### Nuclear Fission

- Usually large unstable nuclei break up gradually by the process of radioactive decay, but a small number (including Uranium-235, a naturally occurring isotope of Uranium) can break up in one big go – a process known as
**nuclear fission** - In order to undergo nuclear fission, a nucleus usually requires some energy which can be given by hitting the nucleus with a neutron

(Neutrons are used because they are chargeless and so are not repelled by the positive charge of the nucleus)

*Nuclear fission: A Uranium-235 nucleus is struck by a neutron, breaking it into two smaller daughter nuclei and 2 or 3 neutrons*

* *

- When this happens, the original nucleus breaks apart into
**two smaller ‘daughter’ nuclei, along with two or three neutrons** - These
**fission products**carry away the energy released in the form of kinetic energy

### Nuclear Fusion

### Nuclear Equations

- Nuclear reactions, such as fission and fusion, can be represented using nuclear equations (which are similar to chemical equations in Chemistry)

For example:

- The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and a Xenon nucleus, releasing two neutrons in the process
- In the above reaction:

The sum of top (nucleon) numbers on the left-hand side equals the sum of top number on the right-hand side:

235 + 1 = 236 = 90 + 144 + 2 × 1

The same is true for the lower (proton) numbers:

92 + 0 = 92 = 38 + 54 + 2 × 0

- By balancing equations in this way, you can determine, for example, the number of neutrons emitted by a process like this

Example:

- In the above example, balancing the numbers on the top shows that 3 neutrons must be released in the reaction (i.e.
*N = 3*)