Naming and Writing Formulas of Inorganic Compounds – Study Notes

Unit Outcomes

• At the end of this unit, students are expected to:
  • Demonstrate competence in the basic concepts and principles of chemistry.
  • Correctly name and write chemical formulas of common inorganic compounds.

Unit Outline

• Binary Ionic Compounds
• Binary Covalent Compounds
• Ionic Compounds with Polyatomic Ions
• Acids

BINARY IONIC COMPOUNDS

• Definition: Compounds composed of a positive ion (cation) and a negative ion (anion).
• General rule: The cation is written first in the formula, followed by the anion.
• In naming these compounds:
  • The cation is named first; the anion second.
  • A monatomic cation takes its name from the element (e.g., Na^+ → sodium).
  • A monatomic anion is named by taking the root of the element name and adding -ide (e.g., Cl^- → chloride).
• Formula example: a binary ionic compound always shows the ion charges balanced to give an electrically neutral formula.

Type I - Binary Ionic Compounds

• Definition: Cations form only one possible charge.
• Nomenclature rule: Name the cation first using the element name; name the anion second.
• Examples with ions and names:
  • NaCl: Na^+ and Cl^- → Sodium chloride, represented as $ext{NaCl}$
  • KI: K^+ and I^- → Potassium iodide, represented as $ext{KI}$
  • CaS: Ca^{2+} and S^{2-} → Calcium sulfide, represented as $ext{CaS}$
  • Li3N: Li^+ and N^{3-} → Lithium nitride, represented as $ext{Li}</em>3 ext{N}$
  • CsBr: Cs^+ and Br^- → Cesium bromide, represented as $ext{CsBr}$
  • MgO: Mg^{2+} and O^{2-} → Magnesium oxide, represented as $ext{MgO}$
• Common cations (Type I): H^+, Li^+, Na^+, K^+, Be^{2+}, Mg^{2+}, Ca^{2+}, Sr^{2+}, Ba^{2+}, Al^{3+} (and other metals that form a single cation).
• Common anions (Type I): F^-, Cl^-, Br^-, I^-, O^{2-}, S^{2-}, N^{3-}, P^{3-}, etc.

Table: Common Monatomic Cations and Anions (selected)

• Cation names: H^+, Li^+, Na^+, K^+, Be^{2+}, Mg^{2+}, Ca^{2+}, Ba^{2+}, Al^{3+}
• Anion names: H^-, F^-, Cl^-, Br^-, I^-, O^{2-}, S^{2-}, N^{3-}, P^{3-}, etc.
• Example pairings:
  • H^+ → Hydride (as anion form: H^- is hydride)
  • Na^+ → Sodium; Cl^- → Chloride
  • Mg^{2+} → Magnesium; O^{2-} → oxide
• Notes on common ions:
  • In many compounds, silver (Ag^+) is treated without a Roman numeral notation (AgCl is commonly called silver chloride).
  • Zinc (Zn^{2+}) also does not require a Roman numeral notation because Zn forms only +2.

