Counting Atoms in a Formula

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 4 of 45.

A formula lists the atoms of a substance. The small number after a symbol counts that element's atoms, and no number means one: H₂O is 2 hydrogen and 1 oxygen.

A number after a bracket multiplies everything inside: Ca(OH)₂ is 1 calcium, 2 oxygen and 2 hydrogen.

When an element appears twice, add both: CH₃COOH holds 2 carbon, 4 hydrogen and 2 oxygen.

Challenge

Limewater is calcium hydroxide, Ca(OH)₂. Count the atoms of each element in it.

Try it yourself

FormulaCount the atoms in Ca(OH)₂

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Atomic Structure

1Matter and Atoms

What Everything Is Made Of Elements, Compounds, Mixtures Chemical SymbolsCounting Atoms in a FormulaRecap - Molecule Kit

4Electron Arrangement

Shells and the First 20 Valence Electrons Ionization Energy as Evidence Light from Atoms Subshells: s, p, d Aufbau, Hund and Pauli Writing Configurations Ions and the Exceptions Recap - Configuration Race

7Practice Problems

Recap - Potassium Oxide Recap - Helium from Rocks Recap - Uranium to Lead

2Models of the Atom

Dalton and Thomson Rutherford's Gold Foil The Nuclear Atom Bohr's Shells Recap - From Ball to Nucleus

5The Periodic Table

Mendeleev's Puzzle Groups and Periods Blocks: s, p, d, f Metals, Non-metals, Metalloids Alkali Metals Halogens Noble Gases Recap - Build the Table

3Inside the Atom

Protons Decide the Element Electrons and Charge Neutrons and Mass Number Nuclide Notation Ions Isotopes Radioactive Decay Relative Atomic Mass Recap - Mystery Particles

6Periodic Trends

Atomic Radius Ionic Radius Ionization Energy Trend Electronegativity Explaining Trends Recap - Mystery Element