look at their oxidation numbers, or their states, here carbon is bonded to four hydrogens, who's giving, who's taking? the carbon, lets go to our periodic table, carbon is here, hydrogen is here, carbon is more electronecative, its kind of, its adjacent to the three musketeers of electronegativity, these guys are the most electronegative, we always ignore the halogens, not the halogens sorry, the noble gasses because they pretty much dont react at all, they are pretty much happy with their 8 valence electrons, these guys love to gain electrons, they're small molecules, their outermost shell is close to the nucleus and they're so close to becoming noble they just love hogging electrons. carbon's almost there, carbon is much further right on the periodic table than hydrogen, so if you have carbon bonding with hydrogen, carbon is going to be the electron hog of this situation, lets go down here, so if carbon is the electron hog, and hydrogen is having its electrond taken away from it, remember this is all kind of hypothetical, its more partial is the reality, but if you had to pick or choose, hydrogen is going to lose an electron each, so its going to have an oxidation state of plus one for hydrogen, you have four hydrogens each giving up an electron, so the carbon must be taking four electrons, so it's oxidation number is minus 4, its taken 4 electrons so its charge will go down by 4, so that's why its negative, fair enough, now whats the oxidation state of this oxygen right there, well its just bonded to itself, there's no reason to believe that one oxygen should be able to take any electrons from another oxygen, so it has a zero oxidation state, its not hogging more than its fair share of electrons that it was initially kind of born with, now after the combustion occurs, what are the oxidation numbers, well i have oxygen bonded with carbon, two oxygens bonded with carbon, now we know oxygen, i mean oxidation was something named after oxygen, it is one of the most electronegative, almost anything bonded with oxygen is going to be giving up its electrons, we also know that oxygen likes to take two electrons because if it takes one, two electrons, then it starts having a, it takes one it gets here, it takes two it gets here, it has 8 valence electrons, so it's typical oxidation number is negative two, so, carbon, in this situation, each oxygen is going to have a minus two oxidation number, each carbon dioxide molecule is neutral, so everything has to add up to zero, so the oxygens, you have two of them, each one of them with a minus two for a total of minus four, so carbon must be plus four, plus four, which means that it has given up four electrons, right, and really it only has four electrons to give up, it has one, two, three, four valence electrons, in its second shell, which is its reactive shell, so its oxidation number is plus four.