2.1.5

# Stable & Unstable Nuclei

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## Strong Nuclear Force in an Atom's Nucleus

The strong nuclear force holds protons and neutrons together in the nucleus of an atom.

### Stability of the nucleus

• The nucleus is positively charged because it contains neutrons, which have no charge, and positively charged protons.
• Without the strong nuclear force, the nucleus would break apart because of the electrostatic repulsion between the protons.

### Short range force

• The strong nuclear force is only significant over a very short distance.
• If two protons are separated by more than about 3 fm (3 ×10-15 m), the strong nuclear force is too weak to overcome the electrostatic repulsion.

### Repulsive at very short distances

• We know that nuclei do not collapse into a point.
• So the strong nuclear force must be repulsive at very small distances.
• The distance at which the strong nuclear force becomes repulsive is about 0.5 fm.

### Same for all nucleons

• The strong nuclear force works in exactly the same way for all nucleons.
• Protons and neutrons will feel the same force.

## Unstable Nuclei

Unstable nuclei will often decay via α (alpha) or β- (beta minus) emission.

### α decay

• Alpha decay happens in very large nuclei.
• An alpha particle, made up of two neutrons and two protons, is released.
• The proton number of the atom decreases by two.
• The nucleon number decreases by four (two protons and two neutrons).

### β- decay

• Beta minus decay happens in nuclei which have too many neutrons.
• One neutron will decay into a proton, releasing a beta particle (an electron) and an antineutrino.
• The proton number increases by one.
• The nucleon number remains the same.

### Energy conservation problem

• When scientists first observed beta decay, they thought that neutrons were decaying into a proton and an electron only.
• They noticed that the energy of the neutron before the decay was larger than the energy of the proton and electron after the decay: energy was not being conserved.

### Antineutrino discovery

• To account for this, scientists hypothesised that a new type of particle was being produced and carrying away some energy.
• This particle must have zero (or almost zero) mass and must be electrically neutral (to obey charge conservation).
• This particle was called a neutrino.
• We now know it to be an antiparticle called an antineutrino.

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