All 33 common polyatomic ions with formulas and charges: flashcards, a 50-question quiz, and the -ate/-ite naming patterns. Free in your browser, no sign-up.
Subject: chemistry
What is the primary structural difference between a polyatomic ion and a monatomic ion?
Answer: A polyatomic ion consists of multiple atoms covalently bonded together with an overall charge, whereas a monatomic ion is a single charged atom.
A polyatomic ion is a group of two or more atoms covalently bonded together that carries an overall electric charge, behaving as a single charged unit. A monatomic ion, like Na⁺ or Cl⁻, consists of a single charged atom.
In the naming system for polyatomic oxyanions, what specific information does the suffix "-ite" convey when compared to the suffix "-ate"?
Answer: The "-ite" form has one fewer oxygen atom than the "-ate" form, and the charge remains the same.
The suffix "-ate" marks the common form of an oxyanion, while "-ite" marks the form with one fewer oxygen atom. Crucially, the charge of the ion stays the same between the -ate and -ite forms (e.g., sulfate SO₄²⁻ and sulfite SO₃²⁻ both have a 2− charge).
Why are parentheses necessary when writing the chemical formula for calcium nitrate, Ca(NO₃)₂?
Answer: To signify that two complete nitrate ions are required to balance the charge of one calcium ion.
Parentheses are used when a compound requires more than one of the same polyatomic ion to balance the charges. The subscript outside the parentheses indicates how many of that entire ion are present. In Ca(NO₃)₂, two NO₃⁻ ions are needed to balance the Ca²⁺ ion.
What is the key difference in chemical significance between ammonium (NH₄⁺) and hydronium (H₃O⁺) ions?
Answer: Ammonium is a common component of fertilizers, while hydronium is the actual form of H⁺ in aqueous solutions.
Ammonium (NH₄⁺) forms when ammonia (NH₃) gains a proton and is significant as a component of fertilizers. Hydronium (H₃O⁺) forms when water (H₂O) gains a proton and is significant as the actual form in which H⁺ exists in aqueous solutions, making it central to acid–base chemistry.
A student needs to write the chemical formula for iron(III) sulfate. Given that iron(III) is Fe³⁺ and sulfate is SO₄²⁻, what is the correct formula and why?
Answer: Fe₂(SO₄)₃, because two iron(III) ions balance three sulfate ions.
To balance the charges, the least common multiple of 3 (from Fe³⁺) and 2 (from SO₄²⁻) is 6. Therefore, two Fe³⁺ ions (2 × 3+ = 6+) and three SO₄²⁻ ions (3 × 2− = 6−) are needed. Parentheses are used around the sulfate ion because more than one is present.
What is the formula for the acetate ion?
Answer: C₂H₃O₂⁻
The acetate ion has the formula C₂H₃O₂⁻ (also written CH₃COO⁻).
What is the formula for the nitrate ion?
Answer: NO₃⁻
The nitrate ion has the formula NO₃⁻.
What is the formula for the nitrite ion?
Answer: NO₂⁻
The nitrite ion has the formula NO₂⁻, which has one fewer oxygen atom than nitrate (NO₃⁻) but the same 1− charge.
What is the formula for the hydroxide ion?
Answer: OH⁻
The hydroxide ion has the formula OH⁻. It is one of the few polyatomic ions ending in -ide.
What is the formula for the cyanide ion?
Answer: CN⁻
The cyanide ion has the formula CN⁻. It is one of the few polyatomic ions ending in -ide.
What is the formula for the permanganate ion?
Answer: MnO₄⁻
The permanganate ion has the formula MnO₄⁻ and carries a 1− charge.
What is the formula for the perchlorate ion?
Answer: ClO₄⁻
The perchlorate ion has the formula ClO₄⁻, indicating one more oxygen atom than the chlorate ion (ClO₃⁻), and carries a 1− charge.
What is the formula for the chlorate ion?
Answer: ClO₃⁻
The chlorate ion has the formula ClO₃⁻, which is the -ate reference form for chlorine oxyanions, and carries a 1− charge.
What is the formula for the chlorite ion?
Answer: ClO₂⁻
The chlorite ion has the formula ClO₂⁻, indicating one fewer oxygen atom than the chlorate ion (ClO₃⁻), and carries a 1− charge.
