An isotope of oxygen has a mass number of $18$. Other isotopes of oxygen will have the same:

Six protons are found in the nucleus of

The atomic number of an element is derived from

The electronic configuration of a dipositive metal $M^{2+}$ is $2, 8, 14$ and its atomic weight is $56 \ a.m.u.$ The number of neutrons in its nuclei would be

The number of electrons and neutrons of an element is $18$ and $20$ respectively. Its mass number is

The metal ion with total number of electrons same as $S^{2-}$ is