MGU Science Series — High School Chemistry
Part III: Bonding, Compounds, and Molecular Structure
Ionic · Molecular · Polyatomic · Acids · Nomenclature
Chapter 11 of 28
MGU Science Series  ·  High School Chemistry  ·  Library Textbook Edition

Chapter 11:
Chemical Names
and Formulas

Ionic compounds, molecular compounds, polyatomic ions, transition metals, binary acids, and oxyacids — the rule-based language chemists use to communicate the identity and composition of every substance

■ Part III: Bonding, Compounds, and Molecular Structure ■

■ 10 Sections  •  Chapter 11 of 28 ■

Follows Chapter 10: Covalent Bonding and Molecular Compounds  ·  Prepares students for Chapter 12: Molecular Geometry and Intermolecular Forces

Chapter 11 — Front Matter

Purpose and Overview

Every substance in chemistry has a name and a formula. These are not decorative labels. They are precise communications — a kind of language chemists use to tell each other exactly what a substance is made of, how many of each atom it contains, and how those atoms are charged and arranged.

When a doctor prescribes sodium chloride, a geologist identifies calcium carbonate, or an engineer specifies dinitrogen tetroxide as a rocket propellant, they are using a naming system that chemists around the world have agreed upon. A chemist in Brazil, Japan, or Kenya can read those names and immediately know what compound is being described, because the naming rules are universal.

This chapter teaches you that language. Chemical nomenclature — the system of naming compounds — is not simply a list of terms to memorize. It is a rule-based system built on the same atomic and bonding knowledge you developed in Chapters 9 and 10. Once you understand the logic behind the rules, you can name thousands of compounds you have never seen before, and you can write a formula from a name you have never encountered.

Opening Question

A chemistry student reads a laboratory procedure calling for "iron chloride." She goes to the stockroom and finds two bottles: one labeled FeCl₂ and another labeled FeCl₃. Which one should she use? Without a Roman numeral in the name, she cannot tell. By the end of this chapter, you will understand why "iron(II) chloride" and "iron(III) chloride" are not the same compound, and you will be able to name any ionic or molecular compound precisely enough that no one could ever be confused about what you mean.

10Sections
12Objectives
18Key Terms
15+Worked Examples

Learning Objectives

  1. 1Explain why systematic chemical names are necessary and what information they communicate.
  2. 2Distinguish between ionic and molecular compounds and identify which naming rules apply to each.
  3. 3Name binary ionic compounds from their formulas and write formulas from their names.
  4. 4Determine the charge of a transition metal ion from a formula and write the correct Roman numeral in the compound's name.
  5. 5Identify common polyatomic ions by name, formula, and charge.
  6. 6Name ionic compounds containing polyatomic ions and write their formulas using charge balance.
  7. 7Name binary molecular compounds using Greek prefixes and write formulas from those names.
  8. 8Identify whether a given name or formula represents an ionic compound, molecular compound, or acid before applying naming rules.
  9. 9Name binary acids and oxyacids and write their formulas.
  10. 10Recognize common chemical names and connect them to systematic names and formulas.
  11. 11Identify and correct the most common nomenclature errors.
  12. 12Apply all naming rules to mixed sets of compounds of different types.
Chapter 11 — Contents

Sections at a Glance

Sections 11.1 – 11.5
The Language of Ionic Compounds
Why Names Matter · Binary Ionic · Transition Metals · Polyatomic Ions · Compounds with Polyatomic Ions
Section 11.1

Why Chemical Names and Formulas Matter

Many substances have common names that have been used for centuries. Table salt, baking soda, chalk, lye, and laughing gas are all familiar to people outside of chemistry. These names are useful in daily conversation, but they create serious problems in science. The name "lye" might refer to sodium hydroxide (NaOH) or potassium hydroxide (KOH) depending on context. The name "muriatic acid" — still found on hardware store labels for hydrochloric acid — tells you nothing about which atoms are present or how they are combined.

Systematic chemical names solve these problems. A systematic name is built according to rules. It tells you what elements are present, how many of each atom are in the formula, and whether the compound is ionic or molecular. The name carries information that any trained chemist in the world can decode.

What a Formula Communicates

The formula NaCl tells you sodium and chlorine are present in a 1:1 ratio. MgCl₂ tells you magnesium and chlorine are in a 1:2 ratio. Ca₃(PO₄)₂ tells you calcium, phosphorus, and oxygen are combined in a 3:2 ratio of calcium to phosphate groups. Formulas are essential for equation writing, stoichiometry, and laboratory work.

