Fission vs Fusion: What Is the Difference?

The distinction, stated once

AP Physics 2 defines both in consecutive essential knowledge statements, and they are almost mirror images of one another.

Fusion. Essential knowledge 15.7.A.6: nuclear fusion is the process by which two or more smaller nuclei combine to form a larger nucleus, as well as subatomic particles.

Fission. Essential knowledge 15.7.A.7: nuclear fission is the process by which the nucleus of an atom splits into two or more smaller nuclei, as well as subatomic particles.

One count goes up, one goes down. Fusion takes several nuclei in and produces one larger one; fission takes one in and produces several smaller ones. Both also produce loose subatomic particles, which the definitions include explicitly, so a reaction equation with only nuclei on the product side is usually incomplete.

That is genuinely all the framework says about what the two processes are. Everything else the course asks of you is about the constraints they share, and those constraints are the same for both: conservation of nucleon number (15.7.A.2), and conservation of energy, energy-mass equivalence, and conservation of momentum (15.7.A.3).

So the honest structure of this pair is unusual. The definitions differ in one word, the direction. The physics you actually do is identical for both. A question about fission and a question about fusion at this level are the same question with the arrows reversed.

Side by side

Most of that table is the same on both sides, and it is more useful to see that than to be handed differences that the framework does not assert. Four rows actually separate the two: the definitions, the direction of the nucleon count, the number of reactant nuclei, and the spontaneity row. Every other row says the same thing twice.

The spontaneity row is worth reading precisely. Essential knowledge 15.7.A.8 says nuclear fission may occur spontaneously or may require an energy input, depending on the binding energy of the nucleus. That statement is about fission only. There is no matching statement about fusion anywhere in the AP Physics 2 framework, so a claim that fusion always requires an energy input, or requires a particular temperature, is not something the course supports. If a question needs a condition, it will state it.

What the CED requires, and four terms it never uses

The pair lives in Topic 15.7, Fission, Fusion, and Nuclear Decay. Learning objective 15.7.A asks you to describe the physical properties that constrain the behavior of interacting nuclei, subatomic particles, and nucleons, and its eight essential knowledge statements are:

• 15.7.A.1: the strong force is exerted at nuclear scales and dominates the interactions of nucleons, protons or neutrons.
• 15.7.A.2: possible nuclear reactions are constrained by the law of conservation of nucleon number.
• 15.7.A.3: the behavior of the constituent particles of a nuclear reaction is constrained by laws of conservation of energy, energy-mass equivalence, and conservation of momentum.
• 15.7.A.4: for all nuclear reactions, mass and energy may be exchanged due to mass-energy equivalence, with the relevant equation $E = mc^2$.
• 15.7.A.5: energy may be released in nuclear processes in the form of kinetic energy of the products or as photons.
• 15.7.A.6 and 15.7.A.7: the definitions above.
• 15.7.A.8: nuclear fission may occur spontaneously or may require an energy input, depending on the binding energy of the nucleus.

The topic's second objective, 15.7.B, covers radioactive decay rather than fission and fusion: half-life at 15.7.B.1.ii, the decay constant related to it by $\lambda = \ln 2 / t_{1/2}$ at 15.7.B.1.iii, and the population equation $N = N_0 e^{-\lambda t}$ at 15.7.B.2. Topic 15.7 carries no boundary statement, checked against both of its pages.

Now the part that matters more than any of it. Four terms that feel indispensable here appear nowhere in the AP Physics 2 course and exam description. Searched from the first page to the last:

• Coulomb barrier: not present.
• Chain reaction: not present.
• Critical mass: not present.
• Mass defect: not present.

The phrase binding energy appears twice in the whole document, and this is the detail worth taking away. Its only definition is in a different unit and a different sense: essential knowledge 15.3.A.5, in Topic 15.3 on emission and absorption spectra, says binding energy is the energy required to remove an electron from an atom, causing the atom to become ionized, and adds that an atom in the lowest energy level, the ground state, will require the greatest amount of energy to remove the electron. Then 15.7.A.8 uses the phrase in the nuclear sense, as the binding energy of the nucleus, without redefining it. So the framework defines an atomic binding energy and then borrows the words for a nuclear one. Neither sense gets an equation: the AP Physics 2 equation sheet has no binding-energy entry, and the framework never plots a binding-energy curve.

