 Covalent bonds and dionic bonds are much stronger than intermolecular forces. But while intermolecular forces are weaker, they are still really important. The strongest intermolecular force is called hydrogen bonding. Hydrogen bonds are all around us and are essential for life. They're at play in water and in our DNA. The structure of our DNA is a double helix. This shape is important for replicating DNA. It is built by having two chains of nucleotides which are held together by hydrogen bonds. The reason water sticks together, for example, to create strong surface tension, is because of hydrogen bonds. Hydrogen bonding requires an intramolecular dipole made up of a hydrogen on one side and an electronegative atom with a lone pair of electrons on the other side. This occurs most often where there is the hydrogen atom covalently bonded with nitrogen, oxygen and fluorine, and sometimes with sulfur. Hydrogen-oxygen, hydrogen-fluorine and hydrogen-nitrogen bonds share electron density unequally. So much so that the hydrogen attracts another oxygen, fluorine or nitrogen that's attached to another hydrogen on a different molecule. For example, in water, hydrogen bonding occurs between the hydrogen of one molecule and the oxygen of another. In ammonia, hydrogen bonding occurs between the hydrogen of one molecule and the nitrogen of another. Note that it's important for the oxygen, nitrogen or fluorine to have a lone pair of electrons. Hydrogen bonding can occur between two molecules that are the same, like water, but also between unlike molecules such as this hydrogen fluoride and water. In larger molecules, it can also occur within the molecule where different areas of the molecule hydrogen bond to each other. If there isn't a lone pair available, for example in ammonium NH4+, it won't be able to hydrogen bond. Here's another situation where hydrogen bonding is important. Drugs. This is aspirin. If you haven't come across these organic molecule structures before, don't worry, you will. Where there's lines meeting, it means there's a satisfied carbon. So a carbon with the right amount of hydrogens to be balanced and not have an overall charge. We can ignore them for now. Looking at aspirin, where could hydrogen bonding occur? Are there any oxygens, fluorines or nitrogens? Yeah, we've got four oxygens. Are any of those oxygens bonded to a hydrogen? Yep, this one. So all of these locations are potentials for hydrogen bonding. The oxygens could bond to a hydrogen of another aspirin molecule and the hydrogen could bond to an oxygen on another aspirin molecule. Or they could bond to water molecules. This is important in the synthesis of medicines. They need to be absorbed by the body and enter the blood. If they have too many or too few potential areas for hydrogen bonding, they may not be absorbed at all. This is ibuprofen. Where on this molecule could hydrogen bonding occur? Hydrogen bonding could occur on these two oxygens to hydrogens of another molecule or on this hydrogen to the oxygen of another ibuprofen. Or in fact to any oxygen, nitrogen or fluorine that's bonded to a hydrogen on another molecule. This one is dichlofenac. Where on this molecule could hydrogen bonding occur? Hydrogen bonding could occur on this nitrogen and these two oxygens to hydrogens of another molecule or on this hydrogen to any oxygen, nitrogen or fluorine that's bonded to a hydrogen on another molecule.