If there are 8 electrons in the last shell if not the first shell, the atom of particular element attain octet state . atoms which have attained the octet state are stable in nature and not react to attain octet state.
The number of electrons in a shell is determined by the formula 2N^2 where N represents the number of orbit. This states 
that for 1st orbit only 2 electrons, for 2nd orbit 8 electrons and so on for 3rd 18, for 4th 32. But how come the 
outermost orbit or the valence shell of an atom can hold only 8 electrons and not more than that?