Two Types of Compounds, Two Sets of Rules

Ionic compounds are made of metal cations and nonmetal anions held together by electrostatic attraction. Names reflect the charges and identities of those ions.

Molecular compounds are made of nonmetal atoms sharing electrons in covalent bonds. Names use prefixes to indicate how many atoms of each element are present.

Acids are a special category treated separately in Section 11.8.

Section 11.2

Naming Binary Ionic Compounds

A binary compound contains exactly two elements. A binary ionic compound contains exactly two elements — one metal and one nonmetal — bonded ionically. The metal forms a cation; the nonmetal forms an anion.

The Naming Rule for Binary Ionic Compounds

Part 1: The name of the metal cation, written exactly as the element's name.

Part 2: The name of the nonmetal anion — take the element's name and replace the ending with the suffix -ide.

The formula subscripts are not stated in the name. The name gives element identity. The formula gives the ratio.

The -ide Suffix

ElementAnion NameElementAnion Name
ChlorineChlorideOxygenOxide
BromineBromideSulfurSulfide
IodineIodideNitrogenNitride
FluorineFluoridePhosphorusPhosphide
CarbonCarbideHydrogenHydride
Example 11.2.1
Naming binary ionic compounds from formulas
  • NaClMetal: sodium. Nonmetal: chlorine → chloride. Name: sodium chloride
  • MgOMetal: magnesium. Nonmetal: oxygen → oxide. Name: magnesium oxide
  • CaBr₂Metal: calcium. Nonmetal: bromine → bromide. Name: calcium bromide (the subscript 2 does not appear in the name)
  • Al₂O₃Metal: aluminum. Nonmetal: oxygen → oxide. Name: aluminum oxide (subscripts 2 and 3 do not appear in the name)

Writing Formulas: The Criss-Cross Method

To write a formula from a name, determine the charge of each ion, then find the subscripts that make the compound electrically neutral. The total positive charge must equal the total negative charge — this is called charge balance.

Example 11.2.2
Writing formulas from binary ionic names
  • Calcium bromideCa²⁺ and Br⁻. One Ca²⁺ (+2) needs two Br⁻ (−2 total). Formula: CaBr₂
  • Aluminum oxideAl³⁺ and O²⁻. Criss-cross: subscript on Al = 2, subscript on O = 3. Check: 2(+3) = +6 and 3(−2) = −6 → balanced Formula: Al₂O₃
  • Calcium oxideCa²⁺ and O²⁻. Criss-cross gives Ca₂O₂ — reduce to lowest ratio. Formula: CaO (not Ca₂O₂)
Always Reduce to Lowest Ratio

After applying the criss-cross method, always check whether the subscripts share a common factor. Ca₂O₂ reduces to CaO. Na₂Cl₂ reduces to NaCl. The formula of an ionic compound must be written in the lowest whole-number ratio.

Section 11.3

Naming Ionic Compounds with Transition Metals

Most main-group metals have a fixed, predictable ion charge — sodium always forms Na⁺, calcium always forms Ca²⁺. Transition metals are different. Many can form ions with more than one possible charge. Iron can form Fe²⁺ or Fe³⁺. Copper can form Cu⁺ or Cu²⁺. Without additional information in the name, a reader cannot know which iron ion is present.

The Roman Numeral System (IUPAC)

When naming a compound with a transition metal that has variable charge, write the metal's ion charge as a Roman numeral in parentheses immediately after the metal's name.

Format: Metal name (Roman numeral) + anion name

Roman NumeralIon ChargeRoman NumeralIon Charge
(I)1+(III)3+
(II)2+(IV)4+
Example 11.3.1
Naming transition metal ionic compounds
  • FeCl₂Chloride is Cl⁻. Two chloride ions = −2 total. Iron must be +2. Iron(II). Name: iron(II) chloride
  • FeCl₃Three chloride ions = −3 total. Iron must be +3. Iron(III). Name: iron(III) chloride
  • CuOOxide is O²⁻. One oxide = −2. Copper must be +2. Copper(II). Name: copper(II) oxide
  • Cu₂OOne oxide = −2. Two copper ions share +2, so each copper = +1. Copper(I). Name: copper(I) oxide

Metals That Always Have Fixed Charges — No Roman Numeral Needed

Metal or GroupAlways FormsExamples
Group 1 metals+1Na⁺, K⁺, Li⁺, Rb⁺, Cs⁺
Group 2 metals+2Mg²⁺, Ca²⁺, Ba²⁺, Sr²⁺
Aluminum+3Al³⁺
Zinc+2Zn²⁺
Silver+1Ag⁺
Do Not Add Unnecessary Roman Numerals

"Sodium(I) chloride" is incorrect. Main-group metals with fixed charges never need Roman numerals. Adding one implies the metal could have other charge states — which is misleading and wrong. The correct name is simply "sodium chloride."

