A nucleus is made up of elementary particles called `text(neutrons)` and `text(protons)`. A neutron is neutral particle carrying no charge. A proton carries a unit positive charge. Neutron is slightly heavier than proton.

Mass of proton (`m_p`) `= 1.6276xx10^(-27) kg`

`=1.007825` `text(a.m.u.)`

Mass of neutron (`m_n`) `= 1.6749xx10^(-27) kg`

`=1.008665` `text(a.m.u.)`

(1 `text(a.m.u.)` when completely converted into energy is equivalent to 931.5 MeV)

Protons and neutrons are together referred as `text(nucleon)`.

`text(Atomic Number :)` It is represented by `Z`.
`text(Mass Number :)` It is represented by `A`.

Number of protons `= Z =` Number of electrons
Number of nucleons `=A`
Number of neutrons `= A - Z`

Nuclide is a specific nucleus of an atom, which is characterized by its atomic number `Z`, and mass number `A`. It is represented as `text( )_ZX^A` or `text( )_Z^AX` where `X` is the chemical symbol of the species. For example, a gold nucleus is represented by `text( )_(79)Au^(197)`. It contains `79` protons and `197-79 = 118` neutrons.