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 sum of the number of neutrons and protons.

The atomic number for F is 9, which means it has 9 protons, and its mass is 19, which means it has 10 neutrons (19 - 9 = 10).

The atomic number for N is 7, which means it has 7 protons, and its mass is 14, which means it has 7 neutrons (14 - 7 = 7).

The atomic number for O is 8, which means it has 8 protons, and its mass is 16, which means it has 8 neutrons (16 - 8 = 8).

The atomic number for Ne is 10, which means it has 10 protons, and its mass is 20, which means it has 10 neutrons (20 - 10 = 10).

The atomic number for Mg is 12, which means it has 12 protons, and its mass is 24, which means it has 12 neutrons (24 - 12 = 12).

Answer F