2.8 Naming Inorganic Compounds

Overview

• Naming in chemistry is called chemical nomenclature, a system to provide informative and unique names that convey composition of a substance.

• There are more than 163 million known chemical substances; many have traditional/common names (e.g., water, H₂O; ammonia, NH₃), but most substances are named using a standardized set of rules.

• The major division is between organic and inorganic compounds; in this section we focus on three inorganic categories: ionic compounds, acids, and molecular (binary) compounds.

Ionic Compounds

• Ionic compounds usually consist of metal cations paired with nonmetal anions.

A. Cations

• Cations formed from metal atoms have the same name as the metal: $ext{Na^+} <br>ightarrow ext{sodium ion},\, ext{Zn^{2+}} <br>ightarrow ext{zinc ion},\ ext{Al^{3+}} <br>ightarrow ext{aluminum ion}.$

• If a metal forms cations with different charges, the positive charge is indicated by a Roman numeral in parentheses after the metal name: $ext{Fe^{2+}} <br>ightarrow ext{iron(II) ion},\ ext{Fe^{3+}} <br>ightarrow ext{iron(III) ion},\ ext{Cu^+} <br>ightarrow ext{copper(I) ion},\ ext{Cu^{2+}} <br>ightarrow ext{copper(II) ion}.$

• Charge variation is common among transition metals (group 3B–2B). In addition, some metals form only one cation (no variable charge): groups 1A and 2A metals and Al³⁺ (group 3A).

  • Other metals commonly found with a single common ion include Ag^+ (group 1B), Zn^{2+} (group 2B), Cd^{2+} (group 2B).

• An older naming method used suffixes -ous and -ic for different cation charges (e.g., Fe^{2+} = ferrous ion; Fe^{3+} = ferric ion; Cu^+ = cuprous ion; Cu^{2+} = cupric ion). Modern texts mostly use Roman numerals, though you may encounter the older names elsewhere.

• Cations formed from molecules composed of nonmetals end in -ium: $ext{NH}<em>4^{+} ightarrow ext{ammonium ion},\ ext{H}</em>3 ext{O}^+ <br>ightarrow ext{hydronium ion}.$

• Ions of the same element with different charges can have different properties (e.g., color). Transition metals often form multiple cations; this leads to many possible ionic formulas.

• The Hg₂^{2+} ion is unusual: it is a diatomic-like cation (mercury(I) ion) that behaves as a cation formed from two Hg^+ ions bound together.

B. Anions

• Monatomic anions: replace the element name’s ending with -ide. Examples:

  • $ext{H^-} <br>ightarrow ext{hydride ion},$

  • $ext{O^{2-}} <br>ightarrow ext{oxide ion},$

  • $ext{N^{3-}} <br>ightarrow ext{nitride ion}.$

  • Other common monatomic anions include $ext{F^-} <br>ightarrow ext{fluoride ion}, ext{Cl^-} <br>ightarrow ext{chloride ion}, ext{S^{2-}} <br>ightarrow ext{sulfide ion}.$

• Polyatomic anions ending in -ide exist, e.g., OH^- (hydroxide) and CN^- (cyanide). A few polyatomic anions also still end in -ide, such as O₂^{2-} (peroxide).

• Polyatomic oxyanions (contain oxygen) end in -ate or -ite:

  • -ate is the most common form (e.g., NO₃⁻ nitrate, SO₄^{2-} sulfate, CO₃^{2-} carbonate, PO₄^{3-} phosphate).

  • -ite has one fewer O atom (e.g., NO₂⁻ nitrite, SO₃^{2-} sulfite).

  • For a series of oxyanions across the halogens, prefixes per- and hypo- are used: per- indicates one more O than the -ate form, hypo- indicates one fewer O than the -ite form.

• The charges and number of oxygens relate to the element's position in the periodic table:

  • In the Cl–O family, charges increase leftward across the period; oxygen count increases downward for a given nonmetal central atom. The second period elements (C, N) form up to three oxygens in oxyanions; heavier elements (third period and beyond) can form up to four oxygens.

• Common oxyanions include:

  • $ext{NO}<em>3^- ightarrow ext{nitrate}, ext{NO}</em>2^- <br>ightarrow ext{nitrite},<br>ext{SO}<em>4^{2-} ightarrow ext{sulfate}, ext{SO}</em>3^{2-} <br>ightarrow ext{sulfite},<br>ext{ClO}<em>4^- ightarrow ext{perchlorate}, ext{ClO}</em>3^- <br>ightarrow ext{chlorate},<br>ext{ClO}_2^- <br>ightarrow ext{chlorite},<br>ext{ClO}^- <br>ightarrow ext{hypochlorite}.</p></li></ul></li><li><p>Naming of acids is closely related to the names of their anions (see acids section).</p></li></ul><h5 collapsed="false" seolevelmigrated="true">C. Names of Ionic Compounds</h5><ul><li><p>Ionic compound names are written with the cation name first, followed by the anion name: e.g.,</p><ul><li><p>$ ext{CaCl}_2
    ightarrow ext{calcium chloride},$</p></li><li><p>$ ext{Al(NO}3)3
    ightarrow ext{aluminum nitrate},$</p></li><li><p>$ ext{Cu(ClO}4)2
    ightarrow ext{copper(II) perchlorate}$(cupric perchlorate).</p></li></ul></li><li><p>When a polyatomic ion is present, parentheses are used around the polyatomic ion if there are multiple such ions in the formula: e.g.,$ ext{Al(NO}3)3,\, ext{Ca(OH)}_2.$</p></li></ul><h5 collapsed="false" seolevelmigrated="true">D. Practice: Naming Ionic Compounds</h5><ul><li><p>Example answers from text:</p><ul><li><p>a)$ ext{K}2 ext{SO}4
    ightarrow ext{potassium sulfate},$</p></li><li><p>b)$ ext{Ba(OH)}_2
    ightarrow ext{barium hydroxide},$</p></li><li><p>c)$ ext{FeCl}_3
    ightarrow ext{iron(III) chloride}.$</p></li></ul></li><li><p>To determine the charge on cations with variable charge, identify the anion charge and balance; if iron appears with three chloride ions, the cation must be$ ext{Fe}^{3+}.$</p></li><li><p>Example practice items in the text include determining names of compounds such as$ ext{NH}4 ext{Br}, ext{ Cr}2 ext{O}3, ext{ Co(NO}3)_2.$</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Names and Formulas of Inorganic Acids</h4><ul><li><p>Acids are hydrogen-containing substances that yield H⁺ in water. Inorganic acids are typically written with hydrogen first in the formula (e.g.,$ ext{HCl}, ext{H}2 ext{SO}4$).</p></li><li><p>An acid is composed of an anion plus enough H⁺ to balance the anion's charge. For example, the sulfate ion SO₄^{2-} forms H₂SO₄.</p></li><li><p>Naming rules for inorganic acids (based on the anion name):</p><ul><li><p>If the anion ends in -ide, the acid name changes -ide to -ic and adds hydro- as a prefix:</p></li><li><p>$ ext{Cl}^-
    ightarrow ext{HCl}
    ightarrow ext{hydrochloric acid},$</p></li><li><p>$ ext{S}^{2-}
    ightarrow ext{H}_2 ext{S}
    ightarrow ext{hydrosulfuric acid}.$</p></li><li><p>If the anion ends in -ate or -ite, change to -ic or -ous respectively, and retain the acid suffix “acid”:</p></li><li><p>$ ext{ClO}4^- ightarrow ext{HClO}4
    ightarrow ext{perchloric acid},$</p></li><li><p>$ ext{ClO}3^- ightarrow ext{HClO}3
    ightarrow ext{chloric acid},$</p></li><li><p>$ ext{ClO}2^- ightarrow ext{HClO}2
    ightarrow ext{chlorous acid},$</p></li><li><p>$ ext{ClO}^-
    ightarrow ext{HClO}
    ightarrow ext{hypochlorous acid}.$</p></li><li><p>The prefixes from oxyanion names are retained in the acid names (e.g., perchloric, chlorous, hypochlorous).</p></li></ul></li><li><p>Examples of acids from common anions:</p><ul><li><p>$ ext{CN}^-
    ightarrow ext{HCN}
    ightarrow ext{hydrocyanic acid};$the pure compound HCN is hydrogen cyanide.</p></li><li><p>$ ext{NO}3^- ightarrow ext{HNO}3
    ightarrow ext{nitric acid}.$</p></li><li><p>$ ext{SO}4^{2-} ightarrow ext{H}2 ext{SO}4 ightarrow ext{sulfuric acid};$$ ext{SO}3^{2-}
    ightarrow ext{H}2 ext{SO}3
    ightarrow ext{sulfurous acid}.$</p></li></ul></li><li><p>Old/ballet naming: some acids from -ite or -ate series historically used names like bicarbonate (HCO₃⁻) or bisulfate (HSO₄⁻).</p></li><li><p>Practice acids naming task examples include naming acids from given formulas such as HCN, HNO₃, H₂SO₄, H₂SO₃;</p></li><li><p>The strong link between acid names and their corresponding anions is essential for understanding acid strength and neutralization reactions.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Binary Molecular Compounds (Molecular Compounds)</h4><ul><li><p>These are compounds formed between two nonmetals; naming rules are similar to those for ionic compounds in structure but differ in details:<br>1) The element farther to the left in the periodic table (closer to metals) is written first. An exception: if the compound contains oxygen and chlorine, bromine, or iodine (any halogen except fluorine), oxygen is written last.<br>2) If both elements are in the same group, the one closer to the bottom (more metallic) is named first.<br>3) The second element is given an -ide ending.</p></li><li><p>Prefixes (Table 2.6) indicate the number of atoms of each element. Common prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca- (note: mono- is not used for the first element).</p></li><li><p>When the prefix ends in -a or -o and the second element begins with a vowel, the -a or -o is often dropped (e.g., monoxide vs. carbon monoxide conventions).</p></li><li><p>Examples from the text:</p><ul><li><p>$ ext{Cl}_2 ext{O}
    ightarrow ext{dichlorine monoxide},$</p></li><li><p>$ ext{NF}_3
    ightarrow ext{nitrogen trifluoride},$</p></li><li><p>$ ext{N}2 ext{O}4
    ightarrow ext{dinitrogen tetroxide},$</p></li><li><p>$ ext{P}4 ext{S}{10}
    ightarrow ext{tetraphosphorus decasulfide}.$</p></li></ul></li><li><p>Rule 4 is important because most molecular formulas cannot be predicted directly from the elements using simple charge balance as with ionic compounds.</p></li><li><p>Hydrogen-containing binary molecular compounds are treated as if neutral substances containing H⁺ ions and anions in some contexts (e.g., hydrogen chloride, HCl). The pure compound is named hydrogen halide (e.g., hydrogen chloride for the pure compound); in water, the solution is called hydrochloric acid.</p></li><li><p>Examples:</p><ul><li><p>$ ext{HCl}
    ightarrow ext{hydrogen chloride} ext{ (pure compound) / hydrochloric acid (in solution)},$</p></li><li><p>$ ext{H}_2 ext{S}
    ightarrow ext{hydrogen sulfide} ext{ (pure compound) / hydrosulfic acid (in solution)}.$</p></li></ul></li><li><p>Practice questions in the text include naming binary molecular compounds and deriving formulas from names (e.g., Cl₂O, NF₃, N₂O₄, P₄S₁₀).</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Quick Reference: Common Names of Ions (Learn These First)</h4><ul><li><p>Cations (common, often monatomic):</p><ul><li><p>$ ext{H^+}
    ightarrow ext{hydrogen ion},
    ext{Li^+}
    ightarrow ext{lithium ion},
    ext{Na^+}
    ightarrow ext{sodium ion},
    ext{K^+}
    ightarrow ext{potassium ion},
    ext{Ag^+}
    ightarrow ext{silver ion},$<br>$ ext{Cu^+}
    ightarrow ext{copper(I) or cuprous ion}, ext{Cu^{2+}}
    ightarrow ext{copper(II) or cupric ion},$</p></li></ul></li><li><p>Anions (selected):</p><ul><li><p>$ ext{Cl^-}
    ightarrow ext{chloride ion},
    ext{O^{2-}}
    ightarrow ext{oxide ion},
    ext{S^{2-}}
    ightarrow ext{sulfide ion},
    ext{OH^-}
    ightarrow ext{hydroxide ion},
    ext{CN^-}
    ightarrow ext{cyanide ion}.$</p></li></ul></li><li><p>Some polyatomic ions are named with -ate/-ite endings (e.g., NO₃⁻ nitrate, NO₂⁻ nitrite, SO₄^{2-} sulfate, SO₃^{2-} sulfite).</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Practice and Check Your Understanding (from the text)</h4><ul><li><p>Naming ionic compounds from formulas: e.g., K₂SO₄ → potassium sulfate; Ba(OH)₂ → barium hydroxide; FeCl₃ → iron(III) chloride.</p></li><li><p>Determining formulas from names (for polyatomic ions and charge balance) often requires recognizing polyatomic ions and the cation's charge.</p></li><li><p>Naming inorganic acids from formulas: HCN, HNO₃, H₂SO₄, H₂SO₃, etc., using the rules above.</p></li><li><p>Binary molecular compounds: apply prefixes, consider exceptions for halogens with oxygen, and remember that the first element’s name is used first; the second element ends in -ide.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Connections to Foundational Principles</h4><ul><li><p>Nomenclature reflects composition and bonding type: ionic vs covalent (molecular) bonds.</p></li><li><p>Roman numerals provide a practical way to indicate variable oxidation states for transition metals, aligning with the concept of multiple possible cations.</p></li><li><p>Polyatomic ions and oxyanions illustrate how electron accounting and resonance structures influence names and formulas.</p></li><li><p>The acid naming rules link the inorganic ion world to aqueous chemistry and acid-base behavior, with practical implications in reactions and stoichiometry.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Real-World Relevance and Implications</h4><ul><li><p>Correct naming ensures clear communication in chemical databases, labs, and industry.</p></li><li><p>Understanding acids and bases through nomenclature helps predict reactions, neutralization processes, and environmental chemistry (e.g., acid rain precursors such as sulfuric and nitric acids).</p></li><li><p>Recognizing common ions and their charges is essential for calculating solution chemistry, precipitations, and redox considerations.</p></li></ul><h4 collapsed="false" seolevelmigrated="true">Notation Recap (LaTeX Formatted)</h4><ul><li><p>Cations:$ ext{Na}^+
    ightarrow ext{sodium ion},\, ext{Fe}^{2+}
    ightarrow ext{iron(II) ion},\, ext{Fe}^{3+}
    ightarrow ext{iron(III) ion}.$</p></li><li><p>Anions:$ ext{O}^{2-}
    ightarrow ext{oxide ion},\, ext{N}^{3-}
    ightarrow ext{nitride ion},\, ext{OH}^-
    ightarrow ext{hydroxide ion},\, ext{CN}^-
    ightarrow ext{cyanide ion}.$</p></li><li><p>Oxyanions:$ ext{NO}3^- ightarrow ext{nitrate}, ext{NO}2^-
    ightarrow ext{nitrite}, ext{SO}4^{2-} ightarrow ext{sulfate}, ext{SO}3^{2-}
    ightarrow ext{sulfite}.$</p></li><li><p>Acids from -ide:$ ext{Cl}^-
    ightarrow ext{chloride}
    ightarrow ext{hydrochloric acid (HCl in solution)},$<br>-ite/-ate:$ ext{ClO}4^- ightarrow ext{perchlorate} ightarrow ext{perchloric acid (HClO}4),$$ ext{ClO}3^- ightarrow ext{chlorate} ightarrow ext{chloric acid (HClO}3),$$ ext{ClO}2^- ightarrow ext{chlorite} ightarrow ext{chlorous acid (HClO}2),$$ ext{ClO}^-
    ightarrow ext{hypochlorite}
    ightarrow ext{hypochlorous acid (HClO)}.$</p></li><li><p>Binary molecular prefixes:$ ext{Cl}2 ext{O} ightarrow ext{dichlorine monoxide},\, ext{NF}3
    ightarrow ext{nitrogen trifluoride},\, ext{N}2 ext{O}4
    ightarrow ext{dinitrogen tetroxide},\, ext{P}4 ext{S}{10}
    ightarrow ext{tetraphosphorus decasulfide}.$$