CHEM051 - Nomenclature: Part 1 – Binary Compounds

Scope & Assumptions

• Focus: nomenclature of binary (two-element) inorganic compounds.
• Audience: first-semester college chemistry; assumed prior knowledge of
  • Electron configuration
  • Electronegativity trends
  • Ionic/covalent bonding fundamentals
  • Acid ionization
• Three naming systems covered
  • Stock (IUPAC recommended; most comprehensive)
  • Prefix (Greek/Latin numerical prefixes; non-metal ⇆ non-metal)
  • US-ic (older, limited to divalent metals)
• A follow-up video handles polyatomic compounds.

Binary Compounds & Formula Writing Basics

• "Binary" = compound containing exactly two different elements.
• A chemical formula conveys
  • Type of each atom (element symbol)
  • Number of each atom (subscripts)
• Rule: never write a subscript “1.”
  • $\text{H}<em>2\text{O}$ not $\text{H}</em>2\text{O}_1$
• Subscripts vs superscripts
  • Subscripts indicate count; superscripts indicate charge/oxidation state.
• Parentheses around a single element are never used; they enclose polyatomic groups only.

Ordering Symbols in a Formula

Electronegativity Criterion

• Less electronegative element is written first.
  • Example: $\text{Na}<em>2\text{S}$ because \chi{\text{Na}} = 1.0 < \chi_{\text{S}} = 2.5
• Metals (low $\chi$) always precede non-metals (high $\chi$) in ionic formulas.

Practice

• 1 S + 2 Na → $\text{Na}_2\text{S}$
• 1 P + 3 Br → $\text{PBr}_3$ (both non-metals; used $\chi$ values 2.1 vs 2.8)
• 1 Mg + 2 Cl → $\text{MgCl}_2$ (recognize metal + non-metal)

Historical / Conventional Exceptions

1. Hydrocarbons (C–H)
   • Carbon precedes hydrogen despite $\chi<em>{\text{C}} > \chi</em>{\text{H}}$.
   • E.g. $\text{CH}<em>4$ (methane), $\text{C}</em>2\text{H}_6$ (ethane).
2. Nitrogen-containing binaries
   • Nitrogen is usually written first regardless of $\chi$.
   • $\text{NH}<em>3$ (ammonia), $\text{N}</em>2\text{O}_5$.
   • Rationale: avoids mis-hinting at acidity ("H" leading implies acid).

Fixed & Variable Oxidation Numbers

• Every pure element: oxidation number $0$.
• ~20 elements possess a single, fixed, non-zero oxidation state in compounds.
  • Group $1A$ metals $\rightarrow +1$
  • Group $2A$ metals $\rightarrow +2$
  • $\text{Al},\;\text{Ga} \rightarrow +3$; $\text{Zn},\;\text{Cd} \rightarrow +2$, etc.
• Non-metals often have one negative but multiple positive states.
  • Halides always $-1$ when acting as anions, but Cl can be $+1,+3,+5,+7$ in oxo-species.

Common Anion Names & Charges (must memorize)

• $\text{H}^-$ : hydride
• $\text{C}^{4-}$ : carbide
• $\text{N}^{3-}$ : nitride ; $\text{P}^{3-}$ : phosphide
• $\text{O}^{2-}$ : oxide ; $\text{S}^{2-}$ : sulfide ; $\text{Se}^{2-}$ : selenide
• $\text{F}^-$ : fluoride ; $\text{Cl}^-$ : chloride ; $\text{Br}^-$ : bromide ; $\text{I}^-$ : iodide

Writing Formulas From Names (Ionic)

1. Write cation & anion symbols with charges.
2. Total positive charge = total negative charge.
3. Use lowest whole-number ratio; show as subscripts.
4. Inverse (criss-cross) rule works but reduce if both numbers share a factor.

Examples

• Calcium chloride: $\text{Ca}^{2+}$ & $\text{Cl}^-$ → $\text{CaCl}_2$.
• Boron bromide: $\text{B}^{3+}$ & $\text{Br}^-$ → $\text{BBr}_3$.
• Magnesium phosphide: $\text{Mg}^{2+}$ & $\text{P}^{3-}$ → $\text{Mg}<em>3\text{P}</em>2$ (LCM = 6).
• Watch reduction: $\text{Be}^{2+}$ with $\text{C}^{4-}$ → initial Be₄C₂ → simplified $\text{Be}_2\text{C}$.

Determining Unknown Oxidation Numbers (Multivalent Metals)

• At least one ion in a binary formula has a known fixed charge.
• Let unknown be $x$; solve so Σ(oxid.# × subscript) = $0$.
• Shortcut: inverse rule (subscript of one = charge of other), then scale per actual subscripts.

Practice

• $\text{MnCl}_2$ : $2(-1)+x=0\Rightarrow x=+2$.
• $\text{Mn}<em>2\text{O}</em>3$ : $3(-2)+2x=0\Rightarrow x=+3$.
• $\text{V}<em>3\text{N}</em>5$ : $5(-3)+3x=0\Rightarrow x=+5$.

Naming Systems for Binary Compounds

1. Stock (IUPAC)

• Format: [cation name] (Roman numeral) [anion root + ide]
• Roman numeral = oxidation state only when element is multivalent.
• No prefixes; name gives no direct count info.
• Examples
  • $\text{FeCl}_2$ : iron(II) chloride (green solid)
  • $\text{FeCl}_3$ : iron(III) chloride (yellow/brown)
  • $\text{Al}<em>2\text{O}</em>3$ : aluminum oxide (no numeral since Al fixed $+3$)

2. Prefix (Greek/Latin)

• Restricted to non-metal ⇆ non-metal (covalent) molecules; occasionally persists elsewhere.
• Always prefix the second element; prefix the first only if >1 atom.
• Common prefixes
  • 1 mono-, 2 di-, 3 tri-, 4 tetra-, 5 penta-, 6 hexa-, 7 hepta-, 8 octa-, 9 nona-, 10 deca-.
• Examples
  • $\text{CO}$ : carbon monoxide
  • $\text{CO}_2$ : carbon dioxide
  • $\text{N}<em>2\text{O}</em>5$ : dinitrogen pentoxide
  • $\text{SF}_6$ : sulfur hexafluoride
• Use to distinguish isomers with same Stock name (e.g. $\text{NO}<em>2$ vs $\text{N}</em>2\text{O}_4$).

3. US-ic (classical) System

• Applies when cation shows exactly two oxidation states.
• Lower state → -ous, higher state → -ic.
• Often uses Latin root when English awkward.
  • $\text{Cu}^+$ : cuprous ; $\text{Cu}^{2+}$ : cupric
  • $\text{Fe}^{2+}$ : ferrous ; $\text{Fe}^{3+}$ : ferric
  • $\text{Sn}^{2+}$ : stannous ; $\text{Sn}^{4+}$ : stannic
  • $\text{Pb}^{2+}$ : plumbous ; $\text{Pb}^{4+}$ : plumbic
  • $\text{Hg}_2^{2+}$ (Hg⁺) : mercurous ; $\text{Hg}^{2+}$ : mercuric
• Example translations
  • $\text{PbCl}_2$ : lead(II) chloride = plumbous chloride
  • $\text{Fe}<em>2\text{O}</em>3$ : iron(III) oxide = ferric oxide

Binary Acids (Hydrogen + Non-metal)

• In gas phase → Stock name hydrogen + ide.
• In aqueous solution → hydro-(root)-ic acid.
  • $\text{HF}$ : hydrogen fluoride → hydrofluoric acid
  • $\text{HCl}$ : hydrogen chloride → hydrochloric acid
  • $\text{HBr}$ : hydrobromic acid
  • $\text{HI}$ : hydroiodic acid
  • $\text{H}_2\text{S}$ : hydrogen sulfide → hydrosulfuric acid (diprotic)

Peroxides

• General anion: $\text{O}_2^{2-}$; oxidation number of each O = $-1$.
• Formulas retain the O₂ unit → not reduced (e.g. $\text{Na}<em>2\text{O}</em>2$, not NaO).
• Stock names: [cation] peroxide (hydrogen peroxide, sodium peroxide, etc.).
• Applications
  • Industrial bleaching (pulp, textiles)
  • Propellants (WWI torpedoes)
  • Oxygen generation: $\text{Na}<em>2\text{O}</em>2 + \text{CO}<em>2 \rightarrow \text{Na}</em>2\text{CO}<em>3 + \tfrac{1}{2}\,\text{O}</em>2$
  • Household disinfectant (3 % $\text{H}<em>2\text{O}</em>2$)

Summary of Key Rules

• Write what you mean: no charges in a final neutral formula.
• Subscript 1 never shown; reduce subscripts to lowest ratio unless peroxide or specific convention demands otherwise.
• Order of elements governed by electronegativity except for historic cases (C-H, N-x).
• Stock: include Roman numeral only when cation is multivalent; otherwise omit.
• Prefix: numeric prefixes; strictly non-metal compounds; always prefix 2nd element.
• US-ic: ‑ous (lower), ‑ic (higher) for divalent cations, often Latin roots.
• Determine oxidation states by balancing charges or by inverse rule; essential for both naming and formula writing.

Concept Connections & Significance

• Reinforces periodic trends: electronegativity, group oxidation patterns, metal vs non-metal behavior.
• Oxidation-state logic underpins redox chemistry, acid-base theory, and electron bookkeeping in reactions.
• Accurate nomenclature crucial for
  • Safety (e.g., distinguishing water vs hydrogen peroxide in lab)
  • Industrial procurement & regulation
  • Communicating stoichiometry for quantitative calculations.

Ethical / Practical Implications

• Misnaming chemicals can lead to dangerous misuse (e.g., confusing ferric vs ferrous salts in medicine, or peroxides vs oxides in lab stocking).
• Understanding peroxide reactivity prevents accidents (strong oxidizers can cause fires/explosions).
• Acid nomenclature links directly to handling protocols (binary acids vs oxoacids have different hazards).