Type II - Binary Ionic Compounds

• Definition: Metals that form more than one type of positive ion (variable valency).
• Naming rule: The charge on the metal ion must be specified in the cation name using a Roman numeral.
• Examples:
  • FeCl2 contains Fe^{2+} → iron(II) chloride; formula shown as $ext{FeCl}</em>2.$
  • FeCl3 contains Fe^{3+} → iron(III) chloride; formula shown as $ext{FeCl}</em>3.$
• Older nomenclature (used in some literature):
  • Higher charge ion ends in -ic, lower charge ion ends in -ous.
  • Example: Fe^{3+} → ferric ion; Fe^{2+} → ferrous ion.
  • Names for chlorides: ferric chloride, ferrous chloride.
• Partial systematic name table examples (for ions and their common oxides/chlorides):
  • Fe^{3+} → Iron(III)
  • Fe^{2+} → Iron(II)
  • Pb^{4+} → Lead(IV)
  • Pb^{2+} → Lead(II)
  • Cu^{2+} → Copper(II)
  • Cu^{+} → Copper(I)
  • Hg^{2+} → Mercury(II)
  • Hg_2^{2+} or Hg^{+} (context-dependent; mercury(I) and mercuric terms appear in older nomenclature)
  • Co^{3+} → Cobalt(III)
  • Co^{2+} → Cobalt(II)
  • Ag^+ → Silver (commonly without Roman numeral in many contexts)
  • Sn^{4+} → Tin(IV)
  • Sn^{2+} → Tin(II)
  • Zn^{2+} → Zinc(II)
  • Cd^{2+} → Cadmium(II)
• Variable valency examples with oxide formulas:
  • Ferrous oxide (Iron(II) oxide): $ext{FeO}$
  • Ferric oxide (Iron(III) oxide): $ext{Fe}<em>2 ext{O}</em>3$
  • Cuprous oxide (Copper(I) oxide): $ext{Cu}_2 ext{O}$
  • Cupric oxide (Copper(II) oxide): $ext{CuO}$
  • Plumbous oxide (Lead(II) oxide): $ext{PbO}$
  • Plumbic oxide (Lead(IV) oxide): $ext{PbO}_2$
  • Stannous oxide (Tin(II) oxide): $ext{SnO}$
  • Stannic oxide (Tin(IV) oxide): $ext{SnO}_2$
  • Mercurous oxide (Mercury(I) oxide): $ext{Hg}_2 ext{O}$
  • Mercuric oxide (Mercury(II) oxide): $ext{HgO}$

Type II - Summary: Variable Valency of Metals

• Decide if the compound contains a Type I or Type II cation:
  • Type I: fixed-charge cation; name the cation using the element name.
  • Type II: metal forms multiple cations; determine cation charge and include a Roman numeral in the cation name.
• Common notes:
  • Group 1A and Group 2A metals (and aluminum) typically form only one type of cation and do not require Roman numerals in standard nomenclature.
  • Some transition metals require Roman numerals due to multiple possible charges.

BINARY IONIC COMPOUNDS WITH POLYATOMIC IONS

• Polyatomic ions are ions composed of more than one atom and carry a net charge (e.g., NH4^+, NO3^-, SO_4^{2-}).
• Example: Ammonium nitrate, NH4NO3, contains the polyatomic ions NH4^+ and NO3^-
• Polyatomic ions have to be memorized for naming compounds containing them.

IONIC COMPOUNDS WITH POLYATOMIC IONS: OXYANIONS

• Oxyanions are polyatomic ions that contain oxygen and another element.
• In a series with the same central element and different numbers of oxygens:
  • The ion with fewer oxygens ends in -ite.
  • The ion with more oxygens ends in -ate.
  • Example: sulfite (SO3^{2-}) and sulfate (SO4^{2-}).
• For a longer series with even more/less oxygens:
  • Use hypo- (fewest oxygens) and per- (most oxygens) prefixes.
  • Examples: hypochlorite (ClO^-), chlorite (ClO2^-), chlorate (ClO3^-), perchlorate (ClO_4^-).

Table: Common Polyatomic Ions (selected)

