NEET Physics Notes Optics-Atoms and Nuclei-Concept of Nucleus
Concept of Nucleus
In every atom, the positive charge and mass is densely concentrated at the centre of the atom forming its nucleus. Nuclear radius is of the order of 10-15 m. In nucleus, the number of protons is equal to the atomic number of that element and the remaining particles to fulfil the mass number are the neutrons i.e. number of protons = atomic number Z (say) and number of neutrons.
Composition and Size of Nucleus
As mass of an atom is very small, hence we define a new unit of mass, called as 1 atomic mass unit (1u),
which is the mass of one atom of carbon-12.
lu = 1.660539 X 10-27 = kg = 1.66 x 10-27 kg
= 931.5 MeV
A nucleus has a structure of its own. It consists of protons and neutrons. Electrons cannot exist inside the nucleus. A proton is a positively charged particle having mass (Mp) of 1.007276u and charge (+ e) = +1602 x 10-39 C.
Number of protons (Z) inside the nucleus of an atom is exactly equal to the number of electrons revolving around the nucleus of that atom. This number is called the atomic number. A neutron is a neutral particle having mass Mn = 1008665 u. The number of neutrons in the nucleus of an atom is called the neutron number N.’ The sum of the number of protons and neutrons is called the mass number A Thus, A = N + Z.
The radius of a nucleus depends only on its mass number A according to the relation r = r0A1/3, where r0 is a constant having a value of 1.2 fm.
Isotopes, Isobars and Isotones
Isotopes of an element are nuclides having same atomic number Z but different mass number A (or different neutron number N). Isotopes of an element have identical electronic configuration and hence, identical chemical properties. and , etc, are isotopes.
Nuclides having same mass number A but different atomic number Z are called isobars. Isobars represent different chemical properties. In isobars number of protons Z as well as number of neutrons N, differ but total nucleon (or mass) number A = N + Z is the same. are
Nuclides with different atomic number Z and different mass number A but same neutron number are called isotones. Thus, for isotones, N = (A – Z) is constant.
are examples of iSotones.
Properties of Nucleus
The nuclear properties are described below
- Nuclear size
(a) Size of the nucleus is of the order of fermi (1 fermi = 1015m )
(b) The radius of the nucleus is given by r = r0A1/3,, where r0 = 1.3 fermi and A is the mass number.
(c) The size of the atom is of the order of 10-19 m.
The volume of nucleus is
- where, Mp = 1.6 x10-27 kg = mass of proton and R0 =1.3 fermi.
- Density of nuclear matter is of the order of 1017kg/m3
- Density of nuclear matter is independent of the mass number.
- Nuclear Binding EnergyThe minimum energy required to Seperate the nucleons upto an infinite distance from the nucleus, is called nuclear binding energy.
Nuclear binding energy per nucleon