Zone of stability

Zone of stability:

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.

Nucleus too heavy

Nucleus too heavy:

All nuclei with atomic number, Z, greater than 82 are unstable: the last stable nucleus is ²⁰⁸Pb which has Z = 82. 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.

Neutron : Proton ratio is wrong

Neutron : Proton ratio is wrong:

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

Examples

Examples:

1. ²¹²Bi has Z = 83. The nucleus is beyond the zone of stability. It is radioactive and undergoes α decay
2. ²³⁰Th has Z = 90. The nucleus is beyond the zone of stability. It is radioactive and undergoes α decay
3. 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.)
4. 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.)
5. 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.
6. 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.

Self test quizzes

Test your understanding using the quiz below.