 All right, I'm going to do a video on electron configurations. Now, the most important thing when writing an electron configuration standard form is to be able to get your orbitals in order, from lowest energy to highest energy. And that's a matter of knowing what orbitals you have on each level and understanding the off-bob principle that tells us that before we can put electrons in a d orbital, we have to put the electrons in the s orbital on the next level first. So again, you've got to know what's on each level. So level one in an atom has only an s orbital, and we call it 1s. First energy level, s orbital. That's all we have room for. It's a very small energy level. Moving on level two, level two gets a little bit bigger, so we get to add the next orbital in the series. So not only do we have an s orbital there, but we have a p orbital as well. Again, I told you in class you had to know your order of orbitals, spdf, and all we're doing is just add in the p orbital. We're adding the next one to it. Move up to level number three, we do the same thing. We know we have an s orbital. We know we have a p orbital, but again we have to add the next one, which would be d. And then moving on to level four, this is the place where we finally have enough room for all four. So we have the s orbitals, the p orbitals, the d orbitals, and the f. And it would work that way in level five. We still have room for all four of them, s, p, d, and f, and then the mystery orbital, whatever comes next in the series. We never have to worry about that because we're not even gonna go into f orbitals. We're never really gonna get any further than 5s. So no reason to panic about what that extra thing is. We just have to know that they're all there. Again, the off-bob principle tells us no electrons in a d orbital until the next s is filled. That's basically what it tells us. No electrons in a d orbital until the next s is filled. So we can make our list just work on our way through. The first orbital is 1s, and then we're full on that level. We move up to the next. Our next orbital is 2s. When that one's full, we move on to the next. 2p. When that one's full, we move on to the next. 3s. When that one's full, we move on to the next. 3p. And this is where we run into a d. And again, the rule is no electrons in a d orbital until the next s orbital is filled. So we don't jump from here to here. We jump from there to there. And we put the electrons in 4s. So we've taken care of that, that, that, that, that, and that at this point. Those are all written down here in my order. Once that 4s is filled, we can come back down to 3d and get that one. That one's full. Now we move on to the next available one, which is 4p. And then we hit a d again. And again, the rule is no electrons in a d orbital until the next s orbital is filled. So instead of going from here to here, we go down to the 5s. That one's filled. Now we can go back to 4d. Again, we're not doing anything with f orbitals, so we don't have to worry about those. 5p would be next, but we're never gonna get that far. So, you know, if you can get your thing, your list, written down to 5s, you've got the standards covered. As the standards say, we're always supposed to go to number 38, we're always supposed to go to strontium. And that's all you need to get to strontium. So if you can get there, just doing this whole thing, just remembering this, don't put anything at d until the s's are filled, you're good to go. Now once you have that down, it's straightforward. We know that s orbitals hold a maximum of two electrons, p's hold a maximum of six, d's hold a maximum of 10, and f's hold a maximum of 14. Again, the way that I told you to remember that is, again, just knowing your orbitals. They're s, p, d, and f in that order. And remember, that's the order you have to learn them in. Once you have them in order to get the sub orbitals, it's just odd numbers starting with one. One, three, five, seven. That's the number of sub orbitals you have in each one of those. So the s is a single orbital. There's no sub compartments or anything. P is actually a collection of three sub orbitals or three separate compartments. D is a collection of five sub orbitals or five separate compartments. And f is a collection of seven sub orbitals or seven separate compartments. So again, it's just the odd numbers, one, three, five, seven. For these, you just double all those. So one times two is two, three times two is six, five times two is 10, and seven times two is 14. That's the poly exclusion principle. It says you can only put two electrons in each of these sub orbitals. So only two electrons in there, because there's only one sub orbital, six in there because you have three of them and each can take two, 10 here because you have five sub orbitals and each one can take two, 14 here because you have seven of them and each one takes two. So again, it's just down to s, p, d, f. Knowing those in order, then your odd numbers, one, three, five, seven, and then double them. That's where all those limits come from. To do the configuration, I tell you what element I want you to write it for. So let's write the electron configuration for calcium. What you need to know is the total number of electrons. We're doing a neutral atom here. There's no charge in that upper right hand corner. Nothing written here, so we know it's a neutral atom. So we just use the atomic number because the protons and electrons will be the same. That's 20. We have 20 electrons that we have to write into our electron configuration. So we just start here and work our way across until we found a happy home for 20 electrons. Our first orbital is one s. It can hold a maximum of two electrons. So we've got two of our 20 taken care of. We've got 18 left to go. My next orbital is two s. It's an s orbital, so it holds two electrons. So I've got four of my 20 taken care of. I've got 16 to go. My next orbital is two p. I can put six electrons in a p orbital. Two plus two plus six, I've got 10 electrons taken care of, I've got 10 more to go. My next orbital is three s. S orbitals hold two electrons. Two plus two is four, plus six is 10, plus two is 12. I've got 12 of my 20 done. I've got eight more to go. Three p is next. And in the three p orbitals, I can put six electrons because p's hold six. Two plus two is four, plus six is 10, plus two is 12, plus six is 18. I've got 18 of my 20. I've got two left, and they'll both fit in four s. When you're all said and done, these exponents, the number of electrons has to equal your total that you started with. Two plus two is four, plus six is 10, plus two is 12, plus six is 18, plus two is 20. So we're good to go. This maps out the location of the electrons in an atom. It tells me on the first energy level, in the s orbital, I have two electrons. On the second energy level, I have two orbitals. I have an s orbital and a p orbital. In the s orbital, there's two, and the p orbital would be six. I have two orbitals on my third level with electrons in them. I have an s orbital with two and a p with six. And on my fourth energy level, there's only one orbital there that has electrons in it. That's the s, and it's got two electrons in it. I can see that this is my highest energy level. So I know I've got two valence electrons. Valence electrons are electrons in the outermost energy level. I just look at these big numbers to see which one's the biggest one. Love my four. And I look to see how many electrons are in it. And it's got two. So I know my valence electrons would be two.