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, to 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 it has 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 2 electrons. The second shell can hold up to 8, 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 round again to put a second electron in each position. This is a bit weird, but you're just adding them in clockwise until you've either filled the shell, or run out of electrons. As we still have 1 electron left, we put it in the third shell. And just like the second shell, the third one can hold a maximum of 8 electrons. In order for atoms to be stable, their ultimate shell, meaning the one furthest from the nucleus, needs to be completely full. You can see here that our sodium atom only has 1 electron in its outer shell, rather than 8. 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. 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 0 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 your 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 it must also have 18 electrons. Then after drawing a quick nucleus we can start to draw the shells and electrons. The third 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 18. 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 third 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 would actually 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. 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 going to have a 2 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 to write 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 two eight. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam-style questions, and past papers, and we track all of your progress so that you always know what to study next. So sign up for free by clicking here, or browse our playlist here on YouTube.