Nuclear Instability
Physics: AQA A Level, topic 8.1.7
Nuclear Instability
Some isotopes of atoms are more stable than others. The unstable isotopes can decay radioactively.
Stability
- Above is a graph of the number of neutrons against the number of protons in an atom
- There is a pattern where some atoms are more stable.
- An unstable atom will decay to a more stable atom.
Types of decay
- Atoms usually decay through alpha or beta radiation untill they reach a stable point.
- Any chain of alpha and beta radiation is possible to reach a stable point.
- Alpha radiation reduces the atom by two neutrons and two protons.
- Beta radiation reduced the atom by one neutron and increases by one proton.
Decay Equations
When an atom decays it may release radiation and a new atom is created. We can write equations for this decay.
Types of radiation
- Alpha radiation is made up of two protons and two neutrons.
- The mass number will reduce by four and the atomic number by two.
- Beta radiation changes a neutron into a proton and releases an electron.
- The mass number will not change but the atomic number will increase by one.
- Gamma radiation is a wave and will only change the energy of the atom, not the form
Equations of decay
- A decay equation shows the mass and atomic numbers for all of the atoms or radiation present.
- Charge and mass must be conserved so we know that the sum of the mass number before and the sum after have to be the same.
- We can work out what type of radiation is produced if we know the mass and atomic number before and after.
- Remember radiation is a product, not a reactant. It should be after the arrow.
Excited States in Nuclei
Just as atoms have excited states, nuclei also have excited states.
Atomic excited states
- In an atom, an electron can gain energy and be promoted to a higher energy level.
- When the electron returns to the original level, the energy is released as electromagnetic waves.
Nuclear excited states
- Nuclei also have excited states. This is because of the different arrangements of protons and neutrons within a nucleus.
- When the protons and neutrons rearrange to a lower energy state, the excess energy is released as gamma radiation.
Technetium-99m
- It is possible to work out the energy of the gamma radiation released.
- Technetium-99m is a valuable resource as it is used in gamma cameras in hospitals. It is produced from the beta decay of molybdenum-99.
Test yourself on Nuclear Instability
What happens to an atom when an alpha particle is emitted?
What happens when an atom decays through beta-minus radiation?
What type of radiation will not change the mass number or the atomic number?
Which of these statements are true about beta emission?
When a nucleus is arranged into a lower energy state what happens?