Section 11.4

Polyatomic Ions

A polyatomic ion is a charged group of covalently bonded atoms that behaves as a single unit in ionic compounds. The atoms within the group are bonded together with covalent bonds, but the group as a whole carries an overall ionic charge. Polyatomic ions appear in minerals, fertilizers, medicines, industrial chemicals, biological molecules, cleaning products, and foods. Learning them is essential for naming and formula writing throughout the rest of your chemistry course.

Common Polyatomic Anions

NameFormulaCharge
HydroxideOH⁻−1
NitrateNO₃⁻−1
NitriteNO₂⁻−1
SulfateSO₄²⁻−2
SulfiteSO₃²⁻−2
CarbonateCO₃²⁻−2
Bicarbonate (hydrogen carbonate)HCO₃⁻−1
PhosphatePO₄³⁻−3
Hydrogen phosphateHPO₄²⁻−2
Dihydrogen phosphateH₂PO₄⁻−1
AcetateC₂H₃O₂⁻−1
PermanganateMnO₄⁻−1
DichromateCr₂O₇²⁻−2
ChromateCrO₄²⁻−2
CyanideCN⁻−1
OxalateC₂O₄²⁻−2

Common Polyatomic Cations

NameFormulaCharge
AmmoniumNH₄⁺+1
HydroniumH₃O⁺+1
The -ate and -ite Pattern

When two related ions differ only in oxygen content, the one with more oxygen ends in -ate (nitrate: NO₃⁻) and the one with less oxygen ends in -ite (nitrite: NO₂⁻). This pattern applies to sulfate/sulfite, chlorate/chlorite, phosphate, and others. These polyatomic ions must be memorized — there is no shortcut for deriving them from rules at this level.

Section 11.5

Naming Compounds with Polyatomic Ions

The Naming Rule

When a polyatomic ion appears in a compound, its name is used directly — exactly as it appears on the reference list — without adding any suffix or modifying the name.

Format: Cation name (+ Roman numeral if needed) + polyatomic ion name

The subscript on a polyatomic ion is never spoken in the name.

Example 11.5.1
Naming compounds with polyatomic ions
  • NaNO₃Sodium (Na⁺, fixed). Nitrate (NO₃⁻). Name: sodium nitrate
  • Ca(OH)₂Calcium (Ca²⁺, fixed). Hydroxide (OH⁻). Name: calcium hydroxide — the subscript 2 is not "dihydroxide"
  • (NH₄)₂SO₄Ammonium (NH₄⁺). Sulfate (SO₄²⁻). Name: ammonium sulfate
  • Fe₂(SO₄)₃Three sulfate ions = −6 total. Two iron ions must provide +6, so each Fe = +3. Iron(III). Name: iron(III) sulfate
  • Mg(NO₂)₂Magnesium (Mg²⁺). Nitrite (NO₂⁻). Name: magnesium nitrite
Example 11.5.2
Writing formulas from names with polyatomic ions
  • Calcium phosphateCa²⁺ and PO₄³⁻. Criss-cross: 3 Ca²⁺ and 2 PO₄³⁻. Check: 3(+2) = +6 and 2(−3) = −6 → balanced Formula: Ca₃(PO₄)₂
  • Ammonium sulfateNH₄⁺ (+1) and SO₄²⁻ (−2). Two ammonium needed. Formula: (NH₄)₂SO₄
Use Parentheses for Polyatomic Ions with Subscripts Greater Than 1

When a polyatomic ion needs a subscript of 2 or more, the entire ion must be enclosed in parentheses. Writing Ca(OH)₂ is correct. Writing CaOH₂ is incorrect because it suggests the subscript applies only to hydrogen, not to the entire hydroxide group.

Sections 11.6 – 11.7
Naming and Writing Molecular Compounds
Greek Prefixes · Binary Molecular Names · Writing Formulas from Any Name
Section 11.6

Naming Binary Molecular Compounds

Binary molecular compounds contain two nonmetal elements bonded covalently. Unlike ionic compounds, they do not form from charges — they form from shared electrons. Because there are no charges to balance, the names of molecular compounds must explicitly state how many atoms of each element are in the molecule. This is done using Greek numerical prefixes.

