Nucleus
The entire positive charge and nearly the entire mass of atom is concentrated in a very small space called the nucleus of an atom.
The nucleus consists of protons and neutrons. They are called nucleons.
Terms Related to Nucleus
( i) Atomic Number The number of protons in the nucleus of an atom of the element is called atomic number( Z) of the element.
( ii) Mass Number The total number of protons and neutrons present inside the nucleus of an atom of the element is called mass number( A) of the element.
( iii) Nuclear Size The radius of the nucleus R ∝ A 1 / 3 ⇒ R = R o A 1 / 3 where, R o = 1.1 * 10-15 m is an empirical constant.
( iv) Nuclear Density Nuclear density is independent of mass number and therefore same for all nuclei.
ρ = mass of nucleus / volume of nucleus ⇒ ρ = 3m / 4π R 3 o where, m = average mass of a nucleon.
( v) Atomic Mass Unit It is defined as 1 / 12th the mass of carbon nucleus. It is abbreviated as arnu and often denoted by u. Thus
1 amu = 1.992678 * 10-26 / 12 kg = 1.6 *
10-27 kg = 931 Me V
Isotopes
The atoms of an element having same atomic number but different mass numbers are called isotopes.
e. g., 1 H 1, 1 H 2, 1 H 3 are isotopes of hydrogen.