So the first thing I want to talk about is just the theory of why this is even possible, why this is a big deal. The Aufbau principle states that electrons can fill orbitals in order of increasing energy. Okay? Remember that my orbitals in order of increasing energy were 1s, 2s, and then the three 2ps.
If we go ahead and draw the diagram for carbon, let's say. Let's just use carbon as an example. What we're going to find is that the 1s gets completely full at the helium stage. Then the 2s gets completely full at the beryllium stage. Then with carbon, we only have 2 electrons in those p orbitals. Remember, there are 3 p orbitals total, and we only have 2 electrons in them. By the way, the notation that I'm referring to here has to do with electron configuration, which is something that you guys are supposed to remember from general chemistry.
So in this case, what that means is I have 2 electrons in the 2p orbitals. So if I were to draw the actual orbital diagrams for both of these, what you would realize or for the carbon, what you would realize is that like I said, 1s is full, 2s is full, and then the ps are just a mess because what I have is basically 2 unfilled orbitals and then one completely empty orbital. So remember that I told you guys earlier when we were talking about molecular orbital theory that unfilled orbitals like to bond with other unfilled orbitals to become more stable.
So when you're looking at carbon, if it only has 2 unfilled, like partially filled orbitals, you would think, well, why wouldn't it just make 2 bonds? Why does it make 4? Remember, this is another way to think about it. We already talked about bonding preferences and stuff, but this actually has to do with orbitals. So why does it make 4? Well, it turns out that this is just a really bad situation for carbon. Carbon does not like to look like this. The reason is because these orbitals are all messed up. The p orbitals, some are partially filled. Some are not filled. So that means that it would never really be fully stable.
So what carbon actually ends up doing is it winds up exciting one electron up into one of the p orbitals. So what that does is it actually violates the Aufbau Principle and it takes one electron from my 2s orbital and it excites it to one of the 2p orbitals. So now what I end up getting is that instead of having one full orbital of lower energy and then three orbitals that kind of just suck. What ends up happening is that I end up getting 4 orbitals of higher energy that all are partially filled. Why is that better? That even sounds worse. Now we have an excited electron. That means it has more energy, so why is this better?