---
title: "Naming Ionic and Simple Molecular Compounds"
description: "Chemical names are built from rules, not memorised one by one: ion charges fix the formula of an ionic compound, and Greek prefixes count atoms in a molecular one. Reading a name backwards to a formul"
canonical: https://lightmysky.com/learn/science/naming-ionic-and-simple-molecular-compounds-mt_GJ267pzxCI
source: https://lightmysky.com/learn/science/naming-ionic-and-simple-molecular-compounds-mt_GJ267pzxCI.md
retrieved: 2026-09-12
---

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# Naming Ionic and Simple Molecular Compounds

Chemical names are built from rules, not memorised one by one: ion charges fix the formula of an ionic compound, and Greek prefixes count atoms in a molecular one. Reading a name backwards to a formula is the skill that makes every later equation legible.

Subject: Science · Area: Matter & Materials · Ages 14 to 15
Page: https://lightmysky.com/learn/science/naming-ionic-and-simple-molecular-compounds-mt_GJ267pzxCI

## Ready when they can

- Write the formula of an ionic compound from the charges of its ions, including common polyatomic ions
- Name a binary molecular compound using counting prefixes and explain why ionic compounds do not use them
- Use a Roman numeral correctly for a metal with more than one possible charge

## Lesson: Reading chemical names both ways

Name an ionic compound metal first, then the nonmetal with its ending changed to ide, as in sodium chloride. Charges must balance to zero, so two potassium ions meet one oxygen ion in potassium oxide, K2O. Use the shortcut: each charge tells you how many of the other ion you need, then simplify, as in BaS.

Some metals form more than one ion, and a Roman numeral says which one. Iron(II) differs from iron(III), and copper(II) chloride, CuCl2, holds copper with a 2+ charge. Read the numeral as the charge before you balance.

Molecular compounds share atoms instead of charging them, so the name counts atoms with prefixes: mono, di, tri, tetra. N2O4 is dinitrogen tetroxide. Ionic compounds never use these prefixes, because the charges already fix the ratio.

**Example.** Polyatomic ions are charged groups that stay together, like hydroxide OH-, sulfate SO4 with 2-, and nitrate NO3-. Balance them like single ions: K2SO4 needs two potassium ions per sulfate. With more than one group, wrap it in parentheses, as in Mg(OH)2 and Sr(NO3)2.

**Recap.** Charges fix ionic formulas, prefixes count molecular atoms, and numerals name the metal charge.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Ionic Bonding: Electron Transfer and Charged Lattices](https://lightmysky.com/learn/science/ionic-bonding-electron-transfer-and-charged-lattices-mt_sf24C4YLtN)
- [The Periodic Table](https://lightmysky.com/learn/science/the-periodic-table-mt_wfEfQuHOG-)

## Opens up

- [Empirical and Molecular Formulas from Composition](https://lightmysky.com/learn/science/empirical-and-molecular-formulas-from-composition-mt_4rN5MQaTf7)
- [Balancing Equations and Conservation of Mass](https://lightmysky.com/learn/science/balancing-equations-and-conservation-of-mass-mt_KUaPCEIA5f)