Menu

Relative Atomic Mass

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 18 of 45.

Chlorine's mass in the periodic table is 35.5: the average of its isotopes, weighted by how common each one is. This is the relative atomic mass, A_r.

Multiply each mass number by its fraction (75% is 0.75) and add:

$A_r = 35 \times 0.75 + 37 \times 0.25 = 35.5$

The average lands nearer the more common isotope. It has no unit: it compares with carbon-12, counted as exactly 12.

Challenge

A sample of chlorine is 75% chlorine-35 and 25% chlorine-37. Here $a$ and $b$ are the mass numbers, and $p$ and $q$ the fractions of each isotope. Find chlorine's relative atomic mass, $M$ .

Try it yourself

Physics diagram: bars

Solve for $M$

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 Symbols Counting Atoms in a Formula Recap - 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 DecayRelative Atomic MassRecap - Mystery Particles

6Periodic Trends

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