Orbital diagrams are pictorial descriptions of the electrons in an atom. Three rules are useful in forming orbital diagrams. According to the Auf Bau Principle, each electron occupies the lowest energy orbital. The Pauli Exclusion Principle says that only two electrons can fit into an single orbital. Hund s rule states that electrons go into different orbitals in the same sub-level before doubling up inside orbitals.
Alright let's talk about orbital diagrams. Orbital diagrams are a pictorial description of electrons in an atom. In order to figure out where electrons go in an atom we have to follow 3 main rules. The first one being the Auf Bau Principle, the Auf Bau Principle states that each electron occupies the lowest energy orbital available. Then we have to think okay with the sublevels, I mean the orbitals how are they falling in terms of like energy which one is lower in energy, which one is higher in energy. So let's look at the Auf Bau diagram which actually show this for us, okay so down here we have the 1s orbital but the 1 dash indicates that there's 1 orbital within the 1s sublevel which makes sense that it is the lowest in energy, it's its first principle energy level. You jump up a little bit in energy and we get the 2s orbital that make it the 2p sublevel. Notice that there's 3 orbitals within the 2p sublevel we have learnt that before, that makes sense. Okay as we go up and we jump up to the 3s orbital then the 3p orbitals then up here it gets all funny like all the things, all the sublevels and all the orbitals kind of get jumbbly in terms of like what you predict. So how are we going to remember which one is lower in energy than the others? Instead of having to carry this around and have to refer back to this actual diagram there is an easy way to remember which way, how electrons fall in the orbitals and in the sublevels.
Alright we're going to make this chart which might seem familiar from the classroom, the first energy level is the 1s orbital then we're going to do the 2s and the 2p then the 3s, 3p, 3d okay then the 4s, 4p, 4d, 4f and so on and so forth. I'm not going to draw the entire thing but you get how it comes out. Then we're going to say "okay electrons are going to fall into the 1s orbital first the lowest in energy as Auf Bau diagram described." Okay falling into the 1s orbital, then they're going to fall into the 2s orbital after that okay great then they're going to fall into the 2p orbital and then the 3s orbital making our diagonal. Then they're going to fall into the 3p orbital and then the 4s orbital notice we skipped the 3d they did not go arcos like that we're following the actual arrows not across. Then they're going to after the 4s then they're going to fall into the 3d and then the 4p and just keep pushing the arrows down if you have more principle energy levels so on and do forth. Okay so I'll actually set this into play and in just a second.
The second rule that we're going to talk about is the Pauli Exclusion Principle which basically states that there are maximum 2 electrons per orbital. So an orbital can only hold 2 electrons and that's it no more. It can hold 1 but it cannot hold more than 2. Lastly the Hands Rule states that they must occupy all orbitals of equal in energy before pairing up. So just like electron are both negative they're all negatively charged they're not going want to be really close to each other, so they're in equal energy they're going to occupy all the energy levels of that same energy first before they pair up because they typically don't like being really close to each other. Let's put all these stuff into play, how this all come together. Okay let's do the orbital diagram for iron, iron we know is on its ground state of 26 electrons, so we know the first electrons are going to go into the 1s orbital and we said 2 electrons can fall into the 1s orbital. After the 1s orbital is a 2s, 2 electrons are going to go in there as well, then you have the 2p and don't forget the p sublevels has 3 orbitals so we're going to draw 3 dashes indicating the 3p sublevel and the electrons are going to go in all of them first before pairing up so right now we have a total of 10 electrons this is a 2p sublevel. After the 2p sublevel we're going to go into the 3s, 2 electrons are going to fall into there making 12 after 3s we're going to go into the 3p, we're gong to draw 3 dashes 1, 2, 3, 4, 5, 6, so 12, 14, 16, 18 electrons. After the 3p we're going to go into the 4s no we're not going to go into the 3d we go right into the 4s which is 1, so you have 2 electrons there, so you have 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, we have 6 electrons left. The next one is a d, d is a 5 orbitals all the same energies we're going to draw 5 dashes 2, 3, 4, 5, all are the 3d, so we're going to draw I need 6 more so 1, 2, 3, 4, 5, 6 and this is how you draw orbital diagrams.