--- title: "Mass, Energy and the Unified Atomic Mass Unit" description: "Mass and energy are one quantity written in two units, tied together by E = mc squared. On the nuclear scale that makes one atomic mass unit worth about 931 MeV, which is why nuclear energies dwarf ch" canonical: https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X source: https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Mass, Energy and the Unified Atomic Mass Unit Mass and energy are one quantity written in two units, tied together by E = mc squared. On the nuclear scale that makes one atomic mass unit worth about 931 MeV, which is why nuclear energies dwarf chemical ones. Subject: Science · Area: Matter & Materials · Ages 17 to 18 Page: https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X ## Ready when they can - Converts a mass in kilograms or atomic mass units into an energy in joules and in MeV - Compares the energy per particle from a nuclear change with that from a chemical one - Explains why the change in mass goes unnoticed in everyday energy transfers ## Lesson: Mass is frozen energy Mass and energy are one quantity in two units, tied by E equals m c squared, which is why a power station can run a city on barely any fuel. The c squared factor is huge: with c at 3.0e8 m/s, c squared is 9.0e16. Losing just 0.004 kg in a reaction releases 0.004 times 9.0e16, which is 3.6e14 J. **Example.** On the nuclear scale one atomic mass unit is worth about 931 MeV. A chemical change shuffles outer electrons and pays only a few eV per particle, like burning coal to heat a home. A nuclear change rebuilds the nucleus and pays millions of eV, roughly a million times more for each particle than coal. The conversion runs both ways. Building a particle of mass 2.0e-28 kg from pure energy costs 2.0e-28 times 9.0e16, which is 1.8e-11 J. The formula only gives the exchange rate, not the machine that performs the swap. Do not expect the scales to notice everyday transfers. Heating water adds energy, so its mass grows by E over c squared, but dividing by that enormous c squared leaves an unweighable speck. The change is real yet far too small for any balance. **Recap.** Lost mass times c squared equals released energy, which is why nuclear changes dwarf chemical ones. ## Practice 13 questions on this page, each with its working shown. ## Needs first - [Standard Form](https://lightmysky.com/learn/mathematics/standard-form-mt_GbZZ2xjBTt) - [Photon Energy and the Electronvolt](https://lightmysky.com/learn/science/photon-energy-and-the-electronvolt-mt_UfmDhGRaYT) ## Opens up - [Particles, Antiparticles and Annihilation](https://lightmysky.com/learn/science/particles-antiparticles-and-annihilation-mt_-Ud20HRNJz) - [Mass Defect and Binding Energy per Nucleon](https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl) - [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY)