NCERT Book Class 12 Physics Chapter 13 Nuclei

Physics

Chapter Thirteen

NUCLEI

13.1 INTRODUCTION
In the previous chapter, we have learnt that in every atom, the positive
charge and mass are densely concentrated at the centre of the atom
forming its nucleus. The overall dimensions of a nucleus are much smaller
than those of an atom. Experiments on scattering of a -particles
demonstrated that the radius of a nucleus was smaller than the radius
of an atom by a factor of about 104. This means the volume of a nucleus
is about 10–12 times the volume of the atom. In other words, an atom is
almost empty. If an atom is enlarged to the size of a classroom, the nucleus
would be of the size of pinhead. Nevertheless, the nucleus contains most
(more than 99.9%) of the mass of an atom.
Does the nucleus have a structure, just as the atom does? If so, what
are the constituents of the nucleus? How are these held together? In this
chapter, we shall look for answers to such questions. We shall discuss
various properties of nuclei such as their size, mass and stability, and
also associated nuclear phenomena such as radioactivity, fission and fusion.

13.2 ATOMIC MASSES AND COMPOSITION OF NUCLEUS
The mass of an atom is very small, compared to a kilogram; for example,
the mass of a carbon atom, 12C, is 1.992647 × 10–26 kg. Kilogram is not
306 a very convenient unit to measure such small quantities. Therefore, a