Difficulty: Introductory to Intermediate | Prerequisites: Part 1 of these notes (Sections 6.1–6.2), specifically ion formation, cation/anion naming, and predicting ion charges from the periodic table.

Big Picture

Now that you know how ions form and what they are called, this part covers two practical skills: writing the chemical formula for an ionic compound when you are given its name, and naming a binary compound when you are given its formula. These are bread-and-butter exam skills. You will use them constantly in later chapters on reactions, stoichiometry, and solutions. The rules are mechanical once you learn them, and almost every general chemistry exam includes several naming and formula-writing questions.

TL;DR

Ionic compounds must be electrically neutral, so you balance cation and anion charges to write formulas. Binary compounds (two elements) are named differently depending on whether they contain a metal + nonmetal or two nonmetals. Metals that form more than one cation type use Roman numerals (the Stock system). Nonmetal + nonmetal compounds use Greek prefixes.

Key Terms

Ionic compound

A compound composed of cations and anions held together by electrostatic attraction. In simple terms, a metal gives electrons to a nonmetal, and the oppositely charged ions stick together. Ionic compounds conduct electricity when dissolved in water.

Binary compound

A compound containing exactly two different elements. Think of "bi" as "two." NaCl (sodium and chlorine) and CO₂ (carbon and oxygen) are both binary.

Stock system

The naming convention that uses Roman numerals in parentheses after the metal name to indicate the charge on the cation. In simple terms, when a metal can form ions with different charges, the Roman numeral tells you which one you are dealing with. Example: iron(II) chloride vs. iron(III) chloride.

Classical nomenclature

An older naming system where, for metals with two possible cation charges, the lower-charge cation gets the suffix "-ous" and the higher-charge cation gets "-ic." In simple terms, ferrous = Fe²⁺ and ferric = Fe³⁺. Less common in modern use but still appears in older texts and some industries.

Greek/Latin prefixes (for binary nonmetal compounds)

Prefixes attached to element names to indicate the number of atoms of each element in a molecule. In simple terms, "di-" means 2, "tri-" means 3, and so on.

Core Content

Section 6.3 – Writing Formulas from Names of Ionic Compounds

The central rule: every chemical compound must have a net charge of zero. The positive charges from cations and the negative charges from anions must cancel out exactly.

Rules for writing formulas of ionic compounds:

• Write the formula for the metal ion (cation) first, followed by the nonmetal ion (anion).

• Combine the smallest whole numbers of each ion needed to make the total charge equal zero.

• Write the formula as the symbol for the metal followed by the symbol for the nonmetal, each with a subscript showing the number determined above. A subscript of 1 is never written.

Worked examples:

The shortcut (criss-cross method): take the magnitude of each ion's charge and use it as the subscript for the other ion. Then simplify to the smallest whole-number ratio. This works because it is just the arithmetic of finding the least common multiple.

Section 6.4 – Naming Binary Compounds

Binary compounds fall into three categories, and each has its own naming rules.

Category 1: Binary Ionic Compounds Where the Metal Forms Only One Type of Cation

These are the straightforward ones. The metal is from Group 1A, 2A, or aluminum (Group 3A), where the charge is always the same.

Naming rules:

• Write the name of the cation (the metal name, unchanged).

• Write the stem of the anion element and add the suffix "-ide."

Examples:

Note that a compound can have more than one atom of an element (e.g., Al₂O₃ still has only two different elements), and it still follows this naming rule. The subscripts do not appear in the name for ionic compounds.

Category 2: Binary Ionic Compounds Where the Metal Forms Two or More Types of Cations

Metals in the middle of the periodic table (transition metals, plus a few others like tin and lead) often form more than one type of cation. You need a way to specify which cation is present.

Stock system (modern, preferred):

• Write the name of the cation (metal name).

• Write the charge on the cation as a Roman numeral in parentheses, immediately after the metal name. No space between the name and the parenthesis.

• Write the stem of the anion and add "-ide."

Examples:

How to figure out the Roman numeral from a formula: use the anion charge to back-calculate. For example, in FeCl₃, each Cl is −1, and there are three of them, so total negative charge is −3. One iron must be +3 to balance. Therefore: iron(III) chloride.

Classical system (older, still encountered):

When a metal has exactly two cation types, the lower-charge version gets the "-ous" suffix and the higher-charge version gets the "-ic" suffix, applied to the Latin root of the metal name.

The Stock system is standard in modern chemistry courses, but you should be able to recognise classical names when you see them.

Category 3: Binary Compounds Containing Two Nonmetals

Compounds between two nonmetals are molecular (covalent), not ionic. The naming system uses Greek or Latin prefixes to indicate the number of atoms of each element.

Common prefixes:

Naming rules:

• The element that appears first in the formula is written and named first. Attach a prefix to indicate how many atoms (mono- is usually dropped for the first element).

• The second element gets its stem + "-ide," with a prefix indicating the count.