The Greek Prefix System

NumberPrefixNumberPrefix
1mono-6hexa-
2di-7hepta-
3tri-8octa-
4tetra-9nona-
5penta-10deca-
The Naming Rule for Binary Molecular Compounds

Part 1: First element is named with its element name, with a prefix for the count. Mono- is not used for the first element.

Part 2: Second element is named with an -ide suffix, with a prefix for the count. Mono- is used for the second element if there is only one atom.

When a prefix ending in a vowel precedes an element name beginning with a vowel, drop the prefix's final vowel: mono + oxide → monoxide (not monooxide).

Example 11.6.1
Naming binary molecular compounds
  • COCarbon (no mono for first element) + one oxygen = monoxide. Name: carbon monoxide
  • CO₂Carbon + two oxygens = dioxide. Name: carbon dioxide
  • N₂O₄Two nitrogens = dinitrogen. Four oxygens = tetroxide. Name: dinitrogen tetroxide
  • PCl₃Phosphorus (no mono) + three chlorines = trichloride. Name: phosphorus trichloride
  • PCl₅Phosphorus + five chlorines = pentachloride. Name: phosphorus pentachloride
Note: PCl₃ and PCl₅ are different compounds. Without prefixes, you could not tell them apart — which is exactly why the prefix system exists.
Example 11.6.2
Writing formulas from molecular names
  • Dinitrogen monoxideDi = 2 nitrogens. Mono = 1 oxygen. Formula: N₂O
  • Sulfur hexafluorideNo prefix on sulfur = 1. Hexa = 6 fluorines. Formula: SF₆
  • Tetraphosphorus decoxideTetra = 4 phosphorus. Deca = 10 oxygens. Formula: P₄O₁₀
Section 11.7

Writing Formulas from Names

Writing formulas from names requires you to first identify which type of compound the name describes, and then apply the appropriate procedure. Reading the name carefully always comes first.

Step 1 — Identify the Compound Type

If the name ends in -ide and contains a metal name with no Greek prefix → likely a binary ionic compound.

If the name contains a polyatomic ion name (nitrate, sulfate, hydroxide, phosphate, ammonium, etc.) → ionic compound with a polyatomic ion.

If the name contains Greek prefixes (di-, tri-, tetra-…) or both elements are clearly nonmetals → molecular compound.

If the name ends in acid with a hydro- prefix or ends in -ic acid / -ous acid without hydro- → acid (see Section 11.8).

Example 11.7.1
Mixed formula-writing practice
  • Potassium sulfideBinary ionic. K⁺ and S²⁻. Two K⁺ balance one S²⁻. Formula: K₂S
  • Iron(III) phosphateIonic with polyatomic ion. Fe³⁺ and PO₄³⁻. One of each. Formula: FePO₄
  • Ammonium carbonateIonic with polyatomic ions. NH₄⁺ (+1) and CO₃²⁻ (−2). Two ammonium needed. Formula: (NH₄)₂CO₃
  • Diphosphorus pentoxideMolecular. Di = 2, penta = 5. Formula: P₂O₅
  • Copper(II) nitrateCu²⁺ and NO₃⁻ (−1). Two nitrate needed to balance Cu²⁺. Formula: Cu(NO₃)₂
Sections 11.8 – 11.10
Acids, Common Names, and Avoiding Errors
Binary Acids · Oxyacids · Common Names · Eight Recurring Errors
Section 11.8

Naming Acids

Acids are compounds that release hydrogen ions (H⁺) when dissolved in water. In their formulas, hydrogen is written first. Acids have their own naming rules that are distinct from both ionic and molecular compounds. At this level, you will learn to name two types: binary acids (hydrogen + one nonmetal) and oxyacids (hydrogen + a polyatomic ion containing oxygen).

Binary Acids

Binary Acid Naming Rule

1. Start with the prefix hydro-

2. Take the nonmetal's name, change the ending to -ic

3. Add the word acid

Note: HCl gas is named "hydrogen chloride." HCl dissolved in water — labeled (aq) — is "hydrochloric acid."

FormulaNonmetalAcid Name
HF(aq)FluorineHydrofluoric acid
HCl(aq)ChlorineHydrochloric acid
HBr(aq)BromineHydrobromic acid
HI(aq)IodineHydroiodic acid
H₂S(aq)SulfurHydrosulfuric acid

Oxyacids

Oxyacid Naming Rule

If the polyatomic anion ends in -ate → the acid name ends in -ic acid

If the polyatomic anion ends in -ite → the acid name ends in -ous acid

The hydro- prefix is not used for oxyacids.

AnionAnion NameAcid FormulaAcid Name
NO₃⁻Nitrate (-ate)HNO₃Nitric acid (-ic)
NO₂⁻Nitrite (-ite)HNO₂Nitrous acid (-ous)
SO₄²⁻Sulfate (-ate)H₂SO₄Sulfuric acid (-ic)
SO₃²⁻Sulfite (-ite)H₂SO₃Sulfurous acid (-ous)
PO₄³⁻Phosphate (-ate)H₃PO₄Phosphoric acid (-ic)
CO₃²⁻Carbonate (-ate)H₂CO₃Carbonic acid (-ic)
C₂H₃O₂⁻Acetate (-ate)HC₂H₃O₂Acetic acid (-ic)
ClO₄⁻Perchlorate (-ate)HClO₄Perchloric acid (-ic)
ClO₃⁻Chlorate (-ate)HClO₃Chloric acid (-ic)
ClO₂⁻Chlorite (-ite)HClO₂Chlorous acid (-ous)
ClO⁻Hypochlorite (-ite)HClOHypochlorous acid (-ous)
Example 11.8.1
Naming oxyacids
  • H₂SO₄Remove H's → SO₄²⁻. Anion name: sulfate (ends in -ate). Acid ends in -ic acid. Name: sulfuric acid
  • HNO₂Remove H → NO₂⁻. Anion name: nitrite (ends in -ite). Acid ends in -ous acid. Name: nitrous acid
Section 11.9

Common Chemical Names

Despite the elegance of systematic nomenclature, many chemical names in common use are historical or trivial names used so long they remain in everyday and even scientific language. As a chemistry student, you must recognize both types of names and understand that the systematic name carries structural information the common name does not.

Common NameSystematic NameFormula
WaterDihydrogen monoxideH₂O
AmmoniaAmmonia (retained)NH₃
Table saltSodium chlorideNaCl
Baking sodaSodium hydrogen carbonateNaHCO₃
Washing sodaSodium carbonateNa₂CO₃
Lye / caustic sodaSodium hydroxideNaOH
Lime (quicklime)Calcium oxideCaO
Slaked limeCalcium hydroxideCa(OH)₂
Chalk / limestoneCalcium carbonateCaCO₃
GypsumCalcium sulfate dihydrateCaSO₄·2H₂O
RustIron(III) oxideFe₂O₃
Laughing gasDinitrogen monoxideN₂O
Dry iceCarbon dioxide (solid)CO₂
Muriatic acidHydrochloric acidHCl(aq)
Vinegar (active component)Acetic acidHC₂H₃O₂
Bleach (active component)Sodium hypochloriteNaOCl
Epsom saltMagnesium sulfateMgSO₄
Fool's goldIron(II) sulfideFeS₂
Section 11.10

Avoiding Naming Errors

Nomenclature errors are among the most common mistakes in introductory chemistry. Most fall into a small number of recognizable categories.

Error 1 — Mixing up ionic and molecular naming rules
CO₂ named as "carbon(IV) oxide"

Correct: "carbon dioxide." CO₂ is molecular — two nonmetals. Use Greek prefixes, not Roman numerals.

Error 2 — Using prefixes on ionic compounds
MgCl₂ named as "dimagnesium chloride"

Correct: "magnesium chloride." Ionic compounds never use Greek prefixes. Subscripts are determined by charge balance, not named.

Error 3 — Forgetting the Roman numeral for variable-charge metals
FeCl₃ named as "iron chloride"

Correct: "iron(III) chloride." When a transition metal with variable charge appears, you must determine and include the Roman numeral.

Error 4 — Adding an unnecessary Roman numeral
NaCl named as "sodium(I) chloride"

Correct: "sodium chloride." Main-group metals with fixed charges never need Roman numerals. Adding one is misleading.

Error 5 — Forgetting parentheses for polyatomic ions with subscripts
Ca(OH)₂ written as CaOH₂

Correct: Ca(OH)₂. Parentheses group the entire polyatomic ion so the subscript applies to the whole group, not just the last element.

Error 6 — Confusing -ate and -ite ions
Calling NaNO₂ "sodium nitrate" (it is sodium nitrite)

Correct: Nitrate is NO₃⁻; nitrite is NO₂⁻. The -ate/-ite distinction must be memorized carefully for every pair.

Error 7 — Confusing -ic and -ous acid names
Calling both HNO₃ and HNO₂ "nitric acid"

Correct: HNO₃ is nitric acid (-ate anion → -ic acid). HNO₂ is nitrous acid (-ite anion → -ous acid). The acid naming system depends on the -ate/-ite distinction.