What is the formula for the hypochlorite ion?
Answer: ClO⁻
The hypochlorite ion has the formula ClO⁻, indicating one fewer oxygen atom than the chlorite ion (ClO₂⁻), and carries a 1− charge.
What is the formula for the bromate ion?
Answer: BrO₃⁻
Bromine forms an analogous series to chlorine. Bromate is the -ate form, corresponding to chlorate (ClO₃⁻), so its formula is BrO₃⁻.
What is the formula for the iodate ion?
Answer: IO₃⁻
Iodine forms an analogous series to chlorine. Iodate is the -ate form, corresponding to chlorate (ClO₃⁻), so its formula is IO₃⁻.
What is the formula for the hydrogen carbonate ion (bicarbonate)?
Answer: HCO₃⁻
The prefix hydrogen- means a hydrogen ion has been added to the anion, reducing its negative charge by one. Carbonate (CO₃²⁻) becomes hydrogen carbonate (HCO₃⁻).
What is the formula for the hydrogen sulfate ion (bisulfate)?
Answer: HSO₄⁻
The prefix hydrogen- means a hydrogen ion has been added to the anion, reducing its negative charge by one. Sulfate (SO₄²⁻) becomes hydrogen sulfate (HSO₄⁻).
What is the formula for the dihydrogen phosphate ion?
Answer: H₂PO₄⁻
Phosphate (PO₄³⁻) can add hydrogen twice. Hydrogen phosphate is HPO₄²⁻, and dihydrogen phosphate is H₂PO₄⁻.
What is the formula for the thiocyanate ion?
Answer: SCN⁻
The prefix thio- means one oxygen atom has been replaced by a sulfur atom. Cyanate (OCN⁻) becomes thiocyanate (SCN⁻).
What is the formula for the carbonate ion?
Answer: CO₃²⁻
The carbonate ion has the formula CO₃²⁻ and carries a 2− charge.
What is the formula for the sulfate ion?
Answer: SO₄²⁻
The sulfate ion has the formula SO₄²⁻ and carries a 2− charge. It is the common '-ate' form.
What is the formula for the sulfite ion?
Answer: SO₃²⁻
The sulfite ion has the formula SO₃²⁻, indicating one fewer oxygen atom than sulfate (SO₄²⁻), and carries a 2− charge.
What is the formula for the chromate ion?
Answer: CrO₄²⁻
The chromate ion has the formula CrO₄²⁻ and carries a 2− charge. It is yellow.
What is the formula for the dichromate ion?
Answer: Cr₂O₇²⁻
The dichromate ion has the formula Cr₂O₇²⁻ and carries a 2− charge. It is orange.
What is the formula for the peroxide ion?
Answer: O₂²⁻
The peroxide ion has the formula O₂²⁻, consisting of two oxygen atoms bonded together with an overall 2− charge. It should not be confused with oxide (O²⁻).
What is the formula for the oxalate ion?
Answer: C₂O₄²⁻
The oxalate ion has the formula C₂O₄²⁻ and carries a 2− charge.
What is the formula for the thiosulfate ion?
Answer: S₂O₃²⁻
The prefix thio- means one oxygen atom has been replaced by a sulfur atom. Sulfate (SO₄²⁻) becomes thiosulfate (S₂O₃²⁻).
What is the formula for the hydrogen phosphate ion?
Answer: HPO₄²⁻
Phosphate (PO₄³⁻) can add hydrogen twice. Hydrogen phosphate is HPO₄²⁻, with one hydrogen and a 2− charge.
What is the formula for the silicate ion?
Answer: SiO₃²⁻
The silicate ion has the formula SiO₃²⁻ and carries a 2− charge.
What is the formula for the phosphate ion?
Answer: PO₄³⁻
The phosphate ion has the formula PO₄³⁻ and carries a 3− charge. It is the common '-ate' form.
What is the formula for the phosphite ion?
Answer: PO₃³⁻
The phosphite ion has the formula PO₃³⁻, indicating one fewer oxygen atom than phosphate (PO₄³⁻), and carries a 3− charge.
What is the formula for the borate ion?
Answer: BO₃³⁻
The borate ion has the formula BO₃³⁻ and carries a 3− charge.
What is the formula for the arsenate ion?
Answer: AsO₄³⁻
The arsenate ion has the formula AsO₄³⁻ and carries a 3− charge. It is analogous to the phosphate ion.
A {0} is a group of two or more atoms covalently bonded together that carries an overall electric charge.
Answer: polyatomic ion
A polyatomic ion is a group of two or more atoms covalently bonded together that carries an overall electric charge, differing from a monatomic ion which is a single charged atom.
A {0}, such as Na⁺ or Cl⁻, consists of a single charged atom.
Answer: monatomic ion
A monatomic ion consists of a single charged atom, unlike a polyatomic ion which is a group of atoms.
Most polyatomic anions are {0}: a central atom bonded to oxygen atoms.
Answer: oxyanions
Oxyanions are polyatomic anions where a central atom is bonded to oxygen atoms.
{0} is SO₄²⁻ and sulfite is SO₃²⁻; both carry a 2− charge.
Answer: Sulfate
Sulfate (SO₄²⁻) is the common form of the oxyanion, while sulfite (SO₃²⁻) has one fewer oxygen atom.
Sulfate is SO₄²⁻ and {0} is SO₃²⁻; both carry a 2− charge.
Answer: sulfite
Sulfite (SO₃²⁻) is the form of the oxyanion with one fewer oxygen atom than sulfate (SO₄²⁻), but the charge remains the same.
{0}, NH₄⁺, forms when ammonia (NH₃) gains a proton.
Answer: Ammonium
Ammonium (NH₄⁺) is a polyatomic cation formed when ammonia (NH₃) gains a proton.
{0}, H₃O⁺, forms when water gains a proton and is central to acid–base chemistry.
Answer: Hydronium
Hydronium (H₃O⁺) forms when water gains a proton and is the form in which H⁺ exists in aqueous solutions.
The prefix per- combined with -ate marks one more oxygen than the -ate form, as seen in {0} (ClO₄⁻).
Answer: perchlorate
Perchlorate (ClO₄⁻) has one more oxygen atom than chlorate (ClO₃⁻), indicated by the 'per-' prefix.
The prefix hypo- combined with -ite marks one fewer oxygen than the -ite form, as in {0} (ClO⁻).
Answer: hypochlorite
Hypochlorite (ClO⁻) has one fewer oxygen atom than chlorite (ClO₂⁻), indicated by the 'hypo-' prefix.
Carbonate, CO₃²⁻, becomes {0} (bicarbonate), HCO₃⁻, when a hydrogen ion is added.
Answer: hydrogen carbonate
Adding a hydrogen ion to carbonate (CO₃²⁻) forms hydrogen carbonate (HCO₃⁻), also known as bicarbonate.
Sulfate, SO₄²⁻, becomes {0}, S₂O₃²⁻, when one oxygen atom has been replaced by a sulfur atom.
Answer: thiosulfate
The prefix 'thio-' indicates that one oxygen atom in sulfate (SO₄²⁻) has been replaced by a sulfur atom, forming thiosulfate (S₂O₃²⁻).
{0} is the most important 3− charge ion, appearing in fertilizers, DNA's sugar-phosphate backbone, and ATP.
Answer: Phosphate
Phosphate (PO₄³⁻) is a crucial ion found in many biological and chemical contexts, including DNA and ATP.
{0}, O₂²⁻, must not be confused with oxide, O²⁻, as it is a two-oxygen unit with an oxygen–oxygen bond.
Answer: Peroxide
Peroxide (O₂²⁻) is a polyatomic ion consisting of two oxygen atoms bonded together, distinct from a single oxide ion (O²⁻).
{0}, CrO₄²⁻, is yellow, and dichromate, Cr₂O₇²⁻, is orange.
Answer: Chromate
Chromate (CrO₄²⁻) is a yellow ion, while dichromate (Cr₂O₇²⁻) is orange, and they interconvert depending on pH.
Chromate, CrO₄²⁻, is yellow, and {0}, Cr₂O₇²⁻, is orange.
Answer: dichromate
Dichromate (Cr₂O₇²⁻) is an orange ion, while chromate (CrO₄²⁻) is yellow, and they interconvert depending on pH.
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