AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Simple calculations from nuclear masses of energy released in fission and…
Full specification wording
Mass and energy (A-level only): Simple calculations from nuclear masses of energy released in fission and fusion reactions.
To find the energy released in a fission or fusion reaction, first write the balanced nuclear equation and identify the masses of all reactants and products.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To find the energy released in a fission or fusion reaction, first write the balanced nuclear equation and identify the masses of all reactants and products. The total mass before the reaction minus the total mass after the reaction gives the mass difference Δm. Convert Δm from atomic mass units to kilograms if necessary, using 1 u = 1.66 × 10⁻²⁷ kg, then calculate energy with ΔE = Δmc². A common method is to work in atomic mass units and use the energy equivalent 1 u = 931.5 MeV, but the same result follows from SI units. The energy appears mainly as kinetic energy of the product nuclei and neutrons, and for fusion also as energy of emitted particles or radiation.
What earns the marks
- Write a balanced nuclear equation for the fission or fusion reaction, ensuring nucleon number and proton number are conserved.
- Calculate the total mass of reactants and the total mass of products using the nuclear masses supplied.
- Find the mass difference Δm = total mass of reactants − total mass of products.
- Convert Δm to kilograms if using SI units, or use the energy equivalent of 1 u, before applying ΔE = Δmc².
- State the energy released with an appropriate unit, such as J or MeV, and link it to the kinetic energy of the products.
Common mistakes and how to put them right
Subtracting masses in the wrong order, giving a negative mass difference.
Put it right: Subtract total product mass from total reactant mass.
Forgetting to convert atomic mass units to kilograms before using ΔE = Δmc² in SI units.
Put it right: Multiply by 1.66 × 10⁻²⁷ kg per u.
Using the mass of a whole atom instead of the nuclear mass when the question specifies nuclear masses.
Put it right: Use the values given and not add or remove electron masses unless instructed.
Exam tips
- Lay out the calculation in clear steps: balanced equation, total reactant mass, total product mass, mass difference, then energy.
- Keep track of units throughout; if masses are in u, either convert to kg or use the u-to-energy equivalence consistently.
- Check that the sign of the energy released is positive and that the magnitude is reasonable for a nuclear reaction.
Try the lesson
The start of the Mass and energy (A-level only): Simple calculations from nuclear masses of energy released in fission and fusion reactions. lesson, made from checked teaching notes for this specification point.
Have a go first
In a fusion reaction, the total mass of the reactants is 5.030 u and the total mass of the products is 5.021 u. Using 1 u = 931.5 MeV, what is the energy released?
Listen
To find the energy released in a fission or fusion reaction, first write the balanced nuclear equation and identify the masses of all reactants and products. The total mass before the reaction minus the total mass after the reaction gives the mass difference Δm. Convert Δm from atomic mass units to kilograms if necessary, using 1 u = 1.66 × 10⁻²⁷ kg, then calculate energy with ΔE = Δmc². A common method is to work in atomic mass units and use the energy equivalent 1 u = 931.5 MeV, but the same result follows from SI units. The energy appears mainly as kinetic energy of the product nuclei and neutrons, and for fusion also as energy of emitted particles or radiation. A strong answer earns marks for these points. Write a balanced nuclear equation for the fission or fusion reaction, ensuring nucleon number and proton number are conserved. Calculate the total mass of reactants and the total mass of products using the nuclear masses supplied. Find the mass difference Δm = total mass of reactants − total mass of products. Convert Δm to kilograms if using SI units, or use the energy equivalent of 1 u, before applying ΔE = Δmc². A common slip: Subtracting masses in the wrong order, giving a negative mass difference. Instead, subtract total product mass from total reactant mass. Exam tip: Lay out the calculation in clear steps: balanced equation, total reactant mass, total product mass, mass difference, then energy.
Key words
Say what each word means before you turn the card.
Mass difference
Try answering aloud or in your head before turning the card.
Energy equivalent
Try answering aloud or in your head before turning the card.
Balanced nuclear equation
Try answering aloud or in your head before turning the card.
Energy released
Try answering aloud or in your head before turning the card.
See every definition
- Mass difference
- The total mass before the reaction minus the total mass after the reaction.
- Energy equivalent
- A value of 1 u = 931.5 MeV used when working in atomic mass units.
- Balanced nuclear equation
- An equation for the reaction in which nucleon number and proton number are conserved.
- Energy released
- The result of applying ΔE = Δmc², appearing mainly as kinetic energy of the product nuclei and neutrons.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Mass and energy (A-level only): Simple calculations from nuclear masses of energy released in fission and fusion reactions.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge