Stable & Unstable Nuclei

Test yourself

Strong Nuclear Force in an Atom's Nucleus

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

Illustrative background for Stability of the nucleusIllustrative background for Stability of the nucleus ?? "content

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.
Illustrative background for Short range forceIllustrative background for Short range force ?? "content

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.
Illustrative background for Repulsive at very short distancesIllustrative background for Repulsive at very short distances ?? "content

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.
Illustrative background for Same for all nucleonsIllustrative background for Same for all nucleons ?? "content

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.

Illustrative background for α decayIllustrative background for α decay ?? "content

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).
Illustrative background for &beta;<sup>-</sup> decayIllustrative background for &beta;<sup>-</sup> decay ?? "content

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.
Illustrative background for Energy conservation problemIllustrative background for Energy conservation problem ?? "content

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.
Illustrative background for Antineutrino discoveryIllustrative background for Antineutrino discovery ?? "content

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.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

Go student ad image

Unlock your full potential with GoStudent tutoring

  • Affordable 1:1 tutoring from the comfort of your home

  • Tutors are matched to your specific learning needs

  • 30+ school subjects covered

Book a free trial lesson