Stable & Unstable Nuclei
Physics: AQA A Level, topic 2.1.5
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.
Test yourself on Stable & Unstable Nuclei
How far can the strong nuclear force reach?
What type of force holds protons and neutrons together in the nucleus of an atom?
Which of these are true of alpha decay?
Which of these are true of beta minus decay?
What gave scientists the clue that neutrinos existed?