Most lightweight atoms are stable if their neutron/proton ratio is a 1:1 ratio, out of the following "light" elements, which one does not have a stable neutron/proton ratio?
F, N, O, Ne, Mg
The periodic table is key to this type of question. You should know that atomic number corresponds to the number of protons (and in a neutral, "uncharged" atom the number of electrons too). But the atomic mass, corresponds top the ...