Q4. Radium-226 (Z = 88) decays by alpha emission. Find A and Z of the product.
A = 226 − 4 = 222 and Z = 88 − 2 = 86 (the counting is beyond the NCERT text).
Q5. An electron and a positron initially at rest in their centre-of-momentum frame annihilate into two gamma photons, releasing approximately 1.02 MeV. How much energy does each photon carry?
About 0.51 MeV each, the rest-mass energy of an electron (NCERT page 316, Eq. 13.14(ii); NCERT Points to ponder 7, page 320).
Q6. Lead-206 has 82 protons. Find its neutron-to-proton ratio. Does it fit NCERT’s rule for heavy stable nuclei?
N = 206 − 82 = 124, so N/Z = 124/82 ≈ 1.51, close to 3 : 2, as NCERT says for heavy stable nuclei (NCERT Points to ponder 8, page 320).
Q7. A nuclide has a half-life of 2 hours. What fraction of a sample is left after 6 hours?
6 hours is 3 half-lives, so 1/23 = 1/8 is left (beyond the NCERT text).
Q8. Why did Becquerel’s plate darken even though the uranium salt was wrapped in black paper?
The salt gave out radiation that passed through the black paper, which stops light, and even through the silver; light from fluorescence could not have done this (NCERT page 314).