Neutrons and Mass Number

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 13 of 45.

Neutrons have no charge but about the same mass as a proton. Electrons are so light that they barely count.

The mass number $A$ is the protons plus the neutrons:

$A = Z+N$

To find the neutrons, subtract: $N = A-Z$ .

Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons. Both are carbon.

Carbon-12 with 6 protons and 6 neutrons beside carbon-14 with 6 protons and 8 neutrons

Same protons, different neutrons: carbon-12 and carbon-14

Challenge

Carbon-14 forms high in the air and ends up in every living thing, which is how scientists date old wood and bone. Build a neutral carbon-12 atom, then a neutral carbon-14 atom.

Try it yourself

Atom builderBuild C, C

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 ChargeNeutrons and Mass NumberNuclide 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