in today's video we're going to take a look at the arrangement of electrons in atoms and see how most atoms like the fluorine that we've shown here have an incomplete outer shell of electrons this is a really key point in chemistry because in order for atoms to be stable they need to have a full outer shell and if they don't then they'll have to react with another atom they gain or lose the electrons that they need let's start by drawing the electron arrangement of sodium the fact that it has an atomic number of 11 tells us that there's 11 protons but also that it has 11 electrons these electrons are going to be arranged in shells around the nucleus which we draw as rings like this and the first one to fill is the one closest to the nucleus as this one will have the lowest energy level this first shell can only hold two electrons the second shell though can hold up to eight and the order in which you add them can sometimes be important the best way to do it is to put a single electron in the top right bottom and left positions before you go around again to put a second electron in each position this might seem a bit weird or unnecessary but really you're just adding them in clockwise until you've either filled the shell or run out of electrons as we still have one electron left we put it in the third shell and just like the second shell the third one can hold a maximum of eight electrons in order for atoms to be stable their outermost shell meaning the one furthest from the nucleus needs to be completely full you can see here that our sodium atom only has one electron in its outer shell rather than eight so we would say that it's unstable and all we mean by that is that it wants to react with another atom so that it can have a full outer shell now most single atoms are like the sodium that we just saw if they weren't bonded to anything then they would have incomplete outer shells and so to fix this most atoms want to react to form molecules or compounds however there are some exceptions to this the noble gases in group zero of the periodic table all have completely full outer shells as we can see with neon and because of this they don't want to lose or gain any electrons and so they don't really react with anything when it comes to exams you could be asked to work out the electron arrangement of any of the first 20 elements in the periodic table for example they could ask what is the electron structure of argon for this we first need to find argon on the periodic table and we can see that argon has an atomic number of 18. so we must also have 18 electrons then after drawing a quick nucleus we can start to draw the shells and electrons the first shell will have two electrons the second shell will have eight and the third shell will also have eight because two plus eight plus eight gives us eighteen we can also show this electron structure with numbers though rather than drawing out all of these shells and to do that we would just do two comma eight comma eight to show there's two electrons in the first shell eight in the second and eight in the third another thing to point out is that when you come to draw these diagrams for yourself you have to draw the electrons either as crosses like this or as dots like this one what if we were asked to do the same for calcium well calcium has an atomic number of 20 so it must have 20 electrons which means working out from our nucleus we have two in the first shell eight in the second eight in the third and two in the fourth and if we were to write this structure it would just be two eight eight two as we can see from this diagram calcium atoms aren't stable because they don't have a full outer shell they only have two out of eight possible electrons in their fourth shell what would help is if these two outer electrons were somehow removed this would mean that the outermost shell which is now the third shell would be full and so the calcium is stable however even though our calcium atom still has 20 positive protons it now has only 18 negative electrons because remember it just lost two of them so overall it's gonna have a two plus positive charge and so we call it a calcium ion rather than a calcium atom with ions we always draw big square brackets around them and write the overall charge in the top right corner because our calcium atom lost two electrons drew right to the electron structure it would now just be two eight eight as a last example let's look at fluorine its atomic number is nine so there are nine protons and nine electrons meaning we'd have two electrons in the first shell and seven in the second which we'd write as two comma seven so here we'd have the opposite problem to what we had with calcium in order to be stable and have a full outer shell fluorine needs to gain one more electron which you would have to get from another atom giving it an overall charge of minus one and making it a one minus fluoride ion and we could write this electron structure as 2 8. anyway that's it for this video if you enjoyed it then please do give us a like and we'll see you next time when we take a look at how to balance chemical equations