---
title: "Mass Defect and Binding Energy per Nucleon"
description: "A nucleus weighs less than its separate nucleons, and that missing mass is the energy it would take to pull it apart. Dividing by the number of nucleons gives the curve that peaks near iron."
canonical: https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl
source: https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl.md
retrieved: 2026-09-12
---

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# Mass Defect and Binding Energy per Nucleon

A nucleus weighs less than its separate nucleons, and that missing mass is the energy it would take to pull it apart. Dividing by the number of nucleons gives the curve that peaks near iron.

Subject: Science · Area: Matter & Materials · Ages 17 to 18
Page: https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl

## Ready when they can

- Calculates the mass defect and the binding energy of a stated nuclide
- Divides by nucleon number and places the nuclide on the binding energy curve
- Reads the curve to say which changes release energy and which would absorb it

## Lesson: The missing mass that holds nuclei together

A nucleus weighs less than its separate parts. The mass defect is the mass of the free nucleons minus the mass of the whole nucleus. For the deuteron, proton 938.28 plus neutron 939.57 minus the bound 1875.61 leaves a defect of 2.24 MeV/c squared. That missing mass is the price of pulling it apart.

**Example.** Turn the defect into energy with E equals m c squared. Helium-4 has a defect of 0.030378 u, and with 1 u worth 931.5 MeV this gives 28.3 MeV of binding energy. Divide by its 4 nucleons to get 7.075 MeV per nucleon, far above deuterium's 1.12.

Plot binding energy per nucleon against mass number and the curve rises, peaks very near iron-56, then tapers off. Values average about 8 MeV. At low mass each new nucleon bonds with all the rest, but past iron new protons feel every other proton's repulsion while the short range attraction stays capped.

**Tip.** Read the curve as a hill that pays you to climb it. Joining light nuclei below iron moves the product upward, so fusion frees energy. Splitting heavy nuclei above iron also moves the fragments upward, so fission frees energy. Iron itself sits at the top, the end of the line.

**Recap.** Missing mass measures binding energy, and the per-nucleon curve peaks at iron, ruling which changes pay out.

## Practice

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

## Needs first

- [Inside the Atom: the Nuclear Model](https://lightmysky.com/learn/science/inside-the-atom-the-nuclear-model-mt_B_PtnvyRIo)
- [Mass, Energy and the Unified Atomic Mass Unit](https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X)

## Opens up

- [Properties of Nuclei and the Nuclear Force](https://lightmysky.com/learn/science/properties-of-nuclei-and-the-nuclear-force-mt_HdMS_muGWa)
- [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY)