A Levels Physics (9702)•9702/14/M/J/25
Explanation
Ion Composition from Atomic Numbers and Charge
Steps:
- Proton number Z = 92, so number of protons = 92.
- Nucleon number A = 235, so number of neutrons = A - Z = 235 - 92 = 143.
- Neutral uranium atom has electrons = Z = 92.
- +2e charge means 2 electrons lost, so electrons = 92 - 2 = 90.
- Total particles = protons + neutrons + electrons = 92 + 143 + 90 = 325.
Why B is correct:
- Total subatomic particles in an ion equal nucleons (A) plus electrons (Z - charge magnitude in units of e), yielding 235 + 90 = 325 per ion definition.
Why the others are wrong:
- A: Counts only nucleons (protons + neutrons = 235), excludes electrons.
- C: Assumes neutral atom total (235 nucleons + 92 electrons = 327).
- D: Overcounts electrons (e.g., 235 + 94 = 329, misapplying charge).
Final answer: B
Topic: Atoms, nuclei and radiation
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