 All right, the last type of electron configuration you have to write is one in a condensed form or abbreviated form, and this is where we relate that electron configuration to that of the noble gas that comes before it on the periodic table. Now this is not like it was with the ion stuff. It's always related to the noble gas that comes before it, so we always have to go backwards on the periodic table to figure out what that is. So, let's say we want to write the condensed configuration for sodium. Sodium is number 11, so we work backwards and we end up doing neon as our abbreviation, as our condensed part. If we were doing selenium, again we go backwards, so 34, 33, 32, 31, and so on until we get to argon. That's the one that comes before it. So our abbreviation would be argon, and then we'd go from there. It's all about knowing the blocks on the periodic table and making sure that you can utilize those blocks to describe atoms. Basically, your first two columns on the periodic table are what are called the S block. They represent electrons being added to the S orbitals, and the number is just the period. That's important to understand. These two boxes would be 4S because we're in the S block and the period is 4. The P block is the next one I'll talk about. Those are the ones that are going in the P orbitals, and again it's still just related to period number. This row is 2P because we are in the P block and we are on period 2. The S block and the P block correspond to the actual period number you're on. When you're in the D block, it's considered N minus 1. It's the period number minus 1. So you see 3D here, and the thing about 3D is it is on period 4. But when we go to label these, we have to subtract one from that, and that would be the 3D block. And this is because of the off-bottom principle, order of orbitals. You would go 3S, then you would go 3P, and the rule is you have to fill up 4S before you can fill up 3D. So it's all about that order of orbitals. It's all about the off-bottom principle. It's making sure that these things are in the right place in the right order. I don't know a better way to show you how to do it than to actually just write one of these. It's do magnesium. So to do the condensed electron configuration for magnesium, the first thing I do is figure out the noble gas. So there's magnesium. We work backwards on the periodic table. 12, 11, 10. Neon is the previous noble gas, so what we do is we put neon in brackets like that. Now for the rest of the configuration, we're using the blocks and period numbers to figure out what to do. Each box on the periodic table is a place to put an electron. That's the way you've got to look at it. You've got to find where magnesium is on your periodic table, and magnesium is right here in that spot. So what we've got to do to finish out this configuration is add two more electrons, and those two more electrons are going to go in the 3S, because we're on period 3 and we're in the S block. That's it. That is the condensed configuration for magnesium. Let me do another one, relatively small one, oxygen, and again it starts the same way. We've got to figure out the previous noble gas. So there is oxygen right there, it's number 8. So we go 7, 6, 5, 4, 3, 2. Helium is the noble gas that came before it. And again, now we've got to find where it is on our periodic table. Helium is right there on the periodic table. So what we've got to do is we've got to work across this until we get to oxygen's place. So we've got 1, 2, 3, 4, 5, 6 more electrons to put in. The first two electrons go here, 1 and 2, and they are in the second period in the S block. So it's 2S2. Now we're over here. 1, 2, 3, 4 electrons we're putting in. And since it's the second period P block, that's 2P. So 2P4. Let's step it up a notch and do a more complicated one, but what you're going to see here is it's not all that much more complicated. Let's go to, oh cobalt, let's do that one. There's cobalt. Now we work backwards to a noble gas, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, argon. That's our previous noble gas, so that's what we put in the parentheses. Now we're going to find it on our periodic table here and it is right there. Now let's do the rest of this configuration, let's put these electrons where they belong. So we work our way across that period. That's 1 and 2 going 4S. And electrons 3, 4, 5, 6, 7, 8, 9, and 9 and we need 1, 2, 3, 4, 5, 6, 7 of them going 3D. Again, it's a matter of remembering that this is one behind the periods. That's not 4D. That's 3D. And we get to go. That's the quick and easy way to do it. Obviously when you're working with a regular periodic table it gets a little bit more challenging. It gets a little bit more difficult because all those S-blocks, P-blocks, D-blocks and stuff that's not on there anymore. So you kind of have to figure that out for yourself. Let's do bromine that way. Again we find bromine on the periodic table and it's right there. We have to work backwards to the noble gas. They 5, 3, 4, 3, 8, we're going to end up at 18. This is the noble gas that comes before bromine, argon. We put it in parentheses. That means it's the same as argon plus all this extra stuff. Now we're going to work across its period to get to it. So we're in period 4. My first two electrons are right there. This is the S-block. The first two are the S-block and we are in period 4, so we write 4S2. Now we're going to go all the way across the D-block. 1, 2, 3, 4, 5, 6, 7, 8, 9. There'll be 10 electrons there in that D-block. Again we're on period 4, but the D is always one behind, so it's 3D, 10. And now we wrap it up. We've got to get the bromine. 1, 2, 3, 4, 5. We're in the P-block now. So it's P5, we're on period 4. So we write 4P5. Students do find this to be the most challenging one to do because it's something different. It's not like the other ones. If all else fails, all else fails. You can figure this out by just doing a little extra work. Let's say I want to do sulfur. My previous noble gas is neon. So what I would do is go ahead and write the electron configuration for sulfur. That is 16 electrons. You want S2, 2S2, 2P6, and then 3S2, and the last four electrons go in 3P. 2 plus 2 is 4, plus 6 is 10, plus 2 is 12, plus 4 is 16. That's the atomic number of sulfur. That's the way you do it. If you don't know how to do those, go back and watch my video on electron configuration. Next thing we've got to do then is look at that previous noble gas. And again, since we're a sulfur, we go backwards, 16, 15, 14, 13, da, da, da, until we get to neon. Write the electron configuration for neon. Neon's got 10 electrons. 1S2, 2S2, 2P6, 2 plus 2 is 4, plus 6 is 10. This is neon right here. That's sulfurs. That's neons. To do the abbreviation, you take the neon out of it. And write 3S2, 3P4, write what's left after the neon. And again, that's the longer, more difficult way to do it. If you don't ever figure out this periodic table thing, if you're not a visual person, this can be challenging. I know it. I understand that. So if you can't figure that out doing the periodic table with the blocks and stuff, just write out both electron configurations. Write out the sulfur one, write out the previous noble gas one, and then just take out the neon. Write out the previous noble gas one, and then just take out that previous noble gas. That's what that bracket means anyways. And just write down what's left over. That's a shortcut way to do it. But one where you don't have to use the periodic table.