Changes During Radioactive Decay (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• A nucleus changes to a different element, during α-decay or β-decay
  • The initial nucleus is often called the parent nucleus
  • The nucleus of the new element produced is often called the daughter nucleus
• The daughter nucleus is a new element because it has a different proton and/or nucleon number than the original parent nucleus
• This can be seen on a graph of N (neutron number) against Z (proton number)
  • For example; when Pu-239 decays by alpha to U-235, it loses 2 protons and 2 neutrons
  • U (uranium) is a completely different element from Pu (plutonium)

Graph of neutron number against proton number

Graph of N against Z for the decay of Pu–239

Alpha decay

• During alpha decay an alpha particle is emitted from an unstable nucleus
• A completely new element is formed in the process

Alpha decay

Alpha decay usually happens in large unstable nuclei, causing the overall mass and charge of the nucleus to decrease

• An alpha particle is a helium nucleus
  • It is made of 2 protons and 2 neutrons

• When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
  • The mass number decreases by 4
  • The atomic number decreases by 2

• The charge on the nucleus also decreases by 2
  • This is because protons have a charge of +1 each

Beta decay

• During beta decay, a neutron changes into a proton and an electron
  • The electron is emitted and the proton remains in the nucleus

• A completely new element is formed because the atomic number changes

Beta decay

Beta decay often happens in unstable nuclei that have too many neutrons. The mass number stays the same, but the atomic number increases by one

• A beta particle is a high-speed electron
• It has a mass number of 0
  • This is because the electron has a negligible mass, compared to neutrons and protons

• Therefore, the mass number of the decaying nucleus remains the same
• Electrons have an atomic number of -1
  • This means that the new nuclei will increase their atomic number by 1 so atomic number is conserved before and after the decay
• During β-emission, a change takes place in the nucleus:
  • A neutron is transformed into a proton and an electron
  • neutron → proton + electron

Gamma decay

• During gamma decay, a gamma ray is emitted from an unstable nucleus
  • This process makes the nucleus less energetic but does not change its structure because gamma radiation has no mass or charge

Gamma decay

Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus

Extended tier only

• Decay equations, use nuclide notation, to show the emission of α-particles, β-particles and γ-radiation
• A decay equation is similar to a chemical reaction equation
  • The particles present before the decay are shown before the arrow
  • The particles produced in the decay are shown after the arrow

• During decay equations, the sum of the mass and atomic numbers before the reaction must be the same as the sum of the mass and atomic numbers after the reaction

Alpha decay equation

• All alpha decay equations have the following form for isotopes X and Y:

• The following decay equation shows polonium-212 undergoing alpha decay
  • It forms lead-208 and an alpha particle
  • An alpha particle can also be written as a helium (He) nucleus 

  

Beta decay equation

• All beta decay equations have the following form for isotopes X and Y:

Gamma decay equation

• All gamma decay equations have the following form for isotope X