 Today we're going to learn how to do electron configurations and how these electron configurations are used to describe how the electrons are arranged within an atom. So if we go back to the periodic table and we just jog our memory about how the elements are arranged in the different levels and sub-levels, the first two columns of elements represent the elements, the N and the S block or S sub-level, the six elements on this side of the periodic table and in what we call the P block or the P sub-level, those in the middle and in the D sub-level and then those at the bottom and up in the F sub-level. So we are going to arrange our electrons by energy level and by sub-level. The period numbers on the side of the periodic table are going to indicate what energy level you're on for the S and the P sub-levels. The D sub-levels, the D block elements, the period number does not correspond to their energy level, their energy level is going to be one less than the period number. And for the F group down here, their energy level is actually going to be two levels lower than what the period number is. So to organize our electrons into an electron configuration, we have to include the energy level, the sub-level designation and the number of electrons that are in each sub-level. So if we pick an element like phosphorus, for example, phosphorus has a total of 15 electrons that we need to organize. So we have to start in level one. Level one has an S sub-level that can hold two electrons. So our first level and sub-level will be one S two because it holds two electrons. The next, after the first level is full, we come to the second level and it's going to be two S and S can hold two electrons. After two S, we follow the periodic table across to two P. Two P can hold a total of six electrons. After two P, we come to three S. S can hold a maximum of two. And after three S, if we follow the periodic table across, we come to sub-level three P. Notice that phosphorus is not at the end of the sub-level. It's in the middle. So we're going to count across one, two, three. So phosphorus is the third element in. So there are only three electrons in the three P sub-level. So this provides the energy level, sub-level and the number of electrons in each energy level and sub-level. To double-check yourself, remember phosphorus had a total of 15 electrons? Add up your superscripts and they should equal your atomic number, which is 15. So two, four, ten, twelve, fifteen. So that means that we must have the correct superscripts if it equaled our atomic number. So let's choose another element from the periodic table. How about something like nickel? Okay? Nickel has atomic number 28, which means it has 28 electrons. So we have to start with the first level and sub-level just as we did before. One S2. The next one to fill will be two S2 and then two P6. So we have finished the second row. We're on to the third. So it's going to be three S2, three P6. After three P6, we start on the fourth row. Now four S is actually going to come before three D. So that is not in order in terms of the levels, but we are going to follow the periodic table across. So four S, the first two, after four S is when we start the D block elements. And as I mentioned earlier, the energy level for the D block elements is one less than the period number. So this is actually going to be three D. And then nickel is the eighth element over in the three D sub-level. So we're going to put our level, sub-level in the number of electrons in that sub-level. And to double check yourself, again, add up your superscripts and make sure that it equals your atomic number. So two, four, ten, twelve, eighteen, twenty-eight. So if we add it up, it equals twenty-eight. That is the same as our atomic number. So we are good. So let's do one more. Let's pick another element. One that's down a little bit farther on the periodic table. How about lead? Lead has eighty-two electrons. Okay? There we go. Eighty-two electrons to organize. So that means we are going to fill everything above lead all the way down to here. So that means all the levels and sub-levels are going to be full until we get to lead on the periodic table. So just to go through it, while we're looking at the periodic table, we're going to have one S-two, two S-two, two P-six, three S-two, three P-six, four S-two. After four S, we fill three D all ten. Now we start back with the P-sub-level again. So this is going to be four P-six. After four P, we come to the fifth row, five S-two. Again the D's are one-level lower, so it's going to be four D all ten, then five P-six down to the sixth row, six S, okay? Now one thing to note, this is where the lanthanides break out on the sixth row. So after barium, you have to come down to LA and across here, okay? The F-block is two lower, energy level is two less than the period number. So we're actually on period six. This is going to be four F. So it'll be six S-two, four F, fourteen of them, back to the D-block. So that's five D, all ten of them. Now we're back to the P-block, and it's the same as the row number, so it's going to be six P, just the first two, because lead only has two electrons here. So let's write that out. One S-two, two S-two, two P-six. Completed the second row, three S-two, three P-six. Completed the third row. Fourth row is when the D start. So after four S-two, we have three D-ten, then four P-six. Completed the fourth row, we're on to the fifth row, five S-two, then for the D, it's four D-ten, then back to five P-six. After five P-six, we're on to the sixth row. So we have six S-two. That's where we break out and go down to the lengthenines. So four F-fourteen, we come back up to the Ds, five D-ten, and then four P-two. That will be our last level and sub-level. As you can see, it gets very long, the more electrons we have. To double check, you would just add up all your superscripts again and make sure that it equaled 82. So for lead, this is a great example to talk about abbreviated electron configurations. Abbreviated electron configurations help you to write the electron configurations in a more concise way. And the way we do it is by locating the noble gas that precedes the element on the periodic table. So for lead, we find the noble gas, remember noble gases are in group 8a, the last column. We find the noble gas that comes before it. So we go up a row and over. So xenon is the noble gas that precedes lead. You're going to write the noble gas in square brackets. That represents all the electrons up to xenon. And then you will continue your electron configuration on the row that your element is on. So we're going to do 6s-2, four F-fourteen, five D-ten, six P-two. And that would be how you would write the abbreviated electron configuration for lead. So the key here is that it must be the noble gas that precedes the element. It can't just be any element from the periodic table before it. It just has to be a noble gas. So that is how you can explain the organization of electrons using electron configuration or abbreviated electron configurations.