 CBSE Sample Papers NCERT Books

# Chapter 13 Nuclei

The thirteenth chapter of NCERT Physics for Class 12 teaches about the nuclei. This chapter explains about the nucleus of an atom and energy, kits size, force, atomic masses, radioactivity, radioactive particles and their properties, and radioactive decay law, isotopes, isobars, isotones, the mass-energy relation, nuclear fission, fusion, the radioactivity properties, etc.

•  Q1 (a) Two stable isotopes of lithium 6Li3 and7Li3 have respective abundances of 7.5% and 92.5%. These isotopes have masses 6.01512 u and 7.01600 u, respectively. Find the atomic mass of lithium.   (b) Boron has two stable isotopes, 10B5 and 11B5 . Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is 10.811 u. Find the abundances of  10B5 and 11B5. Ans: (a) Mass of 6Li3, lithium isotope, m 1 = 6.01512 u Mass of 7Li3, lithium isotope, m = 7.01600 u  Abundance of 6Li3 , η1  = 7.5% Abundance of 7Li3 , η2 = 92.5% The atomic mass of lithium atom is given as: ( b ) Mass of boron isotope 10B5 , m = 10.01294 u Mass of boron isotope 11B5 , m = 11.00931 u Abundance of 10B5, η1 = x % Abundance of 11B5, η2 = (100 − x ) % Atomic mass of boron, m = 10.811 u The atomic mass of boron atom is given as: And 100 − x = 80.11% Hence, the abundance of is 10B5 19.89% and that of 11B5 is 80.11%. Q2 The three stable isotopes of neon: 20Ne10, 21Ne10 and 22Ne10 have respective abundances of 90.51%, 0.27% and 9.22%. The atomic masses of the three isotopes are 19.99 u, 20.99 u and 21.99 u, respectively. Obtain the average atomic mass of neon. Ans: Atomic mass of 20Ne10, m1 = 19.99 u Abundance of 20Ne10, η1 = 90.51% Atomic mass of 21Ne10, m2 = 20.99 u Abundance of 21Ne10, η2 = 0.27% Atomic mass of 22Ne10, m3 = 21.99 u Abundance of 22Ne10, η3 = 9.22% The average atomic mass of neon is given as: 