That is not a gap to be filled in from memory. It is information about what the exam can ask. Nuclear physics is the topic where remembered facts feel most reliable and are least so, and the four terms above are exactly the ones a textbook chapter would use freely. You may still need the underlying ideas: that two positive nuclei repel and so approaching one another costs energy, that the neutrons a fission produces can go on to cause more fissions, that a nucleus weighs less than its separated nucleons. What you should not do is attach those ideas to a framework statement that does not exist, or expect the terminology on an exam. Where this page uses any of those ideas, it says plainly that the framework does not name them.

The case that separates them: count the nucleons in both directions

The one calculation that distinguishes the two processes is bookkeeping, and 15.7.A.2 is the rule: possible nuclear reactions are constrained by the law of conservation of nucleon number. Essential knowledge 15.8.A.2.i, stated for decays, adds that nucleon number, meaning the number of neutrons and protons, along with lepton number and charge, are conserved.

Write a nucleus as $^{A}_{Z}X$, where $A$ counts the nucleons and $Z$ counts the protons. Then both conservation rules become sums you can check by eye: the $A$ values on the left must equal the $A$ values on the right, and the same for the $Z$ values.

A fission, run forwards. A nucleus with $Z = 92$ and $A = 235$ absorbs a neutron, giving $A = 236$ and $Z = 92$, and splits into a fragment with $Z = 56$, $A = 141$ and a fragment with $Z = 36$, $A = 92$, plus some number of free neutrons.

Three neutrons, and both columns now balance. The neutron count came out of the arithmetic rather than out of recall, which is the point: you do not need to know which isotope this is to complete the equation. The Topic 15.8 boundary statement makes that explicit for decays, saying AP Physics 2 does not expect students to memorize the processes by which specific isotopes decay or the half-lives of specific isotopes.

Those three neutrons are also the reason this direction can sustain itself, since each of them can be absorbed by another nucleus and cause another fission. The framework does not name that process, so on an exam describe the mechanism rather than reaching for a term.

A fusion, run forwards. Two nuclei each with $Z = 1$ and $A = 2$ combine into a nucleus with $Z = 2$ and $A = 3$, plus one free particle.

Same method, opposite direction. One nucleus out where there were two in, and the leftover nucleon leaves as a free neutron, which is the "as well as subatomic particles" clause of 15.7.A.6 doing its work.

Notice what the two tables have in common: identical rules, identical procedure, and only the arrow reversed. That is the honest summary of the difference at this level.

Where the energy comes from, and a rounding the sheet does not resolve

Essential knowledge 15.7.A.4 says that for all nuclear reactions, mass and energy may be exchanged due to mass-energy equivalence, and gives $E = mc^2$ as the relevant equation. The AP Physics 2 equation sheet prints it in the modern physics block. Essential knowledge 15.7.A.5 then says where the energy goes: released as kinetic energy of the products, or as photons.

So the recipe is: find how much total rest mass the reaction lost, multiply by $c^2$, and that is the energy released. The framework calls the mass a nucleus is short of its separated nucleons nothing in particular, so this page will not either.

The conversions you need are all on the sheet's constants table:

• $1 \ \mathrm{u} = 1.66 \times 10^{-27} \ \mathrm{kg} = 931 \ \mathrm{MeV}/c^2$
• $c = 3.00 \times 10^8 \ \mathrm{m/s}$
• $1 \ \mathrm{eV} = 1.60 \times 10^{-19} \ \mathrm{J}$

The second form of the first line is the one to use. A mass change quoted in atomic mass units multiplies straight into $\mathrm{MeV}/c^2$, and the $c^2$ cancels against the $c^2$ in $E = mc^2$, so no powers of ten are ever handled.

