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 .