• Ammonium: $ext{NH}_4^{+}$
• Nitrate: $ext{NO}_3^{-}$
• Nitrite: $ext{NO}_2^{-}$
• Carbonate: $ext{CO}_3^{2-}$
• Hydrogen carbonate (bicarbonate): $ext{HCO}_3^{-}$
• Chlorite: $ext{ClO}_2^{-}$
• Chlorate: $ext{ClO}_3^{-}$
• Perchlorate: $ext{ClO}_4^{-}$
• Acetate: $ext{C}<em>2 ext{H}</em>3 ext{O}_2^{-}$
• Permanganate: $ext{MnO}_4^{-}$
• Chromate: $ext{CrO}_4^{2-}$
• Dichromate: $ext{Cr}<em>2 ext{O}</em>7^{2-}$
• Sulfate: $ext{SO}_4^{2-}$
• Sulfite: $ext{SO}_3^{2-}$
• Hydrogen sulfate (bisulfate): $ext{HSO}_4^{-}$
• Hydroxide: $ext{OH}^{-}$
• Cyanide: $ext{CN}^{-}$
• Phosphate: $ext{PO}_4^{3-}$
• Hydrogen phosphate: $ext{HPO}_4^{2-}$
• Dihydrogen phosphate: $ext{H}<em>2 ext{PO}</em>4^{-}$
• Oxalate: $ext{C}<em>2 ext{O}</em>4^{2-}$
• Thiosulfate: $ext{S}<em>2 ext{O}</em>3^{2-}$

BINARY COVALENT COMPOUNDS (Type III)

• Characteristics:
  • No metals present.
  • First element is named first using the full element name.
  • Second element is named as if it were an anion.
  • Prefixes denote the numbers of atoms present.
  • The prefix mono- is not used for the first element.
• Example naming rules (for CO2, N2O, etc.):
  • CO_2 → carbon dioxide (not monocarbon dioxide)
  • N_2O → dinitrogen monoxide
  • NO → nitrogen monoxide
  • NO_2 → nitrogen dioxide
  • N2O3 → dinitrogen trioxide
  • N2O4 → dinitrogen tetroxide
  • N2O5 → dinitrogen pentoxide
• Commonly used common names (not strictly systematic):
  • Water, ammonia, hydrogen peroxide are frequently used instead of their systematic Type III names.
  • Systematic names for these are not typically used in practice.
• Prefix table (Type III):
  • mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-
  • Usage notes: mono- is never used for the first element.

Prefixes Used to Indicate Numbers in Names

• 1 → mono-
• 2 → di-
• 3 → tri-
• 4 → tetra-
• 5 → penta-
• 6 → hexa-
• 7 → hepta-
• 8 → octa-
• 9 → nona-
• 10 → deca-

ACIDS

• Definition: A molecule in which one or more H^+ ions are attached to an anion.
• Naming rules depend on whether the anion contains oxygen.
• If the anion ends in -ide, the acid name uses hydro- prefix and -ic suffix.
  • Example: HCl (aqueous) → hydrochloric acid; HCN → hydrocyanic acid; H_2S → hydrosulfuric acid.
• If the anion contains oxygen, the acid name is formed from the root name of the anion with a suffix of -ic or -ous, depending on the anion.
• If the anion name ends in -ate, the suffix -ic is added to the root name.
  • Examples: $ext{H}<em>2 ext{SO}</em>4 <br /> ightarrow ext{sulfuric acid}$ (sulfate, SO_4^{2-})
  • $ext{H}<em>3 ext{PO}</em>4 <br /> ightarrow ext{phosphoric acid}$ (phosphate, PO_4^{3-})
  • $ext{HC}<em>2 ext{H}</em>3 ext{O}<em>2 ightarrow ext{acetic acid}$ (acetate, C2H3O2^{-})
• If the anion has an -ite ending, replace -ite with -ous.
  • Examples: $ext{H}<em>2 ext{SO}</em>3 <br /> ightarrow ext{sulfurous acid}$ (sulfite, SO_3^{2-})
  • $ext{HNO}<em>2 ightarrow ext{nitrous acid}$ (nitrite, NO2^{-})
• Example acid names from oxyanions:
  • Perchlorate (ClO_4^-) → perchloric acid
  • Chlorate (ClO_3^-) → chloric acid
  • Chlorite (ClO_2^-) → chlorous acid
  • Hypochlorite (ClO^-) → hypochlorous acid
• Names of acids that do not contain oxygen (Table 2.7 reference):
  • HF → hydrofluoric acid
  • HCl → hydrochloric acid
  • HBr → hydrobromic acid
  • HI → hydroiodic acid
  • HCN → hydrocyanic acid
  • HNO_3 → nitric acid (oxygen-containing)
  • HNO_2 → nitrous acid (no oxygen? actually contains O but in nitrite form)
  • H_2S → hydrosulfuric acid (sulfur-containing without oxygen in the acid name form)
  • H2CO3 → carbonic acid (oxygen-containing)
• Examples from oxygen-containing acids (Table 2.8):
  • H2SO4 → sulfuric acid
  • H2SO3 → sulfurous acid
  • H3PO4 → phosphoric acid
  • H2CO3 → carbonic acid
  • HC2H3O_2 → acetic acid