And here is a detail worth knowing before it costs you a mark on a two-route check. The sheet's own constants are not mutually consistent to three figures. Take the kilogram value for one atomic mass unit and push it all the way through:

The same sheet prints $931 \ \mathrm{MeV}/c^2$ on the same line. The two differ by about $0.3$ percent, which is what you should expect from three-significant-figure constants and is not an error in either. The practical rule: use the printed $931 \ \mathrm{MeV}/c^2$ directly, and if you route through kilograms and joules instead, do not be alarmed when the two answers disagree in the third figure.

One thing the CED does not do is tell you which process releases more energy. There is no statement ranking fission against fusion, no binding-energy-per-nucleon curve, and no numbers of any kind attached to either process. If a problem wants an energy release, it has to supply the mass change, and the worked examples below treat every mass change as given data for exactly that reason.

Momentum, and which fragment moves faster

Essential knowledge 15.7.A.3 names conservation of momentum alongside conservation of energy as a constraint on a nuclear reaction, and it is the constraint that decides how the released energy is shared out.

Take a nucleus at rest that splits into exactly two fragments. Total momentum before is zero, so total momentum after must be zero, which means the two fragments carry equal and opposite momenta:

The lighter fragment therefore moves faster, in inverse proportion to its mass. And the kinetic energies are not shared equally either. Writing kinetic energy in terms of momentum, $K = p^2/(2m)$, and noting that the two fragments have the same $\lvert p \rvert$, gives

So the lighter fragment carries the larger share of the kinetic energy, and the shares are in inverse proportion to the masses. That is a general result about a two-body split from rest, and it applies whichever nuclear process produced it.

Two cautions on using it.

Approximating a nuclear mass by its nucleon count is an approximation. A nucleus with $A = 141$ does not have a mass of exactly $141 \ \mathrm{u}$, and the difference is precisely the mass that the energy release came from. Using $A$ as a proxy for mass in a momentum split is fine to two figures and should be labelled as an approximation.

A split into more than two products cannot be handled this way. Fission commonly produces free neutrons as well as two fragments (15.7.A.7), and those neutrons carry momentum and energy too. The clean inverse-mass result needs exactly two bodies, so if the reaction has three or more products, use conservation of momentum as a vector statement and expect the problem to supply enough information to close it.

For the mechanics behind all of this, momentum and kinetic energy are shared with AP Physics 1 and the AP Physics 2 sheet reprints the whole mechanics table: see the conservation of momentum guide for the two-body procedure and the impulse and momentum theorem guide for how the momentum change is delivered.

When it costs a mark

Getting the direction backwards. Fusion combines and fission splits. The words are similar enough that under pressure they swap, and every subsequent line of the answer then inherits the error. A one-word check at the top of the working is worth the time.

Forgetting the subatomic particles. Both 15.7.A.6 and 15.7.A.7 include them explicitly. A fission equation with two fragments and nothing else usually fails to balance nucleon number, and the missing count is telling you how many free particles the products include.

Balancing nucleon number and forgetting charge. They are separate conservation statements, and 15.8.A.2.i lists nucleon number, lepton number and charge as three distinct conserved quantities. A reaction that balances $A$ and not $Z$ is not a possible reaction.

Using a real isotope's numbers from memory. The Topic 15.8 boundary statement says AP Physics 2 does not expect students to memorize the processes by which specific isotopes decay or the half-lives of specific isotopes. A question that needs those numbers must give them, so a recalled value that is not in the question is almost certainly an intrusion.

Claiming fusion requires an energy input because the CED says so. It does not. Essential knowledge 15.7.A.8 makes that statement about fission, and there is no fusion counterpart in the framework. The idea that two positive nuclei must be pushed together against their mutual repulsion is sound physics; it is just not something the AP Physics 2 framework states, and it has no name in that document.

Reaching for chain reaction, critical mass, Coulomb barrier or mass defect as course vocabulary. None of the four appears in the AP Physics 2 course and exam description. Describe the mechanism instead of naming it.

Assuming the heavier fragment carries more kinetic energy. It carries less. Equal and opposite momenta with $K = p^2/(2m)$ put the larger share on the lighter fragment.

Dropping the $c^2$, or squaring it wrongly. $c^2 = 9.00 \times 10^{16} \ \mathrm{m^2/s^2}$ from the sheet value $c = 3.00 \times 10^8 \ \mathrm{m/s}$. Using $3.00 \times 10^8$ where $c^2$ belongs is out by eight orders of magnitude, which at least announces itself. Using $931 \ \mathrm{MeV}$ per $\mathrm{u}$ avoids the exponent handling entirely.

