When you are given a chemical equation in words,how would you know which element gets 2 eg:
Na + Cl -NaCl2,why is there a 2 at the bottom of the Cl
C + O2- CO2-why is the 2 @ the bottom of the O

Whever a number appears after the element it indicates how many atoms of that element are in the molecule. If there is no number then it is assumed that there is just one of that particular atom. Perhaps the most common example is the chemical makeup of water: H2O, meaning that each water molecule has two hydrogen atoms and one oxygen atom. Another example: H2SO4 is sulfuric acid - each molecule contains 2 hydrogen atoms, 1 sulfur atom, and four oxygen atoms. In your example if you combine a carbon atom with two oxygen atoms you get carbon dioxide: C + O2 = CO2. Your other example I don't understand - combining one atom each of sodium and chlorine yields common salt: Na + Cl = NaCl. I don't know why you added the '2' after the Cl. Whenever you show a reaction of elements or compounds combining to yield something else the number of atoms of each element must balance on both sides of the equation - so here the total number of chlorine atoms on the left side must equal the total number on the right side.

Thank You 4 your help