Scientists were always interested to know the smallest particle of matter. Greek Philosopher Democritus in 585 BC postulated that matter is built from small particles called atoms. The atom means indivisible in Greek language. Rutherford in 1911, discovered that atom had a central part called the nucleus. In this unit, we will describe different aspects of atomic and nuclear physics such as radioactivity, half- life, nuclear reactions, fission and fusion.
18.1 ATOM AND ATOMIC NUCLEUS
Rutherford discovered that the positive charge in an atom was concentrated in a small region called nucleus. The nucleus contains protons and neutrons which are collectively called nucleons. Atom also contains electrons which revolve in nearly circular orbits about the positively charged nucleus (Fig. 18.1). The simplest atom is that of hydrogen, nucleus of which is a single proton. We describe an element with respect to its nucleus and use the following quantities:
The atomic number is equal to the number of protons in the nucleus.
The neutron number is equal to the number of neutrons in the nucleus.
The atomic mass number is equal to the number of nucleons (protons + neutrons) in the nucleus i.e.,