Radioactive Decay (Cambridge O Level Physics)

• Some atomic nuclei are unstable
• This is because of an imbalance in the forces within the nucleus
  • Forces exist between the particles in the nucleus
  • This is commonly due to the nucleus having too many protons or neutrons

• Carbon-14 is an isotope of carbon which is unstable
  • It has two extra neutrons compared to stable carbon-12

Isotopes of Carbon-12 and Carbon-14

Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons

• Some isotopes are unstable because of their large size or because they have too many or too few neutrons

• Unstable nuclei can emit radiation in the form of α-particles to become more stable

An Unstable Nucleus Emitting an Alpha Particle

Unstable nuclei decay by emitting α-particles

• As the α-particle moves away from the nucleus, it takes some energy with it
  • This reduces the overall energy of the nucleus
  • It makes the nucleus more stable

• The process of emitting α-particles is called radioactive decay
• Radioactive decay is a random process
  • There is an equal probability of any nucleus decaying
  • It cannot be known which particular nucleus will decay next
  • It cannot be known at what time a particular nucleus will decay
  • The rate of decay is unaffected by the surrounding conditions
  • It is only possible to estimate the probability of a nucleus decaying in a given time-period

• Therefore, the emission of radiation is:
  • Spontaneous 
  • Random in direction

• Radioactive decay events can be shown using a decay equation
• 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 Equations

• 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 Alpha Decay Equation

Alpha decay equation showing the change in mass and atomic numbers

• 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 nucleus (Symbol He)

e.g. 

Beta Decay Equation

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

The Beta Decay Equation

Beta decay equation showing the change in atomic number

Gamma Decay

• The gamma ray that is emitted has a lot of energy, but no mass or charge
• Here is an example of Uranium-238 undergoing gamma decay
  • Notice that the mass number and atomic number of the unstable nuclei remain the same during the decay

The Gamma Decay Equation

Gamma decay equation showing no change in mass or atomic number