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General Chemistry I · Core Concept

Chemical Formulas and Naming

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On this page 6 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools

In 30 seconds

Chemical formulas show composition: molecular formulas give the actual atom count in a molecule, while empirical formulas give the simplest whole-number ratio. Ionic compounds are named cation-then-anion, with Roman numerals for Type II (variable-charge) metals and named polyatomic ions. Molecular compounds use Greek prefixes (mono-, di-, tri-). Acids are named as binary acids (hydro-…-ic acid) or oxyacids (-ate → -ic, -ite → -ous).

Why this matters

Drugs and lab reagents are named by these rules. Sodium chloride (NaCl) is IV saline; potassium chloride (KCl) is a medication dosed carefully because of its effect on the heart. Hydrochloric acid (HCl) is stomach acid; sodium bicarbonate (NaHCO₃) is baking soda and an antacid. Correct naming prevents medication errors—confusing "chloride" with "chlorite," or "sulfate" with "sulfite," could be dangerous.

The college version

1. Molecular vs. Empirical Formulas

  • : the actual number of atoms in a molecule (H₂O₂, C₆H₁₂O₆).
  • : the simplest whole-number ratio (HO, CH₂O).
  • Ionic compounds and network solids are always written as empirical formulas.
  • Structural formulas additionally show how atoms are connected.

2. Naming Ionic Compounds

Ionic compounds are a metal (or polyatomic ) plus a nonmetal (or polyatomic ).

  • Type I metals (fixed charge): Groups 1 and 2, plus Al³⁺, Zn²⁺, Ag⁺, Cd²⁺. Just name the metal: NaCl = sodium chloride, MgO = magnesium oxide.
  • Type II metals (variable charge): transition metals such as Fe, Cu, Sn, Pb, Hg. Use Roman numerals for the charge: FeCl₂ = iron(II) chloride, FeCl₃ = iron(III) chloride, CuO = copper(II) oxide.
  • Polyatomic ions: named as units (sulfate SO₄²⁻, nitrate NO₃⁻, phosphate PO₄³⁻, ammonium NH₄⁺, hydroxide OH⁻, carbonate CO₃²⁻, bicarbonate HCO₃⁻). Examples: Na₂SO₄ = sodium sulfate, NH₄Cl = ammonium chloride.

3. Naming Molecular (Covalent) Compounds

Two nonmetals: use Greek prefixes for the number of each atom, with an -ide ending on the second element.

  • Prefixes: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.
  • "Mono-" is usually omitted on the first element.
  • CO = carbon monoxide; CO₂ = carbon dioxide; N₂O₄ = dinitrogen tetroxide; PCl₅ = phosphorus pentachloride.

4. Naming Acids

  • Binary acids (H + one nonmetal, no oxygen): hydro- + root + -ic + "acid." HCl = hydrochloric acid; HBr = hydrobromic acid; H₂S = hydrosulfuric acid.
  • Oxyacids (H + a containing oxygen):
    • -ate ion → -ic acid: sulfate SO₄²⁻ → H₂SO₄ = sulfuric acid; nitrate NO₃⁻ → HNO₃ = nitric acid.
    • -ite ion → -ous acid: sulfite SO₃²⁻ → H₂SO₃ = sulfurous acid; nitrite NO₂⁻ → HNO₂ = nitrous acid.

How it works

  1. Identify the compound type (ionic, molecular, or acid).
  2. Ionic: name the cation (with a Roman numeral if Type II), then the anion (element -ide or polyatomic name).
  3. Molecular: apply Greek prefixes to each nonmetal, with -ide on the second.
  4. Acid: decide binary (hydro-…-ic) vs. (-ate → -ic, -ite → -ous).
  5. Verify that charges balance to confirm the formula is correct when writing formulas from names.

