Free proton is stable, while free neutron is unstable.

Following nuclear equation for decay of neutron

${ }_{0} n^{1} \rightarrow{ }_{1} p^{1}+{ }_{-1} e^{0}+{ }_{0} v^{0}$ where ${ }_{0} v^{0}$ is antineutrino

Hence, one neutron decays into a proton, an electron and a antineutrino and has a mean life of about $1000 \mathrm{~s}$.

Neutron is stable inside the nucleus.

The composition of nucleus :

$\Rightarrow$ Nucleon $(A)$ : The nucleon is the sum of the number of protons and neutrons presents in

the nucleus of the atom.

$\Rightarrow$ Atomic number $(Z)$ : The number of electrons in the atom and hence the number of proton

in the neutral atom is called the atomic number.

$\Rightarrow$ Mass number $(A)$ : The total mass of proton and neutrons in the nucleus of the $\mathrm{Z}+\mathrm{N}$

atom is called the atomic number $(A)$.

$\Rightarrow$ Neutron number $\mathrm{N}:$ The number of neutron present in the nucleus of a atom is called

the neutron number $\mathrm{N}$.

$\therefore \mathrm{N}=\mathrm{A}-\mathrm{Z}$

$\Rightarrow$ Nuclide : The type of nucleus is called nuclide and is represented by symbol ${ }_{\mathrm{Z}} \mathrm{X}$ or $\mathrm{Z}^{\mathrm{A}}$

where $\mathrm{X}$ is the chemical symbol of element.

For example: Nuclide of gold ${ }_{79}^{197}$ Au in which $197$ nucleon, $79$ proton and $197-79=118$ neutron.