What is the difference between \( {}^{235}_{92}U \) and \( {}^{238}_{92}U \)?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( {}^{238}U \) has 3 neutrons more
The two are isotopes: same atomic number Z, different mass number A. The number of neutrons is \( N = A - Z \).
\( {}^{235}_{92}\mathrm{U}: Z = 92,\; N = 235 - 92 = 143 \)
\( {}^{238}_{92}\mathrm{U}: Z = 92,\; N = 238 - 92 = 146 \)
Both have 92 protons (and 92 electrons in the neutral atom); \( {}^{238}\mathrm{U} \) has 3 more neutrons.
NCERT (2026-27):
Summary – “… The atomic number \( Z \) is the number of protons in the atomic nucleus of an element. The mass number \( A \) is the total number of protons and neutrons in the atomic nucleus; \( A = Z+N \); Here \( N \) denotes the number of neutrons in the nucleus. … Nuclides with the same atomic number \( Z \), but different neutron number \( N \) are called isotopes.”