Year 10 Science · Unit 2 · Lesson 21

Inside the Atom's Nucleus, Nuclear Stability

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Learning Goals

Identify the odd one out in groups about nucleons, isotopes, forces and stability.
Apply nuclide notation to work out protons, neutrons and electrons for real isotopes.
Use the neutron-to-proton ratio and the band of stability to reason about whether a nucleus is stable.

Warm Up

Odd one out

Circle the item that does not belong in each group. Then explain why it doesn't fit in the answer column.

#	Group	Odd one out + reason
1	Proton Neutron Electron Nucleon
2	¹²₆C ¹³₆C ¹⁴₆C ¹⁴₇N
3	Strong nuclear force Electrostatic repulsion Holds nucleons together Short range attraction
4	Protium ¹₁H Deuterium ²₁H Tritium ³₁H Helium ⁴₂He
5	Uranium-238 Polonium-210 Iron-56 Radium-226

Your Turn

Scenario: making medical radioisotopes at ANSTO

At ANSTO's OPAL reactor, low-enriched uranium targets are irradiated. Fission of uranium-235 produces a range of fission products, including radioactive molybdenum-99 (Mo-99), which is then chemically separated and purified. This is fission-product production; some other radioisotopes are made by neutron activation, when a nucleus captures one or more neutrons, but that is not how ANSTO produces Mo-99. Mo-99 is written ⁹⁹₄₂Mo. Two other nuclei in the table below are uranium-235 (²³⁵₉₂U) and the stable nucleus oxygen-16 (¹⁶₈O). Use nuclide notation and the band-of-stability ideas to answer each row.

Count the nucleons For ⁹⁹₄₂Mo, state the number of protons and the number of neutrons. Show how you worked out the neutrons.
Same or different element Are ¹⁶₈O and ¹⁸₈O the same element? Are they isotopes? Justify using their atomic and mass numbers.
The balancing forces Name the two opposing forces inside the nucleus and state which particles each one acts on.
Predict stability Uranium-235 has Z = 92. Predict whether it can be stable and explain your reasoning using the limit at bismuth (Z = 83).
Why neutrons help Explain why adding neutrons helps a large nucleus stay together, but adding too many neutrons can still make it unstable.

Wrap Up

In one sentence, explain why a nucleus is stable only when the neutron-to-proton ratio sits inside the band of stability.