You were asked for the shape of the chlorine dioxide molecule, ClO2, and told that the chlorine-oxygen bonds were both double.The temptation is to think of this as being like carbon dioxide - linear. In fact, if you jump to that conclusion, you would be totally wrong. Incidentally, that is why it is important to have read about the sulfur dioxide case, even though it isn't specifically mentioned by the syllabus.Let's work through it for chlorine dioxide . . .Chlorine is in Group 7, and so has 7 electrons in its outer level. It is forming a total of 4 bonds to oxygens (2 double bonds), and so that adds another 4 electrons to the outer level - making a total of 11. It is a neutral molecule, not an ion, and so that's the final total of electrons.11 electrons will be in five-and-a-half pairs. What that means, of course, is that there are 5 pairs, and an odd single electron. If you haven't come across molecules with unpaired electrons before, that might worry you - but such things are perfectly possible.4 of the 5 pairs will be used to form the 2 double bonds.So, there will be 2 double bond units, a lone pair and a single electron. These will arrange themselves in an approximately tetrahedral fashion to minimise the repulsions. When you think about the shape that the atoms take up, the molecule will be bent (or V-shaped or, as CIE tend to call it, "non-linear").This question was only worth 2 marks - 1 for the shape, and 1 for the explanation. On occasions, CIE certainly make you work for your marks! What should you do if this comes up in an exam? Up to the time of writing (November 2013), CIE have never asked a question about the relative sizes of the various intermolecular attractions. This is almost certainly something you don't need to worry about. They might, of course, expect you to know that a molecule which has dipole-dipole attractionsas well asdispersion forces will probably have a boiling point greater than a similarly sized molecule with only dispersion forces. That is perfectly reasonable. You will find a detailed explanation of theextraction of aluminiumand some notes on recycling it on this Chemguide page.Important: In an exam answer, you wouldn't get any credit for saying that "It is cheaper to recycle aluminium than to produce it from the ore." It is too vague. You would have to explain why it was cheaper, in terms of recycling not needing as much expensive energy. In one question, CIE asked you to balance the following equation:

If you just try to balance this by playing around with the numbers, you will probably still be there an hour later, and still won't have got it right. You have to split these complicated redox equations into the two half-equations, and then combine them.