8.1.9

# Mass & Energy

Test yourself

## Mass and Energy

The nucleus of an atom is made up of protons and neutrons. When protons and neutrons combine to make an atom energy is released.

### Binding energy

• A nucleus is made up of protons and neutrons.
• The mass of a nucleus is less than the mass of the individual protons and neutrons.
• This is because when the protons and neutrons combine they release energy.
• The energy released is called the binding energy.

### Energy with mass

• Energy and mass can be linked through a very famous equation. The equation linking energy and mass is:
• Energy = mass x (the speed of light)$^2$
• $E=mc^2$
• If the change in mass between the individual particles and the mass of the nucleus is known, we can find the binding energy using this equation.

## Fission

Nuclear fission is a reaction in which a nucleus is split (or fissured).

### Fission

• Energy is released when heavy nuclei are split apart.
• The new nuclei (daughter nuclei) have a larger binding energy per nucleon than the parent.
• This means that a large amount of energy is released (about 100 times the energy of a normal nuclear decay).

### Chain reaction

• Fission occurs when an unstable atom splits, we can force this split by making the atom absorb a neutron.
• The fission reaction will produce daughter nuclei but also release energy and some neutrons.
• These neutrons will go on to cause fission reactions themselves, which will produce more neutrons.
• This is now a chain reaction.

## Fusion

Nuclear fusion is a reaction in which two nuclei are combined, or fused, to form a larger nucleus.

### Fusion

• We know that all nuclei have less mass than the sum of the masses of the protons and neutrons.
• When an atom is formed it releases binding energy - the greater the binding energy, the greater the missing mass.
• The binding energy per nucleon is a maximum at iron.
• This means that if two low-mass nuclei can be fused together to form a larger nucleus, energy can be released.

### Making fusion work

• Fusion would release a huge amount of energy but there are many issues which need to be overcome before it will work on earth.
• The repulsion between positive nuclei is very strong and can only be overcome at very high temperatures or pressures.

### Energy from fusion

• We can use the energy and mass equation:
• $E=mc^2$
• If we know the mass of the nuclei before and the mass of the large nucleus afterwards we can find the change in mass.
• If we know the change in mass we can find the energy released.