Some questions about chemistry

What do the numbers represent on the Periodic Table of Elements?
   Well there are numerous numbers present within each element box in the periodic table.

         Upper left number: the atomic number and the number of   protons (+)
         Upper right number: the atomic weight/mass (+,-,=)

Mg, what is this?
         Mg is the element symbol for Magnesium

What is the difference between Na and K?

Sodium atom

         Na (sodium) and K (potassium) are different in numerous ways but one of the most obvious, besides placement within the periodic table, is their reactivity to other elements. In this case potassium is more reactive because the potassium atom has one more shell of electrons with fewer electrons that are more easily lost than those of the sodium atom.