 So, let's determine the oxidation number of chromium in each one of these different chromium species. So, recall for your class, you want to remember that oxygen is always minus 2. So, again, it can be minus 1 in certain situations, but as far as you guys know right now, it's always minus 2. So, in this case, you're going to set the whole thing equal to minus 2 because it's got a minus 2 charge. Oxygen is minus 2 and there's 7 of them, plus 2 chromium. So, 2CR plus minus 14 equals minus 2. 2CR equals, so if we add 14 to both sides, it's going to be plus 12 by 10. And then now we have to divide both sides by 2. So, chromium in the first one is plus 6 oxidation. So, we'll say oxidation number of chromium. So, can I erase that and try the next one? So, if you're able to do that one, then this one should be a breeze, right? This one has a zero overall charge. Again, I like to do these writing them backwards. So, oxygen is minus 2 times 3 of them, and we're looking for chromium. So, chromium plus a minus 6 equals 0 and then add 6 to both sides. So, chromium equals plus 6, again. So, the oxidation state of chromium is plus 6 in this molecule, too. Are we cool with this? Can I erase this one? And then the last one, you've got to remember that chlorine is going to be minus 1. Okay, the halogens are minus 1. So, let's do this one. Minus 1 plus minus 2 times 3 plus chromium. So, chromium plus minus 6 plus minus 1 equals 0. Chromium plus minus 7 equals 0. Add 7 to both sides. Chromium equals plus 7. Can I erase that one? No. Okay, so in that one, an oxidation number of chromium is plus 7. Okay, so again, I think the key that a lot of people have a problem with is recalling that they need to set it equal to one of these charges, okay? So, in these cases, the charge or overall oxidation of the whole complex is 0. Okay, when do I know when to set it to negative 1? When the overall charge is negative 1. Like that one. The total charge is that number? Yeah. The total charge, pure, at the very end, is a negative charge. But then that's the mean. So, let's do another one. So, like that one, I of 3 minus, right? So, like this is the one on the exam. Yeah, we're going to have to set this one equal to negative 1. When we do that, we'll add 6 to both sides. 6 plus minus 1 is plus 5. So, in this case, the oxidation number, does that make sense? Okay, so, every time we're going to set it equal to that number in the top right, or the charge, or oxidation number, if you prefer. Okay?