Ionic Compounds: Nomenclature and Formula Writing (Comprehensive Notes)

Ionic Compounds and Nomenclature: Quick Notes for Exam Prep

• Types of compounds

  • Ionic compounds vs. molecular (covalent) compounds

  • Nomenclature is the system of writing formulas quickly and giving correct names for these compounds

  • Ionic nomenclature is the more challenging; molecular nomenclature will be covered in a later lecture

• Key terms you should know

  • Mineral: an ionic compound that occurs naturally in nature (e.g., table salt, sodium chloride)

  • Sodium chloride formula: $ext{NaCl}$

  • Empirical formula: the simplest whole-number ratio of ions in an ionic compound

• Writing formulas for ionic compounds (criss-cross method)

  • Given ions with charges, balance them to make a neutral compound

  • Example 1: Mn^{6+} and O^{2-}

  • To balance +6 with -2, you need three O^{2-} ions to cancel +6 charge

  • Empirical formula: $ext{MnO}_3$

  • Important note on criss-crossing

  • The criss-cross method assigns the magnitude of each ion’s charge as the subscript of the other ion

  • Example 2: Mg^{2+} and PO_4^{3-} (phosphate)

    • Criss-cross would yield Mg3PO4^2, but you must place the polyatomic ion correctly and reduce if possible

    • Correct formula: $ext{Mg}<em>3( ext{PO}</em>4)_2$

  • Polyatomic ions and parentheses

  • When a formula contains a polyatomic ion more than once, enclose the polyatomic ion in parentheses

    • Example: $ext{Ca(NO}<em>3)</em>2$ (calcium nitrate) has two NO_3^- units

  • If there is only one polyatomic ion unit, no parentheses are needed

    • Example: $ext{BaSO}_4$ (barium sulfate) has one sulfate unit

• Rules for writing and balancing formulas

  • Use the lowest whole-number subscripts (lowest terms) for empirical formulas

  • Always balance charges to achieve neutrality

  • If the compound contains a polyatomic ion and more than one such ion, use parentheses around the polyatomic ion

  • Do not indicate quantity of identical ions with a Roman numeral or other notation in the formula itself

• Naming ionic compounds from formulas

  • Simple binary ionic compounds (metal + nonmetal, fixed metal oxidation state)

  • Name the metal (unchanged) + nonmetal root + suffix -ide

    • Chlorine: Cl^- → chloride

    • Fluorine: F^- → fluoride

    • Magnesium fluoride: formula $ext{MgF}_2$; name: magnesium fluoride

  • Example: $ext{Mg}^{2+} + ext{F}^- <br>ightarrow ext{MgF}_2$ → magnesium fluoride

  • Metals with variable oxidation states (common for transition metals)

  • You must indicate the charge of the metal with a Roman numeral in the name

  • Example: iron can be Fe^{2+} or Fe^{3+}

    • Fe^{2+} with Cl^- → iron(II) chloride

    • Fe^{3+} with Cl^- → iron(III) chloride

  • Silver as an example of a fixed oxidation state

  • Silver commonly forms Ag^+ in ionic compounds

  • Silver oxide: Ag_2O (no Roman numeral needed because Ag has a fixed +1 oxidation state in these compounds)

  • Example work-throughs from the transcript

  • MnO: manganese(II) oxide (inferred from charges, Mn^{2+} and O^{2-}); formula is actually MnO, and the name would reflect Mn^{2+} → manganese(II) oxide

  • TiS_2: titanium(IV) sulfide

    • Sulfide ion: S^{2-}

    • To balance with Ti^{4+}, the Roman numeral indicates +4 oxidation state on titanium

  • When given a name, determine the formula by writing the charges first

  • Important step before constructing the formula

  • This helps avoid common mistakes like CaPO4 instead of Ca3(PO4)2

  • Examples

    • nickel sulfide: NiS (Ni^{2+} and S^{2-})

    • silver oxide: Ag_2O (Ag^{+} and O^{2-}; no Roman numeral needed)

    • calcium phosphate: Ca3(PO4)_2

    • titanium(IV) sulfide: TiS_2 (Ti^{4+}, S^{2-} × 2 → total -4; Ti^{4+} balances)

• Worked examples (summary from the transcript)

  • Mn^{6+} and O^{2-}: formula $ext{MnO}_3$

  • Mg^{2+} and PO4^{3-}: formula $ext{Mg}</em>3( ext{PO}<em>4)</em>2$

  • Ba^{2+} and SO4^{2-}: formula $ext{BaSO}</em>4$ (one sulfate unit, no parentheses needed)

  • Ag^{+} and O^{2-}: formula $ext{Ag}_2 ext{O}$ (silver oxide; silver is +1)

  • Ti^{4+} and S^{2-} (two sulfide ions): formula $ext{TiS}_2$, name: titanium(IV) sulfide

  • Ca^{2+} and PO4^{3-} (calcium phosphate): formula $ext{Ca}</em>3( ext{PO}<em>4)</em>2$

• Quick reference rules to memorize

  • Step 1: Identify charges on ions (including polyatomic ions)

  • Step 2: Use criss-cross to form the initial formula, then simplify to the smallest whole-number subscripts

  • Step 3: If a polyatomic ion appears more than once in the formula, place it in parentheses

  • Step 4: When naming, use the metal name + nonmetal root + ide (for fixed-charge metals) or use Roman numerals for transition metals with variable charge

  • Step 5: For polyatomic ions, remember: one unit does not require parentheses; more than one unit requires parentheses

  • Step 6: Always ensure the final formula is neutral and in the lowest terms

• Connections and relevance

  • Nomenclature underpins chemical communication in labs and industry

  • Understanding charges helps predict compound formation and helps with balancing reactions

  • The concept of minerals links chemistry to geology and natural resources (e.g., salts found in mines)

• Quick TLC (takeaways for exams)

  • If a metal has a fixed oxidation state, name: metal + nonmetal root + -ide (no Roman numeral)

  • If a metal has variable oxidation states, include Roman numeral in the name

  • Always write charges first when solving the “name → formula” direction to avoid errors

  • Use parentheses around polyatomic ions when needed (multiple units) to keep subscripts clear

  • Examples to rehearse: $ext{MnO}<em>3, ext{Mg}</em>3( ext{PO}<em>4)</em>2, ext{BaSO}<em>4, ext{Ag}</em>2 ext{O}, ext{TiS}<em>2, ext{Ca}</em>3( ext{PO}<em>4)</em>2$

• Final notes

  • The next video will cover rules for naming molecular (covalent) compounds, which has a different set of conventions