Predicting Nuclear Decay
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|The solvent is the liquid part of the solution. The solute is dissolved in the solvent to make a solution.
The two factors that determine whether a nucleus is stable or unstable are:
- the size of the nucleus, and
- the composition* of the nucleus: the ratio of protons to neutrons
All stable nuclei fall inside the zone of stability
, shown as the dotted area on the figure opposite. Unstable nuclei decay to change the number of neutrons* (N
) and the number of protons* (Z
) to reach this zone.
The zone has:
- an N:Z ratio close to 1 for light nuclei but
- greater than 1 for larger nuclei and
- no nuclei stable with Z > 82.
All nuclei with atomic number, Z, greater than 82 are unstable:
the last stable nucleus is 208
Pb which has Z
Unstable heavy nuclei decay through a series of steps through unstable intermediates. They use α decay to reduce mass and other mechanisms to change the ratio of protons to neutrons.
This change be achieved in two ways:
- If N/Z is too low, then β+ decay (positron emission) or electron capture occurs,
- If N/Z is too high, then β- decay (beta decay) occurs
- 212Bi has Z = 83. The nucleus is beyond the zone of stability. It is radioactive and undergoes α decay
- 230Th has Z = 90. The nucleus is beyond the zone of stability. It is radioactive and undergoes α decay
- 11C has Z = 6. With 5 neutrons and 6 protons, the N/Z ratio = 5/6 = 0.83. This is too low. It is radioactive and could move towards the zone of stability through β+ decay or electron capture. (β+ decay is observed.)
- 55Fe has Z = 26. With 29 neutrons and 26 protons, the N/Z ratio = 29/26 = 1.11. A nucleus of this size needs more neutrons. It is radioactive and could move towards the zone of stability through β+ decay or electron capture. (Electron capture is observed.)
- 14C has Z = 6. With 8 neutrons and 6 protons, the N/Z ratio = 8/6 = 1.33. This is too high. It is radioactive and undergoes β+ decay.
- 15C has Z = 6. With 9 neutrons and 6 protons, the N/Z ratio = 9/6 = 1.5. This is too high. It is radioactive and undergoes β+ decay.
Each quiz is made up of 10 questions randomly drawn from a large set so you can repeat the quiz many times over. The results are not stored!
Multiple choice practice questions (not an assessment)
*If you are unsure how to extract the number of protons and neutraons from the atomic symbol, review the 'atomic symbols tutorial.
Now you know how to predict which nuclei decay, check the 'radioactive decay equations' iChem resource.