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Current time:0:00Total duration:6:33

Video transcript

we've already spent a lot of time thinking about how awesome carbon is for life that really life as we know it is carbon-based and this all comes out of carbons ability to bond with other carbons and form all sorts of neat structures bonds with oxygen hydrogens and other things in fact we have a whole class of molecules called organic molecules that just are molecules that involve carbon and carbons bonding capabilities really come out of its electron structure we talked about this in the previous video but if we think about carbon it has six protons in a neutral carbon will have six electrons two of them are sitting in its innermost shell and then the other four on its outermost shell and those four the ones that tend to react and we call them the valence electrons and so these valence electrons tend to form four covalent bonds for example if you wanted a bond with four hydrogen's well each hydrogen would contain would contribute an electron to one of these pairs which are essentially going to be a covalent bond and then the hydrogen's the hydrogen's can feel that their outer shell is complete because hydrogen's just trying to get to two to fill its outer shell to try to be a little bit more like helium and carbon is trying to get to eight in its outer shell to be a little bit more like neon neon has to in its inner shell eight in its outer shell you might remember the octet rule that that that that with that atoms try to get to eight or pretend like they have eight electrons in their outer shell to feel stable and here carbon is sharing eight electrons it contributed four and then the four hydrogen's contributed four and it forms it forms methane now those of you who are quite astute might have said okay we you know I know of other elements that tend to form four covalent bonds that have four valence electrons and in particular there's one that's awfully close to carbon on the periodic table and that's right over here this is this is silicon silicon has 14 protons in a neutral silicon would have 14 electrons and so let me draw that so if we have silicon right over here I'm just going to focus on the electrons you would have two electrons sitting in its innermost in its innermost shell so I'll and they're jumping around here not there not these neat well-defined orbits they're just there jumping around in that in that in that lowest energy in that in that in this innermost shell and then you have another eight in its second shell so one I'll just draw three four five six seven eight all jumping around all jumping around in the second shell and as a total of 14 so we already used up 10 so it's going to have four in its outermost shell so one two three four and these four and its outermost shell these are the ones that tend to do the reaction the reacting we call the valence electrons so if we wanted to just draw the valence electrons we could do silicon has four valence electrons four has four one two three four valence electrons just like carbon has a different number of electrons these four valence electrons are actually one shell further out but it has four that tend to react and it does tend to form four covalent bonds so you might be saying well well can't we have life that is dependent that is silicon based as opposed to carbon-based and if you thought that you would not be the first person to think about that science fiction or authors have have theorized that and this right actually this is a this is a a screenshot from the devil in the dark episode 1967 from the original Star Trek where they said hey encounter Kirk encounters the enterprise encounters these creatures that are silicon-based they're called hortas and they have all sorts of interesting properties so there there's you know there's some folks who like to think maybe maybe silicon but when you actually look at the chemistry things start to break down a little bit for example one of the neat things about carbon is it readily forms bonds with other carbons and then these bonds can actually be quite long you can form all sorts of these hydrocarbons and all sorts of other carbon-carbon structures with long chains of carbons so you can do that with carbon but it turns out that silicon silicon bonds are not that strong and silicon is not going to form is not going to form really long chains so silicon is not going to do that even if you look at a simple a simple molecule like methane there is there is a molecule called saline but this does not readily form it does not have the same characteristics so so this is also not going to be as good as methane and even if you think about something like carbon dioxide which is carbon which is carbon bonded having two double bonds so this is carbon having two double bonds two oxygens so two so one double bond to one oxygen another double bond to another oxygen and we're used to thinking about carbon dioxide in our everyday life as a gas plants are using these two to to fix into their structure to actually grow they're taking the carbon out of it their carbon fixation we are we exhale this carbon dioxide it's essential to life and you might think oh what about what about silicon oxide and silicon oxide is actually a fairly common molecule fairly common compound but it does not exist in a gaseous state at the temperatures that we're normally that we normally associate with life most of silicon oxide is in the form of quartz in the ground so that is quartz over there so when you look at the actual ways that that that that silicon forms bonds it actually does not seem actually close to as good as carbon and you know it's something like silicon when silicon bonds with oxygen it's a much not only is it in the solid form at at normal temperatures where we normally associate life but these are very very very strong bonds much stronger than the bonds that carbon is forming with oxygen they're so strong that it make it hard for them to be manipulated with the types of chemical reactions that we're used to seeing inside of organisms so you know it's it's fun to theorize about this and I'd be the last person to rule anything out I think the universe will continue to surprise us with with things that right now we cannot even imagine but based on the chemistry we know and based on life as we know it even though silicon can form these four covalent bonds and does have four valence electrons it still nowhere near as good at carbon as in doing the types of processes that we think are necessary for life
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