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Electron Configurations. The purpose of this tutorial is to help you understand how electrons are arranged in their energy levels. You will learn how to write electron configurations, and how these electron arrangements relate to the shape and lay-out of the periodic table. Because electrons are so important in chemistry, the way in which they are arranged around the nucleus plays a crucial role in determining the chemical reactivity of all the elements.

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First lets review a little: As we have learned, electrons exist in very specific energy levels. And when these electrons absorb energy… They get energized up to higher levels. Actually, the jump to higher levels is not a gradual transition as was just shown. It is a quantum jump, and looks more like this: Quantum means it happens all at once – instantaneously – because the electron can never exist between levels – not even for a second.

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Once it is at this higher level (excited state), it doesnt stay there long. It quickly drops down to a lower level – again as a quantum leap – and as it does, it gives off a distinct band of light energy. Also, notice how the electron doesnt have to drop all the way back down to the lowest level. It can get energized up to any level, and from there it can drop to any lower level. AND the different drops each produce different frequencies of light. See how an electron dropping from the 3 rd level to the 2 nd level produced red light A 4 2 electron drop produces blue light And a 5 2 drop produces violet light

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1 st 2 nd 3 rd 4 th 5 th 6 th 7 th 2p Now lets take a closer look at these electron energy levels. Well color code them to make them easier to distinguish... We will see in a moment that these levels are actually made up of sublevels. The higher up you go, the more sublevels there are. We will represent these sublevels as lines with boxes on them. The boxes represent the orbitals that make up the sublevels. This is where the electrons hang out. A maximum of two electrons can fit in any orbital. The sublevel shown below is part of the 2 nd level and it is called the 2p sublevel. Note that the 2p sublevel is made up of 3 orbitals (see the three boxes). And since two electrons can fit in each orbital, the 2p sublevel is capable of holding a maximum of six electrons: Also, lets not forget about the nucleus… It is positively charged (because of all the protons in it). + After all, it is this positively charged nucleus that holds the negatively charged electrons around it in the first place.

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7s 7p7d7f7g7h 7i 1s 2s2p 3s 3p 4s 3d 4p 4d 4f 5s5p5d5f5g 6s6p6d6f6g6h 1 st 2 nd 3 rd 4 th 5 th 6 th 7 th So lets start at the bottom. The 1 st energy level is comprised of just one sublevel: It is called the 1s sublevel, and it contains just one orbital. Thats it! The 2 nd level is made up of 2 sublevels: The 2s sublevel (which, like the 1s, contains just one orbital), and the 2p sublevel which contains three orbitals. Any guesses about the 3 rd level? The 3 rd is made up of 3 sublevels [Do you see the pattern?] The 3s sublevel (1 orbital), the 3p sublevel (3 orbitals) and the 3d sublevel (5 orbitals) How about the 4 th level? You should have a pretty good idea about everything concerning it, accept what the new letter is. Is this what you were thinking? 4s (1), 4p (3), 4d (5), and 4f (7). See how the number of orbitals is always a consecutive odd number? Also notice how the sublevels start to overlap. The 5 th level is just what you would expect: Five sublevels: 5s (1), 5p (3), 5d (5), 5f (7) and 5g (9). But look at how extensive the overlap becomes. And with the 6 th and 7 th, it just gets worse and worse! The 4s is actually a little lower than the 3d. This overlap is very important, and it becomes more extensive as we move to higher levels. +

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7s 7p 7d 7f 7g 7h 7i 1s 2s 2p 3s 3p 4s 3d 4p 4d 4f 5s 5p 5d 5f 5g 6s 6p 6d 6f 6g 6h 1 st 2 nd 3 rd 4 th 5 th 6 th 7 th Now, it may not look like it, but there is a pattern to the levels shown at left, and the order that they go in from lowest energy to highest (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s…) So lets look at them again, but lets spread them out a bit horizontally, so they are not so crowded. +

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1s2s3s4s5s6s 2p3p4p5p6p3d4d5d6d4f5f6f5g6g 7s 7p7d 8s 8p7f8d 9s 9p 10s This is the order in which they were just introduced to you… There are plenty more, but they go off the screen, so were not going to worry about them! +

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1s2s3s4s5s6s 2p3p4p5p6p3d4d5d6d4f5f6f5g6g 7s 7p7d 8s 8p7f8d 9s 9p 10s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10s 2p 3p 4p 5p 6p 7p 8p 9p 3d 4d 5d 6d 7d 8d 9d 4f 5f 6f 7f 8f 9f 5g 6g 7g 8g 9g 6h 7h 8h 9h 10p10d10f10g10h But check out this simple table at right. If you draw diagonal arrows starting at the bottom like this… It shows the precise order in which the energy levels are arranged from lowest to highest! +