Examples:

• CO = carbon monoxide (mono- dropped on "carbon," but kept on "monoxide")

• CO₂ = carbon dioxide

• N₂O = dinitrogen monoxide

• N₂O₄ = dinitrogen tetroxide

• PCl₅ = phosphorus pentachloride

• SF₆ = sulfur hexafluoride

The prefix "mono-" is rarely used for the first element but is always used for the second element when there is only one atom of it (e.g., carbon monoxide, not "carbon oxide").

Formulas / Key Reference

Charge-balancing formula:

Total positive charge + Total negative charge = 0

(number of cations × cation charge) + (number of anions × anion charge) = 0

Criss-cross shortcut:

For ions A^(m+) and B^(n−), the formula is A_n B_m (then reduce to lowest terms).

Real-World Applications

The Stock system is how pharmaceutical labels, safety data sheets, and industrial chemistry specs identify metal compounds. When you read "iron(II) sulfate" on a supplement bottle, the Roman numeral tells you exactly which form of iron is inside, which matters because Fe²⁺ and Fe³⁺ behave very differently in the body. The Greek prefix system shows up in common chemical names you already know: carbon dioxide (CO₂) in fizzy drinks, carbon monoxide (CO) in exhaust fumes.

Common Misconceptions

• Students frequently forget to include the Roman numeral for transition metals. "Iron chloride" is incomplete; you must specify iron(II) chloride or iron(III) chloride.

• A common error is applying Greek prefixes to ionic compounds. Prefixes are only for two-nonmetal (molecular) compounds. You would never say "disodium monoxide" for Na₂O; it is simply "sodium oxide."

• Students sometimes write subscripts in the name. The name "aluminum oxide" already implies Al₂O₃ through the charge-balancing rules; you do not say "dialuminum trioxide."

• The criss-cross method can mislead if you forget to reduce to lowest terms. For example, Pb²⁺ and O²⁻ criss-crossed gives Pb₂O₂, but the correct formula is PbO.

Why It Matters / Exam Flags

⚠️ Formula writing from compound names is a near-certain exam question. Practise until it is automatic: identify ions, balance charges, write formula.

⚠️ Naming compounds with transition metals requires you to determine the cation charge from the formula. Work backwards from the anion charges. This is tested often.

⚠️ Know which naming system applies to which type of compound: no prefixes for ionic, Roman numerals for multi-charge metals, Greek prefixes for nonmetal + nonmetal.

⚠️ The classical "-ous" / "-ic" system may appear in multiple-choice distractors. Recognise ferrous = Fe²⁺, ferric = Fe³⁺, cuprous = Cu⁺, cupric = Cu²⁺.

⚠️ When writing formulas for polyatomic ions (like CrO₄²⁻ or NH₄⁺), remember to enclose them in parentheses when you need more than one: (NH₄)₃PO₄, not NH₄₃PO₄.

Quick Self-Test

1. True or False: The compound formed by Na⁺ and S²⁻ has the formula NaS.

1. Fill in the blank: FeCl₂ is named iron(__________) chloride.

1. True or False: N₂O₄ is named "nitrogen tetroxide."

1. Fill in the blank: The prefix for 5 is __________.

1. True or False: In the Stock system, the Roman numeral represents the number of metal atoms.

Answers: 1. False (it is Na₂S, because you need two Na⁺ to balance one S²⁻). 2. II. 3. False (it is dinitrogen tetroxide, because the prefix for two nitrogen atoms is "di-"). 4. Penta-. 5. False (the Roman numeral represents the charge on the metal cation, not the number of atoms).

Practice Q&A

Q: Write the formula for aluminum oxide.

A: Al³⁺ and O²⁻. Criss-cross: Al₂O₃. Check: 2(+3) + 3(−2) = 0. Formula: Al₂O₃.

Q: Name the compound CuCl₂ using the Stock system.

A: Each Cl is −1, total anion charge = −2. Cu must be +2 to balance. Name: copper(II) chloride.

Q: Name the compound N₂O₅.

A: Two nonmetals, so use Greek prefixes. Dinitrogen pentoxide.

Q: Write the formula for tin(IV) fluoride.

A: Sn⁴⁺ and F⁻. Four fluorides needed to balance the +4 charge. Formula: SnF₄.

Q: What is the classical name for Fe²⁺ and Fe³⁺?

A: Fe²⁺ = ferrous (lower charge, "-ous" ending). Fe³⁺ = ferric (higher charge, "-ic" ending).

Q: A compound has the formula PbI₂. Name it using the Stock system.

A: Each I is −1, total = −2. Pb must be +2. Name: lead(II) iodide.

Q: Why would you never name MgCl₂ as "magnesium(II) chloride"?

A: Magnesium only forms one type of cation (Mg²⁺), so the Roman numeral is unnecessary. It is simply "magnesium chloride." Roman numerals are reserved for metals that can form more than one type of cation.

Connections to Other Topics

These naming and formula-writing skills feed directly into writing and balancing chemical equations (Chapter 7 and beyond). When you get to acid nomenclature, you will see many of the same patterns extended with new suffixes. The Stock system becomes essential in redox chemistry, where tracking the oxidation state of transition metals is the whole point. If you move on to coordination chemistry or inorganic lab work, the classical Latin names (ferrous, cupric) will reappear.

Related Terms / Search Tags

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