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Video transcript

in other videos we introduced ourselves to the idea of orbitals and these are various orbitals in their various sub shells that you could find in various shells of an atom and in this video we're going a little bit more practice with electron configuration in particular we're going to expose ourselves to the idea of the Alpha bow principle now alfe bow comes from german means the building principle it's a very useful way of thinking about electron configurations past calcium let's just get a little bit warmed up what is the electron configuration of neon pause this video and think about it and as a hint I will give you the periodic table here all right well neon has an atomic number of 10 and if we're talking about a neutral neon atom it's going to have 10 electrons and so the first two will fill that first shell so we have 1 s 2 and then the next two are going to fill the 2 s subshell in your second shell so then you're going to have 2 s 2 and then we have 6 more electrons to get to 10 and that's now going to fill your 2p sub-shell so 2 P 6 and so what's the order of the sub shells that we just filled well first we filled 1s then we filled 2s then we filled 2p and you can also see that in the periodic table of elements in this first row you're filling that first shell in this second row or this second period you are filling that second shell now what's going to happen if we were to go to say argon so if you're going to go to argon what will that electron configuration look like pause the video and think about that well we can use the noble gas configuration or the noble gas notation we could say alright we're going to be building off of neon so we're going to have the electron configuration of neon but then we're going to add electrons into our third shell so from neon we would then add two electrons into the three s subshell three s2 and then to get to 18 electrons we're at 12 right now we're going to have six more that are going to be in the three p subshell so three p6 so on this diagram over here we went from 2s2 2p2 fill-up neon and then as we went to argon we go to 3 s 2 3 P now what would be the electron configuration of calcium pause the video and think about it all right well calcium has 20 protons so a neutral calcium would have 20 electrons so 2 more electrons in argon so we can build off of argon and where are those electrons going to go and this is where the Alpha principle is interesting there is indeed a 3d subshell but in the case of calcium instead of those two electrons being in the 3d subshell they end up in the 4 s subshell so calcium's electron configuration is the same as argon and instead of it being 3d - here on top of that it goes straight to 4s - and so that's why I was drawing this diagram like this and you'll often see that in an introductory chemistry class you fail one ask first no surprises you're filling in that first shell then you fill 2s then you fill 2 P and you filled your second shell then you go to 3s once again no surprises then you go to 3p now this is the surprise and why this alpha diagram is useful for electron configuration purposes if you're thinking about potassium or calcium the extra electrons are now going to go in the 4 s subshell so now let's think about what the electron configuration of scandium would be pause this video and think about that well scandium has one more proton than calcium has 21 protons and if it is neutral it's also going to have one more electron relative to a neutral calcium atom and so it could have a similar electron configuration so we could base it off of argon we have two electrons in the four s subshell so I'll write for s2 and the Aufbau principle would describe that and the Aufbau principle this little diagram would say alright that other electron is going to be in the 3d subshell so you do 3d one and this is indeed an accurate electron configuration for scandium now the Aufbau principle makes you think that you're filling 4s first and then you are starting to fill 3d if you are actually building up a scandium atom and that's actually taught in most chemistry books and in most classes but actually if people start with the scandium nucleus that has 18 electrons so that would have a positive charge when they add that first electron it actually does not go to 4s it goes to 3d so this electron actually gets added first if you're actually thinking about building but I don't want to confuse you too much in this video we're just thinking about the electron configuration and for that alfe bow can be very useful now for electron configuration purposes 3d you then go to 4p and then use then go to 5s and that's why you might see this type of a diagram once again in your traditional first-year chemistry books so the big takeaway here is the Alpha principle that you'll learn this type of diagram it's useful for electron configuration and it might be useful to think about it as you're building these atoms electron by electron but if you really want the precise accurate truth once you get beyond calcium it gets a little bit more complicated now one other thing that I want you to appreciate based on what we just learned is patterns in the periodic table of elements so for which elements are we building out our s subshell well you could see that for all of these elements right over here these first two columns were building out our s subshell now it looks like something is missing there is there something else that builds out our s subshell well from at point of view we could actually think of helium as being right over here because helium were building out that one s subshell and because of that all of these elements right over here we say that they are in the s-block now which elements are building out their p sub shells well all of these elements right over here are building out their p sub shells or have it fully built out and because of that all of these elements we call these the p block and these elements in the middle right over here scandium is one of them they are called the D block now one reason why folks might have called the D block is if you really imagine the Aufbau principle as building up atoms it might be tempting to say oh well we're building in the fourth row here we're building the three d subshell or in the fifth row here we're building the four d subshell now we now know that that actually isn't true but from electron configuration point of view it can appear that way and so that's why it is called the D block and I will leave you there
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