem Definition & Redox Guide ## Definition 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⁺. ## What It Is The chromate ion is CrO₄²⁻, a polyatomic ion built from one chromium atom covalently bonded to four oxygens, carrying an overall 2- [charge](/ap-chem/unit-9 "fv-autolink"). Run the [oxidation number](/ap-chem/key-terms/oxidation-number "fv-autolink") 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](/ap-chem "fv-autolink"), 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. ## Why It Matters Chromate lives in [Unit 4](/ap-chem/unit-4 "fv-autolink") (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](/ap-chem/key-terms/half-reaction "fv-autolink") (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. ## Connections ### Dichromate ion, Cr₂O₇²⁻ (Unit 4) Dichromate is two chromate units fused together, and it's the form chromium(VI) takes in [acidic solution](/ap-chem/key-terms/acidic-solution "fv-autolink"). 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)](/ap-chem/key-terms/oxidation-numbers) 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](/ap-chem/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz "fv-autolink") to Cr³⁺, gaining 3 electrons per atom, which is the first step in writing any half-reaction with it. ### [Conservation of Charge (Unit 4)](/ap-chem/key-terms/conservation-of-charge) 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](/ap-chem/unit-1/atomic-structure-electron-configurations/study-guide/DiW6kVmwDRDakxKodjw5 "fv-autolink") 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. ## On the AP Exam 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 ion vs Dichromate ion (Cr₂O₇²⁻) 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⁻. ## Key Takeaways - 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). ## FAQs ### What is the chromate ion in AP Chem? 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). ### What is the difference between chromate and dichromate? 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. ### Is the charge on chromium in chromate +2 because the ion is 2-? 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. ### Why does the dichromate half-reaction need exactly 6 electrons? 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. ### Has the chromate ion appeared on a real AP Chem FRQ? 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. ## Related Study Guides - [4.9 Oxidation-Reduction (Redox) Reactions](/ap-chem/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-chem/key-terms/chromate-ion#resource","name":"Chromate Ion — AP Chem Definition & Redox Guide","url":"https://fiveable.me/ap-chem/key-terms/chromate-ion","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-chem/key-terms/chromate-ion#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:14.350Z","isPartOf":{"@type":"Collection","name":"AP Chemistry Key Terms","url":"https://fiveable.me/ap-chem/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-chem/key-terms/chromate-ion#term","name":"chromate ion","description":"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. 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