Ans
(a)(i) both have 92 (protons) OR same (number of protons)
(ii) U-235 has (3) fewer neutrons OR U-238 has (3) more neutrons OR U-235 has 143 and U-238 has 146 neutrons
(b) \((2\times 24 =)\) 48 (minutes)
(change in mass takes place over / decay takes) 2 half-lives
\(16\to 8(.0)\to 4(.0)\) OR \(16\times ½\times ½ (= 4(.0))\)
Detailed Solution: (a)(i) Isotopes have the same proton number, so both U-235 and U-238 contain 92 protons. (a)(ii) The neutron number is A – Z, giving 143 for U-235 and 146 for U-238. (b) The mass halves each half-life: 16.0 mg → 8.0 mg (24 min) → 4.0 mg (48 min).
