Common Polyatomic Ions Study Notes

What are polyatomic ions?

• Polyatomic ions are ions that consist of two or more atoms bonded together and carry a net electrical charge.
• They can be positively charged (cations) or negatively charged (anions). In the provided list, most entries are anions, with ammonium as the common cation.
• Understanding these ions is essential for naming salts, writing formulas, and balancing charges in ionic compounds.

Quick reference: List of common polyatomic ions

• Ammonium: $\mathrm{NH_4^+}$
  • Ion name: Ammonium
  • Charge: $+1$
• Carbonate: $\mathrm{CO_3^{2-}}$
  • Ion name: Carbonate
  • Charge: $-2$
• Nitrite: $\mathrm{NO_2^-}$
  • Ion name: Nitrite
  • Charge: $-1$
• Hydrogen carbonate (baking soda form): $\mathrm{HCO_3^-}$
  • Ion name: Hydrogen carbonate
  • Charge: $-1$
• Nitrate: $\mathrm{NO_3^-}$
  • Ion name: Nitrate
  • Charge: $-1$
• Acetate: $\mathrm{C<em>2H</em>3O_2^-}$
  • Ion name: Acetate
  • Charge: $-1$
• Sulfite: $\mathrm{SO_3^{2-}}$
  • Ion name: Sulfite
  • Charge: $-2$
• Oxalate: $\mathrm{C<em>2O</em>4^{2-}}$
  • Ion name: Oxalate
  • Charge: $-2$
• Hydrogen sulfite: $\mathrm{HSO_3^-}$
  • Ion name: Hydrogen sulfite
  • Charge: $-1$
• Permanganate: $\mathrm{MnO_4^-}$
  • Ion name: Permanganate
  • Charge: $-1$
• Sulfate: $\mathrm{SO_4^{2-}}$
  • Ion name: Sulfate
  • Charge: $-2$
• Dichromate: $\mathrm{Cr<em>2O</em>7^{2-}}$
  • Ion name: Dichromate
  • Charge: $-2$
• Hydrogen sulfate: $\mathrm{HSO_4^-}$
  • Ion name: Hydrogen sulfate
  • Charge: $-1$
• Chromate: $\mathrm{CrO_4^{2-}}$
  • Ion name: Chromate
  • Charge: $-2$
• Hydroxide: $\mathrm{OH^-}$
  • Ion name: Hydroxide
  • Charge: $-1$
• Peroxide: $\mathrm{O_2^{2-}}$
  • Ion name: Peroxide
  • Charge: $-2$
• Cyanide: $\mathrm{CN^-}$
  • Ion name: Cyanide
  • Charge: $-1$
• Hypochlorite: $\mathrm{ClO^-}$
  • Ion name: Hypochlorite
  • Charge: $-1$
• Phosphate: $\mathrm{PO_4^{3-}}$
  • Ion name: Phosphate
  • Charge: $-3$
• Chlorite: $\mathrm{ClO_2^-}$
  • Ion name: Chlorite
  • Charge: $-1$
• Hydrogen phosphate: $\mathrm{HPO_4^{2-}}$
  • Ion name: Hydrogen phosphate
  • Charge: $-2$
• Chlorate: $\mathrm{ClO_3^-}$
  • Ion name: Chlorate
  • Charge: $-1$
• Dihydrogen phosphate: $\mathrm{H<em>2PO</em>4^-}$
  • Ion name: Dihydrogen phosphate
  • Charge: $-1$
• Perchlorate: $\mathrm{ClO_4^-}$
  • Ion name: Perchlorate
  • Charge: $-1$

Notes on patterns and naming tips

• -ite vs -ate vs -per-:
  • The -ite ending usually indicates one fewer oxygen atom than the -ate form (e.g., chlorite ClO₂⁻ vs chlorate ClO₃⁻).
  • The -per- prefix indicates one more oxygen atom than the corresponding -ate form (e.g., perchlorate ClO₄⁻).
• Hydrogen-containing forms often appear as HCO₃⁻, H₂PO₄⁻, HPO₄²⁻, HSO₃⁻, HSO₄⁻, etc., showing how protons are added to the base anion.
• Balancing salts: in ionic compounds, the total charge must be zero; combine cations and anions so their charges cancel.

Quick reference: pattern recognition and examples

• Ammonium is a CATION: $\mathrm{NH_4^+}$ (does not contain oxygen in this list but is a polyatomic ion)
• Common anions include: \mathrm{NO3^-}, \mathrm{NO2^-}, \mathrm{CO3^{2-}}, \mathrm{SO4^{2-}}, \mathrm{SO3^{2-}}, \mathrm{ClO4^-}, \mathrm{ClO3^-}, \mathrm{ClO2^-}, \mathrm{ClO^-}, \mathrm{OH^-}, \mathrm{O2^{2-}}, \mathrm{PO4^{3-}}, \mathrm{HPO4^{2-}}, \mathrm{H2PO4^-}, \mathrm{HCO3^-}, \mathrm{HSO4^-}, \mathrm{HSO3^-}, \mathrm{C2H3O2^-}, \mathrm{C2O4^{2-}}, \mathrm{CrO4^{2-}}, \mathrm{Cr2O7^{2-}}, \mathrm{MnO_4^-}
• Example practice:
  • What is the formula for the nitrate ion? \mathrm{NO_3^-}
  • What is the name of \mathrm{CO_3^{2-}}$$? Carbonate
  • How many oxygens does the chlorate ion have? 3 (ClO₃⁻)

Connections to foundational principles

• Ionic compounds require overall charge neutrality; the sum of positive charges must equal the sum of negative charges.
• Polyatomic ions act as a single unit when balancing equations and naming compounds in inorganic chemistry.
• The same element can form multiple oxyanion species (e.g., chlorine forms hypochlorite, chlorite, chlorate, perchlorate) with varying numbers of oxygen atoms and different charges.

Practical implications

• In laboratory preparation of salts, knowing these ions helps predict solubility, stability, and reactivity.
• In environmental chemistry, these ions appear in water chemistry (e.g., nitrate and sulfate levels affect water quality).