Error 8 — Not reducing formulas to lowest whole-number ratios
Calcium oxide written as Ca₂O₂

Correct: CaO. After applying the criss-cross method, always check whether subscripts share a common factor and reduce them.

Chapter 11 — Back Matter

Chapter Summary

Key Terms Review

acid
A compound that produces hydrogen ions (H⁺) when dissolved in water. Named using conventions distinct from ionic and molecular compounds.
binary acid
An acid containing hydrogen and one nonmetal. Named using the prefix hydro- and the suffix -ic acid. Requires (aq) label to distinguish from the pure gas.
binary compound
A compound containing exactly two elements.
binary ionic compound
An ionic compound containing exactly two elements — one metal and one nonmetal — bonded ionically.
binary molecular compound
A molecular compound containing exactly two nonmetal elements bonded covalently. Named using Greek prefixes.
charge balance
The condition in which total positive charge equals total negative charge in an ionic compound, giving it no net charge.
chemical formula
A representation of a compound using element symbols and subscript numbers to show the types and ratios of atoms present.
chemical nomenclature
The systematic, rule-based method used to name chemical compounds so that any trained chemist can determine composition from the name.
criss-cross method
A shortcut for finding ionic formula subscripts by swapping the absolute values of the ion charges, then reducing to lowest ratio.
-ide suffix
The ending added to a nonmetal's name when it forms a simple anion. Chlorine → chloride; oxygen → oxide; sulfur → sulfide.
ionic compound
A compound formed from the attraction between positively charged cations and negatively charged anions, typically metal and nonmetal.
molecular compound
A compound formed when nonmetal atoms share electrons through covalent bonds. Named using Greek prefixes rather than charge-based rules.
oxyacid
An acid containing hydrogen, oxygen, and another element (as a polyatomic oxyanion). Named based on whether the anion ends in -ate or -ite.
polyatomic ion
A charged group of covalently bonded atoms that acts as a single unit in ionic bonding. Must be memorized by name, formula, and charge.
Roman numeral system
The IUPAC method of indicating the charge of a variable-charge metal cation by placing the charge in Roman numerals in parentheses after the metal's name.
systematic name
A chemically precise name built according to IUPAC rules, as opposed to a common or trivial name.
trivial name
A traditional or informal name for a compound that does not follow systematic naming rules (for example, "baking soda" for sodium hydrogen carbonate).
variable charge
The property of transition metals that allows them to form cations of more than one charge, requiring Roman numerals in compound names.

Naming Rule Review

Binary Ionic Compounds — fixed-charge metal

Name: metal name + nonmetal name with -ide suffix

Formula: determine subscripts from charge balance; always reduce to lowest ratio

Binary Ionic Compounds — variable-charge metal

Name: metal name + (Roman numeral) + nonmetal name with -ide suffix

Roman numeral: determined by working backward from the anion's total charge

Ionic Compounds with Polyatomic Ions

Name: cation name (+ Roman numeral if needed) + polyatomic ion name (unchanged)

Formula: charge balance; use parentheses when polyatomic ion subscript > 1

Binary Molecular Compounds

Name: (prefix)first element + (prefix)second element with -ide suffix

Prefixes: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca

Drop mono- for the first element; keep mono- for the second element

Binary Acids

Name: hydro- + nonmetal stem + -ic acid

Formula: H followed by the nonmetal symbol, labeled (aq)

Oxyacids

Anion ends in -ate → acid ends in -ic acid

Anion ends in -ite → acid ends in -ous acid

No hydro- prefix for oxyacids

Worked Example Review

Worked Example 1
Name an ionic compound from its formula — Al₂S₃
  • Step 1Identify elements: aluminum (metal) and sulfur (nonmetal). Binary ionic compound.
  • Step 2Aluminum is a main-group metal with fixed charge (+3). No Roman numeral needed.
  • Step 3Sulfur forms the sulfide ion. Apply the -ide suffix.
Answer: aluminum sulfide
Worked Example 2
Write a formula from an ionic name — iron(III) oxide
  • Step 1Iron(III) means Fe³⁺.
  • Step 2Oxide means O²⁻.
  • Step 3Apply charge balance: 2 Fe³⁺ = +6 and 3 O²⁻ = −6. Balanced. Formula: Fe₂O₃
Answer: Fe₂O₃
Worked Example 3
Name a compound with a polyatomic ion — Pb(NO₃)₂
  • Step 1Lead (Pb) is a transition metal with variable charge.
  • Step 2Nitrate is NO₃⁻ (charge −1). Two nitrate ions = −2 total.
  • Step 3Lead must be +2 to balance: Pb²⁺. Roman numeral: (II).
Answer: lead(II) nitrate
Worked Example 4
Name a binary molecular compound — N₂O₅
  • Step 1Two nonmetals: nitrogen and oxygen. Binary molecular compound.
  • Step 2Two nitrogens = di-; five oxygens = penta-.
Answer: dinitrogen pentoxide
Worked Example 5
Name an oxyacid — H₂SO₃
  • Step 1Remove hydrogens to identify the anion: SO₃²⁻.
  • Step 2Anion name: sulfite (ends in -ite).
  • Step 3-ite anion → -ous acid.
Answer: sulfurous acid

