Stable & Unstable Nuclei

This lesson covers: 

  1. Why some atomic nuclei are unstable
  2. The types of particles emitted in nuclear decay
  3. The changes to the nucleus during alpha, beta minus, and beta plus decay
  4. How discoveries during beta decay led to hypotheses about fundamental particles like the neutrino

Nuclear instability

The strong nuclear force can only bind nuclei up to a certain size together. In larger nuclei, the strong force is too weak and the nucleus is unstable.


Unstable isotopes undergo nuclear decay - emitting particles to become more stable.


The main types of nuclear decay are:

  • Alpha decay - emission of an alpha particle
  • Beta minus decay - emission of an electron (β− particle)
  • Beta plus decay - emission of a positron (β+ particle)

Alpha decay

Alpha particles are Helium nuclei, comprising two protons and two neutrons. The are very ionising but only have a range of a few cm in air.


They are emitted from very large nuclei, like uranium, to reduce size. 


The general equation for alpha decay is represented as:


ZA​X→Z−2A−4​Y + 24​α


Where: 

A = mass number

Z = atomic number

X = element symbol of the parent nuclei

Y = element symbol of the daughter nuclei

α = alpha particle 


Changes to nucleus during alpha decay:

  • Proton number decreases by 2
  • Nucleon number decreases by 4

Beta minus decay

Beta minus particles are high speed electrons with a range of several meters in air.


During beta minus decay a neutron in a neutron rich nucleus decays into a proton, emitting an electron (beta minus particle) and an electron antineutrino.


The general decay equation for beta-minus decay is:


ZA​X→ Z+1+1A​Y + −110​β− + νˉe​


Where:

A = mass number

Z = atomic number

X = chemical symbol of parent nucleus

Y = chemical symbol of the daughter nucleus

β- = beta particle

νe​​  = electron antineutrino


Changes to nucleus:

  • Proton number increases by 1
  • Nucleon number stays the same

Beta plus decay

A beta plus particle is a positron, the antiparticle of the electron. 


During beta-minus decay a proton in a proton rich nucleus decays into a beta-plus particle and an electron neutrino.


The general formula for beta-plus decay is:


ZA​X→ Z−1+1A​Y + +110​β+ + νe​


Where:

A = mass number

Z = atomic number

X = chemical symbol for parent nuclei

Y = chemical symbol for daughter nuclei

β+= beta-plus particle

νe​ = electron neutrino 


Changes to nucleus during beta-plus decay:

  • Proton number decreases by 1
  • Nucleon number stays the same

Discoveries in beta decay

Originally scientists thought only electrons were emitted. But energy appeared to disappear during beta decay.

In 1930 Wolfgang Pauli hypothesised an additional near-massless neutral particle was produced - later named the neutrino.


Observing the antineutrino decades later provided evidence for Pauli's particle and led to revisions in theories of fundamental particles and processes within the atom.