# What is the difference between nuclear fission and nuclear fusion?

I always mix up the definition of nuclear fission and nuclear fusion. So, what is nuclear fission and nuclear fusion, and an example for each?

Fusion is an increase in the number of protons in the nucleus by fusing two nuclei. You know fusing means joining together, right? So that can help you remember which is which. "Confusion" comes from the same root word, and happens when you mix things up together in your mind - the "fusion" in "confusion" is the bringing things together.

Fission is a decrease in the number of protons by splitting the nucleus: "fission" means a split.

Ron's given you the classic examples for fission and for fusion.

Either process can absorb energy, or release it.

Iron is the key element here.

For elements with a lower proton count (i.e. a lower atomic number) than iron, fusion releases energy, but fission absorbs energy.

Whereas, for elements with a higher proton count (i.e. a higher atomic number) than iron, fusion absorbs energy, and fission releases energy.

(it's not quite that simple, otherwise we might find a way to fuse two lighter-than-iron nuclei to release energy and create a heavier-than-iron nucleus which we then split again back to the two original nuclei and release even more energy. That's impossible)

Nuclear fission involves the splitting of a heavy nucleus into two lighter fragments plus the release of energy. As an example here is one route by which $\ce{^235U}$, a fissile material, can split apart.

$$\ce{^{235}_{92}U + ^{1}_{0}n -> ^{141}_{56}Ba + ^{92}_{36}Kr + 3^{1}_{0}n + 200 MeV}$$

Note the 3 neutrons generated in the fission process, they are part of the chain reaction; they allow the reaction to propagate at an even faster rate as they go on to react with 3 other $\ce{^{235}_{92}U}$ nuclei.

Nuclear fusion involves the fusing together of nuclei to form a heavier element. Again, energy is released in the process. Here is an example of a fusion reaction involving the combination of 2 deuterium nuclei.

$$\ce{^{2}_{1}H + ^{2}_{1}H -> ^{4}_{2}He + ^{1}_{0}n + 17.6 MeV}$$

Some time ago I made this rudimentary infographics to help the students remember which is splitting and which is merging, maybe it can also assist you: 