Concept Check

Concept Check
Answer these questions before moving to practice — they test naming logic, not just procedural steps
Understanding the Rules
  1. Why do molecular compounds use Greek prefixes in their names, but ionic compounds do not?
  2. What information does a Roman numeral in a chemical name communicate? Why is it necessary for some compounds but not others?
  3. A student names CaO as "calcium(II) oxide." What error did they make?
  4. A student names N₂O₃ as "nitrogen oxide." What is wrong with this name, and what is the correct name?
  5. How can you tell from the name "sulfurous acid" that the anion is sulfite rather than sulfate?
Vocabulary
  1. What does the suffix -ide tell you about the ion it describes?
  2. Explain the difference between a polyatomic ion and a simple ion. Give one example of each.
  3. Why does Ca(OH)₂ have parentheses in its formula, but CaCl₂ does not?

Naming Practice

Part ABinary Ionic Compounds

Name each compound. Identify the cation and anion before writing the full name.

  1. LiF
  2. K₂O
  3. MgBr₂
  4. BaS
  5. AlN
  6. Na₂S
  7. CaI₂
  8. SrO
  9. Cs₂O
  10. RbCl
Part BTransition Metal Ionic Compounds

Determine the metal's charge from the formula, then write the full name.

  1. CuCl₂
  2. CuCl
  3. FeO
  4. Fe₂O₃
  5. MnO₂
  6. SnCl₄
  7. PbO
  8. PbO₂
  9. CoCl₃
  10. NiS
Part CCompounds with Polyatomic Ions
  1. NaOH
  2. Ca(NO₃)₂
  3. NH₄Cl
  4. Al₂(SO₄)₃
  5. K₂CO₃
  6. Mg(HCO₃)₂
  7. (NH₄)₂SO₄
  8. Fe(OH)₃
  9. Cu(NO₂)₂
  10. Pb₃(PO₄)₂
Part DBinary Molecular Compounds
  1. SO₂
  2. SO₃
  3. NO
  4. NO₂
  5. N₂O
  6. CCl₄
  7. PCl₃
  8. PCl₅
  9. SF₆
  10. Cl₂O₇
Part EAcids
  1. HCl(aq)
  2. HBr(aq)
  3. HNO₃
  4. HNO₂
  5. H₂SO₄
  6. H₂SO₃
  7. H₃PO₄
  8. HClO₄
  9. HClO
  10. HC₂H₃O₂

Formula-Writing Practice

Write the correct formula for each compound.

  1. Potassium bromide
  2. Magnesium nitride
  3. Calcium sulfide
  4. Aluminum oxide
  5. Sodium oxide
  6. Iron(II) chloride
  7. Iron(III) chloride
  8. Copper(I) oxide
  9. Copper(II) sulfide
  10. Manganese(IV) oxide
  11. Sodium carbonate
  12. Potassium nitrate
  13. Calcium hydroxide
  14. Ammonium phosphate
  15. Iron(III) sulfate
  16. Lead(II) nitrate
  17. Calcium hydrogen carbonate
  18. Aluminum sulfate
  19. Ammonium dichromate
  20. Chromium(III) hydroxide
  21. Dinitrogen trioxide
  22. Tetraphosphorus decoxide
  23. Sulfur trioxide
  24. Carbon tetrachloride
  25. Dichlorine heptoxide

Name-to-Formula and Formula-to-Name Mixed Practice

Some items are formulas to be named; others are names requiring a formula. Identify which is which, then complete each.

