 As you're already aware, the group of atoms that is the distinctive feature of aldehydes and ketones is the carbonyl group, a carbon atom with an oxygen double bonded to it. In an aldehyde, this group is on an end carbon, while in a ketone it's in the middle of the molecules surrounded by other carbons. This means that the smallest possible aldehyde has just one carbon. This is called methanol, and it's more commonly known as formaldehyde. Although it's a gas at room temperature, it dissolves easily in water and has long been used in medicine as a preserving fluid for bodies. The smallest possible ketone has three carbons. This is propanone, or it's more commonly known as acetone. Because oxygen is more electronegative than carbon, the carbonyl bond is polar. The oxygen has a partial negative charge, and the carbon has a partial positive charge. Now this bond is not so polar that hydrogen bonding is possible, but it is polar enough for there to be dipole-dipole attractions between neighboring molecules. You should recall that dipole-dipole attractions are intermediate in strength between hydrogen bonds, which are stronger, and van der Waals forces, which are weaker. For instance, here are three methanol molecules. Remember each carbon has a slight positive charge, and each oxygen has a slight negative charge. This means that neighboring molecules are attracted to each other, the oxygen of one to the carbon of another. So dipole-dipole attraction there, and there, and of course with the many other molecules that I haven't drawn. A similar situation exists for propanone. The oxygen atom on one molecule is attracted to the carbon atom of another. Note that it's only the carbonyl carbon that takes part in this bonding, because it has the partial positive charge because of the polar bond. The CH3 groups, including these guys, are relatively non-polar and don't participate in this bonding. But they do, however, participate in the van der Waals bonding that happens for all molecules. Although aldehydes and ketone molecules cannot form hydrogen bonds with themselves, they are able to accept hydrogen bonds from other molecules. For instance, we know that water molecules are able to form hydrogen bonds among themselves, since the OH bond is sufficiently polar for this to happen. In water, for instance, we're getting hydrogen bonds forming between the oxygen of one. In fact, specifically between the lone pairs of the oxygens and the hydrogen of another molecule. Here's the lone pairs on this oxygen. They form hydrogen bonds. Again, just to make it explicit, partial negative charge on the oxygen and partial positive charges on the hydrogen. Now, if methanol is added to water, methanol also has lone pairs on its oxygen atom. And that oxygen is therefore able to form hydrogen bonds with neighbouring water molecules. So, for instance, we could have a hydrogen bond between that oxygen atom and the hydrogen of the neighbouring water molecule. However, it doesn't work the other way. So, if I've got a water molecule down here, it is not able to form a hydrogen bond with this carbon here. It cannot form a hydrogen bond there. The reason is that the carbonyl bond is not sufficiently polar for there to be a strong attraction between the oxygen of the water and the carbonyl carbon. However, there is a slight attraction and what you get instead of a full H bond is, in fact, a dipole-dipole attraction. There could also be van der Waals forces, of course, since all molecules participate in these. Similarly for ketones, the oxygen of the carbonyl is able to accept a hydrogen bond from a neighbouring water molecule. So, there are the lone pairs on the oxygen. It can form a hydrogen bond with the hydrogen of a neighbouring water molecule. But the carbonyl carbon can only have dipole-dipole attractions with neighbouring water molecules.