 Here we have a toddler's building block and as you can see it has four studs on it Which means we could take these single studs and add one two three or four on top or If we use one of these two bricks we have two left over so far so obvious But there is an interesting analogy here to how atoms bond together to make molecules. So let's Lay down some rules for this. Our aim is to build a wall It needs to be two blocks high with no gaps around the edges and the sides need to be straight no Uncovered studs on one side or overhanging bricks on the top So if I stick to these blue bricks together there are six remaining Slots and these can be filled With six white studs and this is a representation of a very simple molecule Ethane each blue block has four slots just like the carbon atom which can bond to four other atoms and The single studs are like hydrogen which only bond to one other atom and easily fill up the spare slots that are left over If we overlap the blue blocks by two studs Then we only have four slots left over for the white studs just like with Ethene there's a double bond between the two carbon atoms that take up two of carbon's other slots Think a little further with this because we can build methane acetone ethanol Acetic acid or methyl ethanoate this pink three block could represent nitrogen so we could build ammonia acetone nitrile glycine and even more if you put the blocks together somewhat randomly providing You obey all those rules. You have two layers of bricks and square sands You'll have a molecular structure if you have any gaps You know you need to put something there like a hydrogen atom or something else that's appropriate That's fair enough, but not really anything particularly special Any system where you limit the number of connections between building blocks and then lay down a couple of rules Will reproduce molecular structures? We have molecular modeling kits designed to do the exact same thing and even simple card games have been built to do just this The posh term for this will be that there is an isomorphism between these bricks and molecular modeling kits and Roughly with sub microscopic molecules the limits on connectivity remain to a rough approximation Unchanged the interesting connection to chemistry with these bricks However, is that it resembles a wave representing molecular structures Invented and published by August Kekelay back in the 1860s Kekelay is mostly famous for working out the structure of benzene in 1865 his story if you believe what he said about it years later goes that he was daydreaming and Imagined a snake eating its own tail or the Oroboros symbol and then he decided that benzene must have a ring-like structure Because that's the only structure that satisfies everything we know about it and but it does it's quite an insight Made all the more amazing by what we had to go on at the time So a brief bit of history. What was chemistry like back in 1865? Well, no periodic table. Mendelio's modern version was another four years away We were fairly sure there were atoms at least philosophically, but the structure we know about today would be at least 50 years away and Certainly no fancy instruments like magnetic resonance or mass spectrometry or even infrared spectroscopy And definitely no x-ray crystallography most chemistry was very wet and With minimal health and safety procedures backing it up to prove anything chemists of the day would have to react materials together and Carefully check the amounts used or any changes in weight or volume to try and figure out How much of each element had reacted in which proportion? Once you had that information and all those proportions you just had to well Guess it what the molecular structure must have been that's the world that Kekele was in and before we came up with the structure of Benzene that made him famous He came up with a way of representing the proportions or equivalences between elements when they reacted He would represent hydrogen as a single circle and then molecules that were equivalent to two Hydrogens as two overlapping circles Then three and then the carbon atom which would be represented by four overlapping circles These would then be drawn as long elongated Sausages actually according to Oswald Walker writing and J. K. Med back in the 1970s They were originally called roll formulas as in bread rolls and the German for bead or bulge was accidentally misread as versed and Translated into English as sausage these formulas are a pretty elegant way of representing the proportions The elements must exist in in order to form stable organic molecules Exactly like this brick method here You can't really go wrong. You have to fill up every slot or it won't be right It's really easy to see If you've gone wrong But Kekele's method didn't take off. It was never popular He kept at it though only using the structural representations We recognize today a handful of times in his whole career But what did stick around was his terminology elements react in certain equivalents and Kekele was key popularizer of the term We know today as valence or valency Carbon has an equivalence to four hydrogen atoms. It has a valence of four So why was Kekele's sausage formula unpopular? Well in addition to being a complete painter right out and giving us no real indication of the actual geometric structure of a molecule and only the idea of a rough connectivity between the atoms These have a few structural limitations We can do ammonia just fine We can then substitute one hydrogen for carbon to make a primary amine or substitute another for a secondary amine and Then we're a little stuck if we want to tertiary amine Do we stick it sideways? Can we cram an extra carbon in there in some other way? Either way, it's ugly and a little ambiguous when it comes to whether the valence has been satisfied But we can with building blocks because we have another dimension to branch off into In fact as far as I can tell this handles branching structures really well this extra dimension really helps here Whereas on a page gets very crowded very quickly So ultimately the sausage formula just is not that good at what it really needs to do If you want to teach valency great the usual way of representing chemical structures doesn't build that in You need to learn valency first and then read that into the structures if you get it wrong It's not immediately obvious unless you've gone through first-year undergraduate chemistry already But this method whether with sausages or building blocks or bulging rolls Forces you to engage with valency without knowing about it in advance There's a strong connection or isomorphism when it comes to connectivity but not to arrangement in space Whereas the conventional method of drawing lines and connecting letters and dots maps on to actual structure in most cases at least and using wedges and dashes we can expand on it to show a bit of 3d information or spatial awareness But this is not really remotely possible in Kekele's sausage formula Both methods of drawing molecules struggle to properly convey benzene however Even Kekele had no choice but to abandon his own method in favor of the molecule drawing conventions that were developing at the time But benzene isn't this hexagon of alternating single and double bonds each bond is supposed to be Identical so Kekele later in the 1870s proposed that they alternate rapidly because there's nothing particularly special about this Configuration you can swap them around no problem Modern organic chemistry keeps that with a concept called resonance Which became more formalized a few decades after Kekele's benzene structure was proposed Ultimately resonance throws its hands up and just straight up admits that our representations are flawed There are an approximation and the rigidity proposed by Kekele sausage structures and even by our modern chemical shorthand Doesn't really exist. So resonance allows us to move bonds around Be a little more flexible with what valence means and making multiple representations of the same molecule That remain completely valid within the rules even though we know that this isn't literally true when it comes to the actual Molecules themselves, but it's still a model that can make useful predictions about chemical reactivity So that's really the point here What method is best for the job? What makes the best predictions? If we just want to know about valences and how many equivalences of atoms will react Then the sausage formula can help us But if you want to know about structures and get a feel for what the molecules actually look like Well, I think Kekele's awkward attempt at drawing benzene with it Quite clearly says we should stick with the usual method