Binding energy is a key concept in IB Chemistry Topic 12 (Atomic Structure), especially when studying nuclear reactions such as fission and fusion. It explains why nuclei stay together, why some isotopes are stable while others are radioactive, and how tiny differences in mass translate into enormous amounts of energy. Understanding binding energy also helps students interpret nuclear stability graphs and solve mass–energy conversion problems.
What Is Binding Energy?
Binding energy is the energy required to separate a nucleus into its individual protons and neutrons.
Alternatively:
It is the energy released when the nucleus forms from separate nucleons.
Because binding energy is released when nucleons come together, the resulting nucleus has less mass than the sum of its parts—this is the mass defect. Binding energy and mass defect are two sides of the same nuclear process.
Why Binding Energy Is Important
Binding energy tells us:
- How stable a nucleus is
- How much energy is stored in nuclear reactions
- Why some isotopes undergo radioactive decay
- Why fusion and fission release so much energy
A nucleus with high binding energy per nucleon is more stable and less likely to undergo decay.
The Link Between Mass and Energy: E = mc²
Binding energy is directly related to mass defect through Einstein’s famous equation:
E = mc²
Where:
- m = mass defect
- c = speed of light (3.00 × 10⁸ m/s)
Because c² is huge, even a tiny amount of mass becomes a massive amount of energy.
This explains why nuclear reactions release far more energy than chemical reactions.
Binding Energy vs Binding Energy per Nucleon
A crucial distinction in IB Chemistry:
Binding Energy
- Total energy holding an entire nucleus together.
Binding Energy per Nucleon
- Total binding energy ÷ number of nucleons.
- This value determines nuclear stability.
Nuclei with higher binding energy per nucleon are more stable.
Binding Energy per Nucleon Graph
This graph is one of the most important in nuclear science.
Key features:
- It rises sharply from hydrogen to iron.
- Peaks at iron-56 (most stable nucleus).
- Slowly declines for heavier nuclei.
Implications:
- Fusion of light nuclei releases energy (moving up the curve).
- Fission of heavy nuclei releases energy (moving down the curve toward iron).
This single graph explains the energy source of stars and nuclear reactors.
Binding Energy in Fusion
Fusion occurs when two light nuclei combine to form a heavier nucleus with a higher binding energy per nucleon.
Example:
²H + ³H → ⁴He + energy
Because the helium nucleus has:
- Less mass
- Higher binding energy
The mass difference becomes energy.
This is why fusion powers stars.
Binding Energy in Fission
In fission, a heavy nucleus splits into smaller nuclei that are more stable.
Example:
U-235 splits into Ba and Kr fragments.
The products:
- Have higher binding energy per nucleon
- Have lower total mass
Again, the mass difference becomes energy.
This is the basis of nuclear power and nuclear weapons.
Calculating Binding Energy (IB Level)
Steps:
- Calculate mass defect (Δm):
Δm = (mass of protons + mass of neutrons) – (mass of nucleus) - Convert mass defect into kilograms (if needed).
- Use E = mc² to find binding energy.
- Divide by number of nucleons for binding energy per nucleon.
IB provides values for proton and neutron masses in the data booklet.
Why Binding Energy Matters
Binding energy explains:
- Nuclear stability
- Energy release in nuclear reactions
- Radioactive decay
- Fusion in stars
- Fission in reactors
- Why mass appears to “disappear”
This concept ties together nuclear physics and chemistry.
Common IB Misunderstandings
“Binding energy adds mass.”
Incorrect—binding energy reduces mass because it is released during formation.
“High total binding energy means stable.”
It is binding energy per nucleon that indicates stability.
“All nuclei have similar binding energies.”
Binding energies vary widely between isotopes.
“Mass defect violates conservation of mass.”
Mass becomes energy; nothing is lost.
FAQs
Why is iron the most stable nucleus?
It has the highest binding energy per nucleon, meaning its nucleons are most tightly bound.
Can a stable nucleus have low binding energy?
Not usually—low binding energy means weaker nuclear forces and more instability.
Why do heavy nuclei undergo fission?
Splitting increases binding energy per nucleon and releases energy.
Conclusion
Binding energy is the energy that holds the nucleus together, and it arises from the mass defect created when nucleons bind. Higher binding energy per nucleon means greater nuclear stability and explains why both fission and fusion release enormous amounts of energy. Mastering binding energy helps IB Chemistry students understand nuclear reactions and mass–energy relationships at a deep level.
