Uranium-238 undergoes alpha decay. What are the mass number and atomic number of the daughter nucleus?
AMass 234, atomic 90
BMass 236, atomic 92
CMass 238, atomic 90
DMass 234, atomic 92
Alpha decay emits a helium-4 nucleus (mass number 4, atomic number 2). So the daughter has mass number 238 − 4 = 234 and atomic number 92 − 2 = 90, which is thorium-234. This is a direct application of conservation of mass number and charge.
Question 2 True / False
In beta-minus decay, the electron emitted comes from the innermost electron shell of the atom, which is disrupted by the nuclear instability.
TTrue
FFalse
Answer: False
The electron in beta-minus decay is created at the instant of decay when a neutron converts to a proton. There are no electrons inside the nucleus — they cannot exist there. The emitted electron (and antineutrino) are newly created particles, not pre-existing ones that were stored in the nucleus or electron shells.
Question 3 Short Answer
A nucleus undergoes gamma decay. How do its atomic number and mass number change, and why?
Think about your answer, then reveal below.
Model answer: Neither changes. Gamma decay releases energy as a high-energy photon from an excited nuclear state but does not change the number of protons or neutrons.
Gamma emission is purely an energy transition — like an atom emitting light when an electron drops to a lower shell, except it happens at the nuclear level. No particles are added or removed, so both the proton count (atomic number) and nucleon count (mass number) remain the same. The element and isotope are unchanged.