12th physics. NCERT notes

Basic Properties of Neutron

  • mn = 1.00866 u = 1.6749 × 10-27 kg
  • A free neutron, unlike a free proton, is unstable
  • It decays into a proton, an electron and an antineutrino (another elementary particle), and has a mean life of about 1000s. It is, however, stable inside the nucleus.

Composition of Nucleus

The composition of a nucleus can now be described using the following terms and symbols:

Z – Atomic number = number of protons

N – Neutron number = number of neutrons

A – Mass number = Z + N = total number of protons and neutrons

One also uses the term nucleon for a proton or a neutron. Thus the number of nucleons in an atom is its mass number A. Nuclear species or nuclides are shown by the notation where X is the chemical symbol of the species. For example, the nucleus of gold is denoted by. It contains 197 nucleons, of which 79 are protons and the rest118 are neutrons.

Size of the Nucleus

It has been found that a nucleus of mass number A has a radius

where R0 = 1.2 × 10-15 m.

This means the volume of the nucleus, which is proportional to R3 is proportional to A. Thus the density of nucleus is a constant, independent of A, for all nuclei. Different nuclei are likes drop of liquid of constant density.

The density of nuclear matter is approximately 2.3 × 1017 kg m-3. This density is very large compared to ordinary matter, say water, which is 103 kg m-3.


The nuclei of isotopes of a given element contain the same number of protons, but differ from each other in their number of neutrons.


All nuclides with same mass number are called isobars.


Nuclei of different atom containing same number of neutrons are called isotones.


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