Formula-Writing Notes

Writing Formulas for Ionic Compounds

  • 3–step procedure

    • Identify ions

    • Cation = positive, Anion = negative (use periodic table or ion sheet to find charges)

    • Balance charges

    • Write subscripts (cation written first, anion second)

  • Charges = electrons lost/gained ⇒ net charge of compound

CDR (Cross, Drop, Reduce) Method

  • Cross charges, drop signs, reduce whole-number ratio if needed

    • Ex: Fe^{3+} & Cl^- ⇒ cross \rightarrow FeCl_3 (no reduction)

    • If Pb^{4+} & O^{2-} ⇒ cross → Pb2O4 → reduction → PbO2

Polyatomic Ion Rules

  • Treat entire ion as a unit

  • If >1 needed, enclose in parentheses and add subscript

  • Only charges may be reduced, never subscripts inside polyatomic

    • Eg: Ca²+NO³- → Ca(NO3)2

Naming Ionic Compounds

Binary Ionic (metal + non-metal)

  • Metal name stays, non-metal suffix \rightarrow –ide

  • Type I (fixed-charge metals)

    • Groups 1, 2, 13, Zn^{2+}, Ag^+, Cd^{2+}

    • Examples

    • NaCl → sodium chloride

    • CaI_2 → calcium iodide

  • Type II (variable-charge metals)

    • Transition & post-transition; give Roman numeral

    • CuBr → copper(I) bromide (Cu⁺)

    • CuBr_2 → copper(II) bromide (Cu²⁺)

    • Old Latin (not required in AP): cuprous/cupric

Naming Covalent (Molecular) Compounds

  • Two non-metals; use Greek prefixes (mono-, di-, tri- …)

  • First element keeps full name (no “mono-” on first), second becomes –ide

    • N2O4 → dinitrogen tetroxide

    • PCl_3 → phosphorus trichloride

    • N_2S → dinitrogen monosulfide

How to Decide Which System to Use

  1. Is the first element a metal?

    • No → covalent naming

    • Yes → ionic naming

  2. If ionic: metal in Gr 1,2,13,Ag,Zn,Cd (Type I) else Type II

  3. Polyatomic present? keep name of ion

Practice Identification & Naming Highlights

  • CaF_2 → calcium fluoride (Type I)

  • K_2S → potassium sulfide (Type I)

  • CoI_2 → cobalt(II) iodide (Type II)

  • SnF_4 → tin(IV) fluoride

  • OF_2 → oxygen difluoride (covalent)

  • SrS → strontium sulfide (Type I)

  • LiBr → lithium bromide

Acid Nomenclature

Binary Acids (H + non-metal, no O)

  • Rule: "My ride has hydrolics"

    • Hydro + stem + “ic” + acid

    • HCl → hydrochloric acid

Oxyacids (ternary) – contain polyatomic O-anion

  • "I ate something -icky" (-ate → ‑ic)

  • "Sprite is delicious" (-ite → ‑ous)

  • No "hydro" prefix

    • H2SO4(sulfate) → sulfuric acid (add back –ur for pronunciation)

    • H2SO3 (sulfite) → sulfurous acid

    • HNO_3 (nitrate) → nitric acid

    • HC2H3O_2 (acetate) → acetic acid

Multiple-Choice Error Example (Concept Check)

  • TiO_2 named "titanium(II) oxide" is WRONG; O is 2- so Ti must be 4+ → titanium(IV) oxide

Hydrocarbons: Classification & Formulas

  • Hydrocarbon = C & H only

  • Families & General Formulas

    • Alkanes (single bonds): CnH{2n+2), suffix –ane

    • Alkenes (≥1 double): CnH{2n}, suffix –ene

    • Alkynes (≥1 triple): CnH{2n-2}, suffix –yne

    • Aromatic: alternating double/single (e.g., benzene)

  • Carbon forms 4 σ-bonds; double = 1 σ + 1 π; triple = 1 σ + 2 π

Carbon-Count Prefixes (1–10)

  • meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-

Representing Hydrocarbon Structures

  • Structural formula: every bond drawn (tedious)

  • Condensed: group H’s (e.g., CH3CH2CH_3)

  • Line-angle (skeletal): vertices/endpoints = C; H’s implied

    • Each C assumed to have enough H to reach 4 bonds

Branched Hydrocarbon (Substituent) Nomenclature

  1. Find longest continuous chain (base name)

  2. Number chain to give first point of difference the lowest locant; double/triple bonds outrank substituents for numbering

  3. Name substituents (alkyl groups): drop –ane add –yl; list alphabetically

    • Multiple identical groups: di-, tri-, tetra- (do not alphabetize prefixes)

  4. Assemble: locant-substituent + base + suffix (incl. locant for unsaturation)

    • Example: 2,3,4-trimethylpentane

  5. Unsaturations

    • Put locant of first C in double/triple bond: 1-hexyne or hex-1-yne

    • Multiple π bonds: use di-, tri- (e.g. 2,4-hexadiene)

Substituent Examples

  • CH_3- methyl

  • CH3CH2- ethyl

  • Complex alkyls in parentheses when cited in formula (e.g., C(CH3)3)

Cycloalkanes

  • Ring hydrocarbons; prefix cyclo + base

  • Draw by removing 2 terminal H’s and bonding ends together

  • 5- & 6-member rings most stable (≈ 109.5^{\circ} bond angle)

  • One substituent: no locant needed; ≥2: number ring to give lowest set, consider alphabetical priority

    • Example: 1-ethyl-3-methylcyclopentane

Sample Advanced Naming Walk-Throughs

  • 4-ethyl-2,4-dimethyldecane (10-C chain, ethyl + two methyl substituents)

  • 4,4-dimethyl-1-hexyne (6-C, terminal alkyne, two methyls on C-4)

  • methylcyclobutane (single CH₃ on 4-member ring)

Practical/Real-World Connections & Exam Tips

  • Correct names/formulas prevent lab mix-ups (safety critical)

  • Hydrocarbon nomenclature underpins petroleum, polymers, pharmaceuticals

  • On AP & college exams, expect: write formula from name, name from formula, spot errors, identify compound type

  • Always cross-check: total charge =0 (ionic) & carbon valence of 4 (organics)

  • Mnemonic recap

    • "my ride has hydro-ics" (-ide acids)

    • "I ate something -icky" (-ate → ‑ic)

    • "Sprite is delicious" (-ite → ‑ous)

    • "If you're not trying, you're dying" – practice problems daily!