Naming flow for acids (summary)

• Does the anion contain oxygen?
  • No → use hydro- + root of anion + -ic acid (e.g., HCl → hydrochloric acid; HCN → hydrocyanic acid).
  • Yes → if the anion ends with -ate, use -ic; if it ends with -ite, use -ous.
• Examples:
  • HClO_4 → perchloric acid
  • HClO_3 → chloric acid
  • HClO_2 → chlorous acid
  • HClO → hypochlorous acid
  • H2SO4 → sulfuric acid
  • H3PO4 → phosphoric acid
  • HC2H3O_2 → acetic acid

Quick study links and practical notes

• Systems of naming help ensure clear communication across languages and labs; different systems (I, II, III) reflect charges and composition.
• The Roman numeral in Type II conveys oxidation state and guides formula writing via charge balance.
• Polyatomic ions require memorization because their names do not decompose into simple element-name rules.
• Some elements (Ag, Zn) have naming conventions that deviate from the general Type I/II rules in common practice.
• Common names (water, ammonia, hydrogen peroxide) are widely used despite not following systematic Type III naming rules.

Connections to foundational principles

• Charge balance: Neutral formulas arise from opposite charges balancing; guides both naming and formula writing.
• Oxidation states: Roman numerals indicate the oxidation state of the metal in Type II compounds.
• Electronegativity trends: Distinguishes ionic (often metal + nonmetal) from covalent bonding (nonmetals share electrons) and informs naming conventions (Type III).
• Real-world relevance: Accurate naming ensures unambiguous communication in chemical industries, pharmaceuticals, and academic research.

Practical implications and ethics

• Misnaming compounds can lead to dangerous misinterpretations in synthesis, dosing, and safety data.
• Using standard naming conventions supports reproducibility and safety across laboratories.
• Awareness of exceptions (e.g., Ag^+ roman-numeral omission, Zn^{2+} always fixed) is essential for accurate communication.

Quick reference formulas and examples (LaTeX)

• Sodium chloride: $ext{NaCl}$
• Potassium iodide: $ext{KI}$
• Calcium sulfide: $ext{CaS}$
• Lithium nitride: $ext{Li}_3 ext{N}$
• Magnesium oxide: $ext{MgO}$
• Iron(II) chloride: $ext{FeCl}_2$
• Iron(III) chloride: $ext{FeCl}_3$
• Copper(I) oxide: $ext{Cu}_2 ext{O}$
• Copper(II) oxide: $ext{CuO}$
• Ammonium nitrate: $ext{NH}<em>4 ext{NO}</em>3$
• Sulfate ion: $ext{SO}_4^{2-}$
• Nitrate ion: $ext{NO}_3^{-}$
• Acetate ion: $ext{C}<em>2 ext{H}</em>3 ext{O}_2^{-}$
• Hydrochloric acid: $ext{HCl} <br /> ightarrow ext{hydrochloric acid}$
• Sulfuric acid: $ext{H}<em>2 ext{SO}</em>4 <br /> ightarrow ext{sulfuric acid}$
• Phosphoric acid: $ext{H}<em>3 ext{PO}</em>4 <br /> ightarrow ext{phosphoric acid}$

End of notes