The answer is that “chlorine has a greater electron affinity
than sodium does”. Sodium has a lesser electron affinity than Chlorine, which
is why it is deliberated that Chlorine is much more suitable to be existent as
an anion than sodium, due to the lack of its electron affinity.
Well, some things I would do is to know the Periodic Table font and back. Know they elements and their class and atomic number and weight. Know how many atoms make up each element and how many protons, neutrons, and elements are in each atom.