Putting Electrons into Orbitals

Reminder from Last Time So we have these orbitals: regions of space where we should look for electrons. They come in various sizes (1,2,3,4…etc) and various shapes (s,p,d,f). s orbitals are always a set of 1, p orbitals a set of 3, d orbitals 5, and f orbitals 7. Each individual one can hold two electrons—one spinning each way.

Reminder from Last Time So we have these orbitals: regions of space where we should look for electrons. They come in various sizes (1,2,3,4…etc) and various shapes (s,p,d,f). s orbitals are always a set of 1, p orbitals a set of 3, d orbitals 5, and f orbitals 7. Each individual one can hold two electrons—one spinning each way.

Little Houses Think of the orbitals as a bunch of empty little houses. As long as nobody’s living in them, they don’t really contribute to the neighborhood.

Start Simple With Hydrogen We’ll only do neutral elements for now, and worry about ions later. Hydrogen has one electron to put in. Now we encounter the two rules of electrons: 1. They’re “lazy,” and will always go to the lowest open spot 2. They're “antisocial”. Don’t like being around other electrons unless that have to 1s 2s 2p 3s 3p 3d

Start Simple With Hydrogen Electron goes in the lowest open spot—1s. Choice of spin direction (arrow) is arbitrary. This ‘electron configuration’ is recorded as a list: H: 1s 1 (one electron in the 1s orbital) 1s 2s 2p 3s 3p 3d

Helium Two electrons to put in. First one still goes in 1s. Second takes the open space in 1s spinning the other way (remember, up to 2 per orbital) He: 1s 2 (two electrons in the 1s orbital) Note that the list doesn’t specify that they’re opposite spin—you’re expected to just understand that. 1s 2s 2p 3s 3p 3d

Lithium Three electrons to put in. First one still goes in 1s. Second takes the open space in 1s spinning the other way. Third has to go somewhere else…the 2s! Li: 1s 2 2s 1 (two electrons in the 1s orbital, one in the 2s) 1s 2s 2p 3s 3p 3d

Beryllium Starting to see a pattern here? Be: 1s 2 2s 2 1s 2s 2p 3s 3p 3d (Turns out is was actually Galaxy Quest that I remembered beryllium spheres from, not Star Trek)

Boron Now we have a choice: which p orbital to use? They're all the same energy, so just pick one. B: 1s 2 2s 2 2p 1 1s 2s 2p 3s 3p 3d

Carbon Now what? Double up, or put in separately? C: 1s 2 2s 2 2p 2 ….but where are those two? 1s 2s 2p 3s 3p 3d

Carbon Refer back to rule number two: electrons don't like each other. C: 1s 2 2s 2 2p 2 (This is called “Hund's Rule”, by the way. It also states that they go in with the same spin) 1s 2s 2p 3s 3p 3d

Nitrogen Just keep going N: 1s 2 2s 2 2p 3 1s 2s 2p 3s 3p 3d

Oxygen Now we can start doubling up O: 1s 2 2s 2 2p 4 1s 2s 2p 3s 3p 3d

Something Weird Happens Around 3d If you think of level 1 to 2 to 3 to 4 as “big steps”, and s to p to d as “little steps”, when you get to level three, the sum of the little steps ends up being more than the big step, and the 4s orbital sneaks in underneath the 3d.

Something Weird Happens Around 3d This means that we will fill orbitals from 3p to 4s, then 3d, then 4p. This overlap gets even worse as you go up into f orbitals and such. Fortunately, there's a way to not have to memorize the order

Diagonal Lines Write the orbitals out in normal order, then draw diagonal lines down and to the left. Follow the lines from top to bottom. 1s 2s2p 3s3p3d 4s4p4d4f 5s5p5d5f 6s6p6d6f 7s7p7d7f If you follow these lines in order, then start the next line, nothing changes until you get to 3p, which you follow to 4s, then start back at 3s, 4p, 5s, 4d, 5p, 6s, etc.

Let’s Use a Big Atom Neutral Selenium has 34 electrons to put into orbitals 1s 2s2p 3s3p3d 4s4p4d4f 5s5p5d5f 6s6p6d6f 7s7p7d7f Each s can hold up to 2, each p can hold up to 6, d up to 10, and f up to 14 So: Se: 1s 2 2s 2 2p 6 3s 2 ….12 in, 22 to go… Se: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 … 4 left to go into 4p Se: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 4 all 34 in We didn’t fill the 4p up to 6, because we only had 4 more electrons. Note that I followed the arrows from 3p to 4s, then back to 3d, then 4p

Simplifying a Little These things can get long and ugly fast. 1s 2s2p 3s3p3d 4s4p4d4f 5s5p5d5f 6s6p6d6f 7s7p7d7f Kr: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 Ag: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 9 So to save a bit of writing, they are frequently written in “noble gas abbreviated form”. Note that everything in Ag is the same as Kr up until the 5s. Therefore, Ag can be written as [Kr]5s 2 4d 9 The “[Kr]” just means “all electrons the same as Krypton, and plus these next ones I’m listing.” This is always done based off the last noble gas before your element.

Vocabulary This idea that each element is the same as the last, but with just one more thing added, is called the ‘aufbau principle’ It’s the German word that means “building up” or “construction” Other fun to say German words that have nothing to do with chemistry: Todesberührung Leichen-Überraschungsangriff Grauenhafte Geschwindigkeitsbegrenzung Rindfleischetikettierungsüberwachungsaufgabenübertragungsgesetz Schweinefleisch

Summary Put electrons in orbitals in this order that gets a little funny around 3d and 4s. The ‘electron configuration’ is just a list of which orbitals the electrons are in. Hund’s rule says for things like p,d, and f orbitals, fill each one separately first before doubling up. You can shorten electron configurations based off the last noble gas. German words sound funny to many English speakers.