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1s2s3s4s5s6s 2p3p4p5p6p3d4d5d6d4f5f6f5g6g 7s 7p7d 8s 8p7f8d 9s 9p 10s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10s 2p 3p 4p 5p 6p 7p 8p 9p 3d 4d 5d 6d 7d 8d 9d 4f 5f 6f 7f 8f 9f 5g 6g 7g 8g 9g 6h 7h 8h 9h 10p10d10f10g10h Watch … +

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s p d f He H So what does all this have to do with chemistry? Below is a rough sketch of the periodic table. For the sake of this discussion, we are going to move He over so it is in that little open space next to H: And now we are going to number the periods 1 through 7: As you will see, these periods correspond (more or less) to the energy levels weve just been discussing. And we are going to designate the four distinct rectangular blocks by the type of sublevel they match up with: s, p, d and f: Now lets take the first ten sublevels (1s, 2s, 2p…) and see how the electrons filling these sublevels takes us row by row across the periodic table, and allows us to read electron configurations right off the table… And lets also number the groups within each block (1,2,3,4…)

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f OK, so were going to use arrows pointing up or down to represent the electrons. Can you guess into which box the first electron would go given that it is attracted to the nucleus? s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s Thats right: it goes in the 1s sublevel. And its el. config is 1s 2. Notice in the table above where H is – in the area designated as 1s. So where does the next electron go? s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s s If you were thinking it went in the 2s, then you forgot that each orbital can hold up to two electrons. Note how He is right here in the area designated as 1s 2 and so its el. config. is 1s 2. +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s s Now that the 1s is filled, the next electron goes in the next sublevel – the 2s. Again note how Li is in 2s 1. Its full el. conf. is 1s 2 2s 1. What is Bes el. conf? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s s Is this what you were thinking? Good. Now look at the periodic table above, what comes after the 2s sublevel? The 2p sublevel. So what will the next el. conf. be? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s Is this what you were thinking? Notice how B is in the 2p 1 spot. So its full el. conf. is 1s 2 2s 2 2p 1. Whats next? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s Is this what you were thinking? Notice how C is in the 2p 2 spot. So its el. conf. is 1s 2 2s 2 2p 2 Notice also how when we fill a sublevel… +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s …we put one electron in each orbital until the sublevel is half filled… +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s … and then we go back and start pairing off This is called Hunds Rule, but it also referred to as the bus seat rule. Can you figure out why? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s Look at F. Its just one electron away from having filled 2p sublevel… And its just one square away from the end of the 2p block. +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p s And then Ne has a completely filled outer level. Na is next. Can you guess where the next electron is going to go? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s s Thats right: in the 3s sublevel. Right now, write down in your notebook what you think the next three el configs will be. +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s s Did you get this one right? +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s How about Als? See how Al is in the 3p 1 spot on the per table and its el config ends with 3p 1 +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s Now just advance through the next 23 slides, but as you do, make sure you are understanding +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s Exactly what is going on… how the el configs simply follow the sequence of the periodic table. +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s Your goal by the end of this slide show is to be able to write el configs for any element using just the periodic table – and your brain! +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p s +

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1s 2s 3s 4s 2p 3p 4p 3d 4d 5s s p d f 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s

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1s 2 2s 2 3s 2 4s 2 5s 2 6s 2 7s 2 2p 6 3p 6 4p 6 5p 6 6p 6 3d 10 4d 10 5d 10 6d 10 4f 14 5f So, the pattern for reading the electron configurations right off the periodic table is this: If you are wanting to write the electron configuration for any element, just follow this pattern and remember to stop at the element youre representing.

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1s 2 2s 2 3s 2 2p 6 3p For example, Cl (#17) which is right here on the table: So the answer would be 1s 2 2s 2 2p 6 3s 2 3p 5 The short cut would be: [Ne]3s 2 3p 5

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1s 2 2s 2 3s 2 4s 2 2p 6 3p 6 3d 8 4f 14 5f Or how about Ni (#28) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 8 Short cut: [Ar] 4s 2 3d 8

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1s 2 2s 2 3s 2 4s 2 5s 2 6s 2 2p 6 3p 6 4p 6 5p 6 6p 3 3d 10 4d 10 5d 10 4f Lets try Bi (#83) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 3 (dont forget the 4f 14 !) Short cut: [Xe]6s 2 4f 14 5d 10 6p 3

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By the way, the orbitals are not really little empty boxes on a line: 2p Instead, they are specific three-dimensional shapes called probability clouds that show where you are most likely to find the electron around the nucleus. The s sublevels are all spherical in shape: And they just get larger and larger as you move to higher levels 1s 2s 3s The p orbitals are a bit more complicated. They are peanut shaped! And within the 2p sublevel, the three orbitals are oriented at right angles to each other. They are referred to as the 2px, 2py and 2pz orbitals. And they fit together around the nucleus like this:

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Now try some of the problems from the Electron Configuartion worksheet. The End

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