Fission and Fusion Model

proton (positive) neutron (no charge)

Fission and fusion compared

	Fission	Fusion
What happens	A large nucleus splits into two smaller nuclei	Two light nuclei join to make a heavier nucleus
Starts with	A heavy nucleus, such as uranium-235, and a neutron	Light nuclei, such as isotopes of hydrogen
Needs	The nucleus to absorb a neutron	Very high temperature, to overcome the repulsion between the nuclei
Total mass	Decreases slightly	Decreases slightly
Energy	Released	Released
Where	Nuclear power stations	The Sun and other stars

How to write it in the exam

Fission is the splitting of a large nucleus into two smaller nuclei, with the release of energy.
Fusion is the joining of two light nuclei to form a heavier nucleus, with the release of energy.
In both, the total mass after the reaction is slightly less than before. The mass lost has been converted into the energy released.
In a nuclide equation, the nucleon numbers (top) must add up to the same total on both sides, and so must the proton numbers (bottom).
Fusion needs very high temperatures because the nuclei are both positive and repel each other.
Stable stars such as the Sun are powered by the fusion of hydrogen into helium.