Mixing up $\lambda$. In this topic $\lambda$ is the decay constant, related to half-life by $\lambda = \ln 2 / t_{1/2}$ (15.7.B.1.iii). Elsewhere on the same equation sheet $\lambda$ is a wavelength. The symbol is reused and the two have nothing to do with each other.

What the two share, and why that lulls you

This pair is unusual among comparisons: almost everything is shared, and the shared parts are the parts you are examined on.

Both obey the same conservation laws, and the framework states them once for both. 15.7.A.2 covers nucleon number for all nuclear reactions, and 15.7.A.3 covers energy, energy-mass equivalence and momentum for all of them. There is no fission version and no fusion version.

Both use the same equation. $E = mc^2$ is the only relevant equation attached to either process, and the sheet prints nothing else for them.

Both release energy in the same forms. 15.7.A.5 gives kinetic energy of the products or photons, without distinguishing which process produced them.

Both are governed by the same force at the nucleon scale. 15.7.A.1 gives the strong force for the interactions of nucleons generally.

So a student who has learned to do fission problems can already do fusion problems, which is genuinely good news and also the reason the two get conflated. The lull is that the identical procedure makes the direction feel unimportant, and the direction is the only thing the definitions actually say.

The distinction turns on three questions, and they are the three that a question can actually test.

1. How many nuclei go in and how many come out? One in and several out is fission (15.7.A.7); several in and one out is fusion (15.7.A.6).
2. Does the nucleon count balance, including the free particles? The same check for both, and the one that most reaction-completion questions are really asking.
3. Is the statement you are about to make one the framework actually makes? For spontaneity, 15.7.A.8 covers fission and nothing covers fusion. For binding energy, one mention and no definition. For the four familiar terms above, nothing at all.

That third question is not usually part of a comparison page. It belongs on this one, because nuclear physics is where a confidently remembered fact is most likely to be a fact about a different course.

Where this sits on the AP exam

Both processes are in Topic 15.7, inside Unit 15, Modern Physics, which the CED weights at 12 to 15 percent of the multiple-choice section over a suggested 14 to 22 class periods. The unit's Progress Check is listed as about 24 multiple-choice questions and 4 free-response questions.

The suggested skills the CED lists for Topic 15.7 are 1.B, create quantitative graphs with appropriate scales and units, including plotting data; 2.B, calculate or estimate an unknown quantity with units from known quantities; 2.C, compare physical quantities between two or more scenarios or at different times and locations in a single scenario; 3.A, create experimental procedures that are appropriate for a given scientific question; and 3.B, apply an appropriate law, definition, theoretical relationship, or model to make a claim. Skill 2.C is worth noting for this page specifically, since comparing two scenarios is exactly what a fission-against-fusion question does.

Unit 15's exam guidance says the first free-response question on the AP Physics 2 exam is the Mathematical Routines question, which focuses on creating and using mathematical models, and that while Unit 15 offers content well suited to it, the question can draw on any of the seven units. The unit's stated purpose is also worth knowing: the CED says Unit 15 lays the groundwork for the study of modern physics by resolving the conflicts and unanswered questions from Units 13 and 14, and that students will make connections between this content and the fundamental principles of physics and principles of conservation used earlier in the course. That is a direct instruction to bring conservation of energy and momentum with you rather than treating nuclear reactions as a separate subject.

The neighbouring topic is 15.8, Types of Radioactive Decay, which covers alpha, beta-minus, beta-plus and gamma decay (15.8.A.2), and whose boundary statement is the one quoted above about not memorising specific isotopes, along with excluding neutron emission and electron capture, types of neutrinos, the characteristics distinguishing neutrinos from antineutrinos, and any explanation or application of the weak force. Reading that boundary statement in full is the fastest way to see how tightly the nuclear content of this course is scoped.

For the energy bookkeeping that both processes rely on, the conservation of energy guide covers the framework, and the conservation of momentum guide covers the two-body split used above.