Common Polyatomic Ions Flashcards

A polyatomic ion is a group of two or more atoms covalently bonded together that carries a net electric charge. Common examples: NH4+ (ammonium), OH- (hydroxide), NO3- (nitrate), SO4^2- (sulfate), CO3^2- (carbonate), PO4^3- (phosphate), HCO3- (bicarbonate), CH3COO- (acetate), MnO4- (permanganate), ClO4- (perchlorate). The atoms inside the ion are held by covalent bonds, but the ion as a whole behaves as a single charged unit. It forms ionic bonds with oppositely charged ions in compounds — for example, Na2SO4 = 2 Na+ + SO4^2-, NH4Cl = NH4+ + Cl-, Ca3(PO4)2 = 3 Ca^2+ + 2 PO4^3-. Polyatomic ions show up across biology (PO4^3- in DNA), agriculture (NO3- and SO4^2- in fertilizers), and industry.

A compound containing a polyatomic ion has two kinds of bonding at once: covalent bonds inside the polyatomic group, and ionic bonds between that group and an oppositely charged ion. Take Na2SO4 as the classic example. Inside the SO4^2- ion, sulfur is covalently bonded to four oxygens; the group as a whole carries a net 2- charge. Two Na+ cations balance that charge through electrostatic (ionic) attraction, giving Na2SO4. The general procedure is: form covalent bonds within the polyatomic group, assign the group's net charge from the electron count, then balance positive and negative charges in the formula unit (Al2(SO4)3 balances 2 × +3 against 3 × -2 = 0). On dissolution, the polyatomic ion stays intact: Na2SO4(s) → 2 Na+(aq) + SO4^2-(aq).

Both, at different scales. Within the polyatomic ion the bonds are covalent — atoms share electrons. In SO4^2-, for instance, sulfur shares electron pairs with each of four oxygens, and the group carries a net 2- charge. When that polyatomic ion combines with an oppositely charged ion to form a compound, the bond between them is ionic. So Na2SO4 has covalent S-O bonds inside the SO4^2- unit and ionic Na+ ↔ SO4^2- attractions between units. Other examples follow the same pattern: NH4Cl (covalent N-H, ionic NH4+ ↔ Cl-); KMnO4 (covalent Mn-O, ionic K+ ↔ MnO4-); Ca(NO3)2 (covalent N-O, ionic Ca^2+ ↔ NO3-).

Yes — NH4+ is the most common polyatomic cation. Structure: 1 N + 4 H, tetrahedral with H-N-H bond angles near 109.5°, net charge +1. It forms when ammonia accepts a proton through its lone pair: NH3 + H+ → NH4+ (the new N-H bond is a coordinate covalent bond). Common compounds: NH4Cl (used in dry cells and as a soldering flux), (NH4)2SO4 (a major nitrogen fertilizer), NH4NO3 (fertilizer and explosive), NH4HCO3 (a leavening agent in some baked goods). Lab test: warming an ammonium salt with NaOH releases ammonia, identifiable by its pungent smell and by turning moist red litmus blue: NH4Cl + NaOH → NaCl + NH3↑ + H2O.

Neither — those terms classify elements, not ions. Most polyatomic ions are built from nonmetal atoms: NH4+, OH-, CO3^2-, SO4^2-, PO4^3-, NO3-, ClO3-, CN-. Some polyatomic ions do contain a metal as the central atom — usually a transition metal in a high oxidation state: MnO4- (permanganate), CrO4^2- (chromate), Cr2O7^2- (dichromate), VO3-, MoO4^2-. Complex ions such as [Fe(CN)6]^3-, [Cu(NH3)4]^2+, and AlO2- also include metals. The classification 'metal vs nonmetal' applies to atoms; for ions, the relevant categories are cation/anion and monatomic/polyatomic.

A monatomic ion is a single atom that gained or lost electrons (Na+, Mg^2+, Cl-, O^2-, S^2-). A polyatomic ion is a group of atoms covalently bonded that collectively carry a charge (NH4+, NO3-, SO4^2-). Differences worth memorizing: (1) Number of atoms — one vs two-or-more. (2) Internal bonding — none vs covalent bonds inside. (3) Naming — monatomic anions take an -ide suffix (chloride, oxide); polyatomic anions usually end in -ate or -ite (sulfate/sulfite, nitrate/nitrite). (4) Charge origin — single-atom electron transfer vs net electron count over the whole group. Compounds can mix the two types: NaCl is two monatomic ions, Na2SO4 has a monatomic Na+ paired with a polyatomic SO4^2-, and (NH4)2SO4 has two polyatomic ions.

Yes — the carbonate ion is CO3^2-. There is no neutral CO3 molecule under ordinary conditions. Structure: 1 C + 3 O, trigonal planar with O-C-O angles of 120°. The 2- charge is delocalized equally over the three oxygens by resonance. Common compounds: CaCO3 (limestone, marble, eggshells, antacid tablets), Na2CO3 (washing soda), K2CO3, FeCO3 (siderite). Diagnostic reactions: with dilute acid a carbonate fizzes from CO2 evolution (CaCO3 + 2HCl → CaCl2 + H2O + CO2); on strong heating most metal carbonates decompose (CaCO3 → CaO + CO2, the basis of lime production). In water, carbonate hydrolyzes weakly: CO3^2- + H2O ⇌ HCO3- + OH-, which is why Na2CO3 solutions are basic.

Yes — NH4+ (ammonium) is a polyatomic cation. There is no stable neutral NH4 molecule. Structure: 1 N + 4 H, tetrahedral, bond angles ≈ 109.5°, net charge +1. It forms from ammonia by coordinate covalent bonding: NH3 + H+ → NH4+. Common compounds and uses: NH4Cl (dry cells, expectorant in cough syrups), (NH4)2SO4 (nitrogen fertilizer), NH4NO3 (fertilizer, explosive), (NH4)3PO4 (NPK fertilizer). Properties: ammonium salts are generally soluble; NH4+ is a weak acid (Ka ≈ 5.6 × 10^-10), so its solutions are slightly acidic; warming an ammonium salt with a base releases ammonia (NH4Cl + NaOH → NaCl + NH3↑ + H2O), which is the standard lab test.

Yes — NO3- (nitrate) is one of the most familiar polyatomic anions. There is no stable neutral NO3. Structure: 1 N + 3 O, trigonal planar with O-N-O angles of 120°. The single negative charge is delocalized over the three oxygens by resonance, and nitrogen is in oxidation state +5. Common compounds: NaNO3 (Chile saltpetre, used historically as a fertilizer and food preservative), KNO3 (saltpetre, in fireworks and gunpowder), NH4NO3, AgNO3 (silver nitrate), Ca(NO3)2. Properties: every common nitrate salt is water-soluble, and concentrated nitric acid (HNO3) is a strong oxidizer that can dissolve copper, silver, and many other metals. Lab test: the brown ring test — careful addition of concentrated H2SO4 to a mixture of FeSO4 and a nitrate solution gives a brown ring of [Fe(NO)]^2+ at the layer interface.

Yes — PO4^3- (phosphate) is a polyatomic anion central to biochemistry. There is no stable neutral PO4. Structure: 1 P + 4 O, tetrahedral with O-P-O angles of 109.5°; the 3- charge is delocalized over the four oxygens. Phosphorus is in oxidation state +5. Phosphate family in equilibrium with water: PO4^3- ⇌ HPO4^2- ⇌ H2PO4- ⇌ H3PO4, depending on pH. Biological roles: the sugar-phosphate backbone of DNA and RNA, the high-energy bonds of ATP, and the hydroxyapatite (Ca5(PO4)3OH) of bone and tooth enamel. Industrial: phosphate rock (Ca3(PO4)2) is converted to soluble fertilizer by reaction with sulfuric acid: Ca3(PO4)2 + 2H2SO4 → Ca(H2PO4)2 + 2CaSO4 (single superphosphate).

Yes — SO4^2- (sulfate) is a polyatomic anion. There is no stable neutral SO4 molecule. Structure: 1 S + 4 O, tetrahedral with O-S-O angles of 109.5°. The 2- charge is delocalized equally over the four oxygens; sulfur is in oxidation state +6. Common sulfate compounds: Na2SO4 (Glauber's salt), CaSO4·2H2O (gypsum and plaster of Paris), CuSO4·5H2O (blue vitriol), FeSO4·7H2O (green vitriol, in iron supplements), MgSO4·7H2O (Epsom salt), (NH4)2SO4 (fertilizer), BaSO4 (insoluble; used as X-ray contrast). Lab test: adding BaCl2 to a sulfate-containing solution gives a white BaSO4 precipitate that does not dissolve in dilute HCl. Solubility: most sulfates are water-soluble, with BaSO4, PbSO4, and SrSO4 as the main exceptions and CaSO4 only slightly soluble.