btec applied science unit one chemistry electron configuration so this is basically whereabouts are the electrons in an atom an atom's got electrons whizzing around it uh the electrons are in shells uh the shells are n equals one two three etc uh and then this the shells have sub shells and the subshells are called s p d and f we won't worry too much about f but s p d and f are the subshells and then each subshell is made of things called orbitals and then each orbital can hold a maximum of two electrons as we'll see so atoms have shells the shells have subshells spd and f the subshells have orbitals and the orbitals contain electrons okay so here's the first three shells uh n equals one is the lowest energy n equals two n equals three these are the first three shells then the subshells the n equals one shell just has s n equals two has s and p and then n equals three has s and p and d and then each subshell is made of orbitals the s sub just has uh one orbital the p subshell has three orbitals and the d sub shell has five if you remember one three five uh and then guess what spdf guess how many orbitals f has got and each orbital can hold two electrons sounds a little bit tricky but you get used to it it's pretty straightforward once you've got your head around it so s is one pair of electrons the first shell can only hold two electrons you probably remember from gcse so it only has an s orbital the second shell can hold eight electrons so it has an s and three p orbitals so that's two plus six is eight uh the third shell can hold 18 electrons so it has an s uh three p and five d orbitals okay let's look at a few examples so fluorine has got nine electrons so what we're going to do is we're going to fill up all the available spaces um and its electron configuration is 1s2 2s2 2p5 and that basically fill up the bottom first so in n equals one the first shell fills up first and there's two electrons you see that orbital is full then you go to n equals two and you fill up the s orbital up down the the electrons are that imagine them spinning opposite to each other so you represent one of them with an up arrow and the other one with a down arrow that they spin opposite to each other and then how many electrons have we got left well we've got five electrons left so in the n equals two shell it's up down up down up you actually fill the up ones first um the chemistry teacher told me a nice little trick imagine you go on a bus and there's lots of pairs of seats now if there's if there's a pair of seats free people don't sit down next to somebody else they'll always go for a seat which is part of a free pair and it's the same with the electrons in this p sub shell they'll do up up up and then down down down or they could do down down down and then up up up doesn't matter but you fill up the ups before you start filling up the downs okay uh few little complicated bits calcium has got 20 electrons what would its electron configuration be now uh you just really need to know about potassium and calcium here we only do we will only do the first 20 elements and calcium is the 20th 20th element its atomic number is 20. don't worry about anything beyond calcium okay so calcium if we filled it up i'm using the mouse now it'll be up then down then up then down then up up up down down down how many is that that's two and then another two is four and then 6 8 10 we're up to 10 okay so let's go here so up down that's 12 up up up down down down that takes me to 18 and then as i said before we start filling in 3d we fill up this so up down okay a little bit tricky as i said but the the 4s fills up before the 3d so its electron configuration is 1s2 s two two p six three s two three p six and then four s two okay not not three d two four s two the next one would be 3d1 okay a magnesium atom has got 12 electrons what would the electron configuration for a a magnesium ion now magnesium is in group two so you should be able to figure out that it's mg2 plus okay so basically a magnesium ion has got 10 electrons so where would the 10 electrons be well that'd be 1 2 3 4 5 6 7 8 9 10 so it would be 1 s 2 2s2 2p6 okay it would be the same as number 10 which is harry henry likes be about couples not overflowing nelly neon it has the same electron configuration as neon okay right what you should do is practice this and to really get this into your head draw an electron configuration diagram for the first 20 elements okay and then that will really really get this into your head you'll be able to do it in your sleep which you know would come in handy in the exam and also draw a diagram for the ions of the for the ions of the first 20 elements if it's a group one element then it's just plus if it's group two then it's two plus if it's group seven then it's going to be uh two minus if it's group eight then it'll just be minus saying that you'll be struggling to get an ion of a group eight element sorry group zero i beg your pardon the inert gas is a group zero no such thing as group eight you can tell i'm not a chemist okay so have a go do the electron configuration diagram for the first 20 elements that's up to calcium and their ions