  1. Ba(OH)₂
  2. Tin(IV) oxide
  3. Mn₂O₇
  4. Nickel(II) nitrate
  5. (NH₄)₂Cr₂O₇
  6. Calcium acetate
  7. Cr₂(SO₄)₃
  8. Ammonium permanganate
  9. HI(aq)
  10. Phosphoric acid
  11. N₂O₄
  12. Chlorine trifluoride
  13. Fe(C₂H₃O₂)₃
  14. Magnesium oxalate
  15. NaH₂PO₄

Error Analysis

Each item contains one or more naming or formula-writing errors. Identify the error and write the correct name or formula.

Problem 1
The formula for sodium nitrite is written as NaNO₃

Identify the error. NaNO₃ contains which polyatomic ion? What is the correct formula for sodium nitrite?

Problem 2
The name for FeCl₂ is written as "iron chloride"

What is missing? How do you determine what goes there? Write the correct name.

Problem 3
The name for MgO is written as "magnesium(II) oxide"

Why is the Roman numeral incorrect here? Write the correct name.

Problem 4
The formula for dinitrogen pentoxide is written as N₂O₃

What prefix was mistranslated? Write the correct formula.

Problem 5
CuSO₄ is named "copper sulfate" (no Roman numeral)

Determine the copper ion's charge from the formula. Write the correct name.

Problem 6
The formula for calcium hydroxide is written as CaOH₂

What structural problem does this create? Write the correct formula.

Problem 7
HNO₂ is named "nitric acid"

Identify the anion in HNO₂. What does its name tell you about the acid name? Write the correct name.

Problem 8
PCl₅ is named "phosphorus chloride"

What is missing from this name? Is this ionic or molecular? Write the correct name.

Problem 9
The formula for iron(III) sulfate is written as Fe₂SO₄

Apply charge balance. How many sulfate ions are needed to balance two Fe³⁺ ions? Write the correct formula.

Problem 10
N₂O is named "dinitrogen oxide" (no prefix on oxygen)

What rule was violated? What is the correct name?

Problem 11
The formula for ammonium sulfate is written as NH₄SO₄

Apply charge balance. How many ammonium ions are needed? Write the correct formula with proper parentheses.

Problem 12
Hydrofluoric acid is named "hydrogen fluoride acid"

What is wrong with the format? Write the correct name using the binary acid naming rule.

Mixed Nomenclature Review

These problems mix all naming types from the chapter. Determine compound type before applying any rule.

  1. AlPO₄ — name this compound
  2. Chromium(III) carbonate — write the formula
  3. XeF₄ — name this compound
  4. Lithium hypochlorite — write the formula
  5. Co(OH)₂ — name this compound (cobalt has variable charge)
  6. Manganese(II) bromide — write the formula
  7. (NH₄)₃PO₄ — name this compound
  8. Barium acetate — write the formula
  9. HCN(aq) — name this acid (CN⁻ is cyanide; apply binary acid naming)
  10. Diarsenic pentoxide — write the formula
  11. SnBr₄ — name this compound (tin has variable charge)
  12. Gold(III) chloride — write the formula
  13. Ag₂SO₄ — name this compound (silver is fixed at +1)
  14. Zinc phosphate — write the formula
  15. N₂H₄ — name this compound (dinitrogen tetrahydride)
  16. Strontium nitrite — write the formula
  17. Bi₂O₃ — name this compound (bismuth charge: +3)
  18. Nickel(IV) oxide — write the formula
  19. H₃BO₃ — name this acid (borate is BO₃³⁻; apply oxyacid naming)
  20. Tetraphosphorus hexoxide — write the formula
Looking Ahead
Chapter 12: Molecular Geometry and Intermolecular Forces

In Chapter 11, you learned to name and write formulas for ionic and molecular compounds. You now know the symbols and ratios — but molecules are not flat. They occupy three-dimensional space. The arrangement of atoms in space determines a molecule's shape, and shape determines properties.

Chapter 12 will show you why molecules are three-dimensional, how electron pairs push atoms into predictable shapes, and how molecular shape connects to polarity, boiling point, solubility, biological function, and material design. The Lewis structures you practiced in Chapter 10 will become the starting point for understanding molecular geometry in Chapter 12.

MGU Library Connection
  • Common Ions Chart — Appendix C of this textbook
  • Polyatomic Ions Reference — Appendix D of this textbook
  • Chemical Nomenclature Guide — MGU companion resource
  • Chapter 9: Ionic Bonding and Ionic Compounds — foundation for ionic names and charge balance
  • Chapter 10: Covalent Bonding and Molecular Compounds — foundation for Lewis structures and molecular formulas
  • Chapter 12: Molecular Geometry and Intermolecular Forces — where named formulas gain three-dimensional structure and physical meaning