Na, Sodium, is a group one element. It is a positive ion because it loses one electron in a chemical reaction. Its charge is +1.
Cl, Chlorine, can have many oxidation numbers, including +7, +1, +5, -1, and +3. Because it is the second element in a binary ionic compound, you know that its charge has to be negative.
Its oxidation number is -1, bringing the overall charge of NaCl to be 1+(-1), which equals 0!
