Q4. Which nuclide with Z = 80 is an isotone of 19779Au?
Gold-197 has N = 197 − 79 = 118 neutrons. With Z = 80 and N = 118, A = 198: 19880Hg (NCERT page 309).
Q5. An element has two isotopes, of masses 10.01 u and 11.01 u, with abundances 19.9% and 80.1%. Find its average atomic mass.
19.9 × 10.01 ≈ 199.2 and 80.1 × 11.01 ≈ 881.9, so the average is (199.2 + 881.9)/100 ≈ 10.81 u (worked as NCERT does for chlorine, page 307).
Q6. Using 1 u = 1.660539 × 10−27 kg and NA = 6.022 × 1023 per mole, find the mass of one mole of carbon-12 atoms.
12 u × NA = 12 × 1.660539 × 10−27 × 6.022 × 1023 ≈ 1.200 × 10−2 kg = 12.00 g, approximately 12 g at this precision. Since the 2019 SI revision the mole fixes the Avogadro constant exactly; carbon-12’s molar mass is no longer defined as exactly 12 g/mol (NIST, Redefining the Mole) (reasoned from NCERT Eq. (13.1), page 307).
Q7. By how much is the neutron heavier than the proton? Express it in u and in electron masses.
1.00866 − 1.00727 = 0.00139 u, about 0.00139/0.00055 ≈ 2.5 electron masses (NCERT Eqs. (13.2)–(13.3), pages 307–308).
Q8. Deuterium and hydrogen nuclei each contain one proton, yet the deuterium nucleus has about twice the mass. What did this suggest?
That the deuterium nucleus also holds neutral matter of about one proton mass. Chadwick’s discovery of the neutron in 1932 confirmed it (NCERT page 308).