[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the limitations of using different diagrams to represent a molecule or a giant ionic structure now during this topic we've seen several different ways that we can represent molecules and one of these ways is a DOT and cross diagram I'm showing you here the dot and cross diagram for the molecule water Dot and cross diagram like this have one advantage because we use dots to represent the electrons from one atom and we use crosses to represent the electrons from another atom it's very clear where the electrons are coming from however Dot and cross diagram such as this one don't tell us about the shape of the molecule now another way to show a molecule is a two-dimensional stick diagram such as this one the problem with these diagrams is that because a Cove valent bond is shown as a stick we cannot tell which electron in the calent bond came from which atom stick diagrams also give us no idea of outer electrons that are not in bonds for example in the water molecule four outer electrons on the oxygen atom are not in Cove valent bonds and finally two dimensional stick diagrams do not give us accurate information on the shape of the molecule another way of representing molecules is to use a three-dimensional stick diagram like this one this is for the molecule ammonia the benefit of a three-dimensional stick diagram is that it shows us the shape of the molecule okay now we've also seen diagrams to show giant ionic luses I'm showing you here the giant ionic Lattis for sodium chloride this is called a ball and stick diagram now the benefit of a ball and stick diagram is that it allows us to clearly see the ions in three dimensions however in ball and stick diagrams the ion are shown as widely spaced when in reality the ions are packed together instead of using a ball and stick diagram we can use a space filling diagram like this one space filling diagrams give us a better idea of how closely packed the ions are however it can be difficult to see the three-dimensional packing with a space filling diagram now there is one problem with both ball and stick diagrams and space filling diagrams and that is that they only show a tiny part of the giant crystal lce in reality a crystal lce is a giant structure so both of these diagrams give the impression that these structures are much smaller than they actually are you'll find plenty of questions on this topic in my vision workbook which you can get by clicking on the link above [Music]