Common confusions

Do not confuseWithDifference
Molecular formulaEmpirical formulaMolecular = actual count; empirical = reduced ratio
-ide-ate / -ite-ide = single-element anion; -ate/-ite = polyatomic with oxygen
-ic acid-ous acid-ic from -ate (more O); -ous from -ite (fewer O)
Roman numeralGreek prefixRoman = metal charge (ionic); Greek = atom count (molecular)
Chloride (Cl⁻)Chlorite (ClO₂⁻) / chlorate (ClO₃⁻)Different oxygen content and charge

Memory aids

"Ate-ic, ite-ous" — when the polyatomic ion ends in -ate, the acid ends in -ic; when it ends in -ite, the acid ends in -ous (sulfate → sulfuric acid; sulfite → sulfurous acid).

Quick review

Topic Recap

Molecular formulas give actual atom counts; empirical formulas give the simplest ratio. Ionic compounds are named cation-then-anion, with Roman numerals for variable-charge metals and named polyatomic ions. Molecular compounds use Greek prefixes; acids are named as binary (hydro-…-ic) or oxyacids (-ate/-ite → -ic/-ous).

Knowledge Check

  1. Name FeCl₃.
  2. Write the formula for magnesium nitride.
  3. Name P₂O₅.
  4. Name H₂SO₄ and H₂S.
  5. What is the empirical formula of C₆H₁₂O₆?

Answers and Rationales

  1. Iron(III) chloride — Fe has a variable charge; three Cl⁻ ions make Fe +3.
  2. Mg₃N₂ — Mg²⁺ and N³⁻ balance as Mg₃N₂ (6+ and 6−).
  3. Diphosphorus pentoxide — two nonmetals, so Greek prefixes are used.
  4. H₂SO₄ = sulfuric acid (sulfate → -ic); H₂S = hydrosulfuric acid (binary, no oxygen).
  5. CH₂O — divide all subscripts by 6.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of chemical formulas as recipes. A molecular formula is the exact ingredient list with counts: glucose is C₆H₁₂O₆ (6 carbons, 12 hydrogens, 6 oxygens). The empirical formula is the same recipe reduced to its lowest ratio: CH₂O (just the proportions). For ionic compounds, the "recipe" is simply the ratio of positive and negative ions that balances charge—NaCl means one sodium for every chlorine.

Naming is like giving each compound a precise address: the cation (positive part) first, then the anion (negative part). For metals that can take more than one charge (like iron, Fe²⁺ or Fe³⁺), we add a Roman numeral to say which one—FeCl₂ is iron(II) chloride and FeCl₃ is iron(III) chloride.

Where it stops being exact: ionic "molecules" aren't individual molecules at all—they're extended lattices of ions, so an ionic formula is always an empirical (simplest-ratio) formula. And some names are historical conventions rather than systematic rules (H₂O is "water," not "dihydrogen monoxide").

Simple Example

NaCl = sodium chloride. CO₂ = carbon dioxide (one carbon, two oxygens). H₂SO₄ = sulfuric acid (sulfate → sulfuric). Fe₂O₃ = iron(III) oxide.

Key takeaways

  • High yield: Ionic formulas are written cation first, anion second, with charges balanced.
  • High yield: A Roman numeral gives the metal's charge, not the number of atoms (FeCl₃ = iron(III), not "iron trichloride").
  • High yield: "hydro-…-ic" is used only for acids WITHOUT oxygen; oxyacids use -ate/-ite → -ic/-ous.
  • High yield: Mono- is dropped on the first element (CO = carbon monoxide, not "monocarbon monoxide").
  • NH₄⁺ (ammonium) is the only common polyatomic cation and behaves like a metal in naming.
  • Ionic formulas are empirical (simplest-ratio) formulas.

Keep learning

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Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Distinguish molecular formulas from empirical formulas.
  • Name ionic compounds, including Type I and Type II metals and polyatomic ions.
  • Name binary molecular (covalent) compounds using Greek prefixes.
  • Name binary acids and oxyacids correctly.

Key vocabulary

Molecular formula
Actual atom count per molecule
Empirical formula
Simplest whole-number ratio
Ionic compound
Metal + nonmetal (ions)
Cation
Positive ion
Anion
Negative ion
Type I metal
Fixed-charge metal
Type II metal
Variable-charge metal
Polyatomic ion
Charged group of atoms
Binary molecular compound
Two nonmetals
Binary acid
H + nonmetal, no oxygen
Oxyacid
H + oxygen-containing ion
Hydrate
Ionic compound with water

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