# Fission and Fusion

## Key Questions

• Fusion is the reaction in which atoms are banged together to form heavier elements.

The most basic fusion reaction is between two hydrogen atoms:

H + H -> He

The atomic number of hydrogen is 1, so banging two hydrogen nuclei together creates a mass number of two: thus, helium is formed.

A great amount of energy is needed to allow fusion to take place. This is because that the nuclear charge of the nucleus is positive: if you bring two of them close together they will repel (much like if you put two positive poles of a magnet together). Fusion does, however, create a vast amount of energy: far more than fission.

The best way to separate fission from fusion is that fission reactions break down atoms into smaller atoms and fusion bangs atoms together to create heavier atoms.

• Fusion and fission are similar in that they both release large amounts of energy.

Nuclear fusion is a process in which two nuclei join to form a larger nucleus.

$\text{_5^10"B" + _2^4"He" → _7^13"N" + _0^1"n}$ + energy

Nuclear fission is a process in which a nucleus splits into two smaller nuclei.

$\text{_92^235"U" + _0^1"n" → _56^142"Ba" + _36^91"Kr" +3_0^1"n}$ + energy

At first sight, it doesn’t make sense that both fission and fusion release energy.

The key is in how tightly the nucleons are held together in a nucleus. If a nuclear reaction produces nuclei that are more tightly bound than the originals, then the excess energy will be released.

It turns out that the most tightly bound atomic nuclei are around the size of iron-56.

Thus, if you split a nucleus that is much larger than iron into smaller fragments, you will release energy because the smaller fragments are at a lower energy than the original nucleus.

If instead you fuse very light nuclei to get bigger products, energy is again released because the nucleons in the products are more tightly bound than in the original nuclei.