 Let's try the next one. Aluminium plus oxygen goes to aluminium oxide. So aluminium is an element, symbol is AL, oxygen is also an element and it's one of the diatomic seven, so it's going to be O2 and they react to give aluminium oxide. Again this is an ionic solid, a metal and a nonmetal. We'll work out the ions, the first one is going to be aluminium, which is in group three, so it's going to be three plus. Oxide is oxygen and it's in group six, so it needs to gain two electrons to get a full octet, which means that it has a charge of two minus. When we do the crossover for that, that gives us AL2O3. Okay, let's balance it. On the left we have one aluminium and on the right we have two aluminium, so let's make that a two there. Okay, now for oxygen we've got two on the left and we've got three on the right. Okay, this could be a little bit tricky, but the way that we approach it is to look for the lowest common multiple of these two numbers. And the lowest common multiple of two and three is six, so we're going to swing things so that there are six oxygens on either side. So oxygen molecule has two atoms in it. To make six atoms we need three molecules. And on the right we've got three oxygen atoms, so that means we need two molecules. So now on the left we've got three times two, which is six oxygen atoms. And on the right we've got two times three, which is six oxygen atoms. Now let's just go back and double check the aluminium. We've got two on the left, but now because we put this two in front of the aluminium oxide, we've got two times two, which is four aluminium's on the right. So that means we need to go back and make a modification and make that four. So I'll just clean that up. There we go. Four aluminium's on the left, four on the right, six oxygens on the left, six on the right. Now notice that that balancing required a little iteration. We had to first go and then we had to make some modifications and go back and change things. That frequently happens when you're balancing equations. Don't be afraid of that. It's just a normal part of the process.