Introduction to electron configurations | AP Chemistry | Khan Academy

In a previous video, we've introduced ourselves to the idea of an orbital. Electrons don't just orbit a nucleus the way that a planet might orbit a star, but really, in order to describe where an electron is at any given point in time, we're really thinking about probabilities—where it's more likely to be found and less likely to be found. An orbital is a description of that: where is it more or less likely to be found.

This diagram shows us the types of orbitals which can be found in the various subshells that are found in the various shells. So, you have the s subshell, the p subshell that has three different orbitals in it, you have the d subshell that has one, two, three, four, five different orbitals in it, and then you have the f subshells. Now, each orbital can fit two electrons.

So, if you're thinking about the subshell, the s subshell can fit two electrons. The p subshell can fit six electrons. The d subshell can fit ten electrons, and the f subshell can fit fourteen electrons—two per orbital. Now, the goal of this video is to think about electron configurations for particular atoms. To help us with that, we will look at a periodic table of elements.

So, first, let's just think about the electron configuration of the simplest element. If we're talking about a neutral hydrogen atom, a neutral hydrogen atom has an atomic number of one, which tells us that it has one proton. If it's neutral, that means it has one electron. Now, where would that one electron be? Well, it would be in the lowest energy level or the first shell.

That first shell has only one subshell in it; it only has one type of orbital. It only has an s subshell, and so that one electron in that neutral hydrogen atom would go over there. So, we would say its electron configuration is 1s¹ in the first shell, which is made up only of an s subshell. It has one electron.

Now, what happens if we go to helium? A neutral helium atom is going to have two electrons. So, instead of just having one electron in that first shell, we can fit up to two there. So, its electron configuration would be 1s². Now, what do you think is going to happen when we go to lithium? Well, lithium, a neutral lithium, will have three electrons in it.

So, the first two could go to the first energy level, the first shell. So, the first two go 1s², and then the third electron is going to go into the second shell. The subshell that's going to fill first is the s subshell, so then it'll go to the second shell and start filling up the s subshell. Notice two electrons in the first shell and one electron in the second shell.

Now, what about beryllium? Well, that's going to look a lot like lithium, but now it has four electrons. So, two of them are going to go into the first shell 1s², and then the next two are going to fill up the s subshell in the second shell. I know it's a bit of a mouthful: 2s².

Notice we have 2, we have 4 total electrons, which would be the case in a neutral beryllium atom. But what about boron? Boron gets interesting. A neutral boron would have five electrons, so the first two are going to fill the first shell: 1s². Now, the second two are then going to go to the second shell and fill up the s subshell: 2s², and then we're going to start filling up the p subshell.

So let's see, we have one more electron, so we go 2p. You're going to have one electron in one of these p orbitals. And then what happens when we go to carbon? Well, it's going to look a lot like boron, but now we have one more electron to deal with. If we have a neutral carbon atom, it's going to have six electrons.

So that extra electron is once again going to fall into the p subshell in the second shell because that can fit six electrons. So, we're going to fill the first shell with two electrons, then the 2s subshell with two electrons, and then we have two more electrons for the 2p subshell.

Now, you can imagine as we get to larger and larger atoms with more and more electrons, this can get quite complex. So, one notation folks often use is noble gas configuration. Instead of saying, okay, this is carbon, they could say that, hey, look, carbon is going to have the electron configuration of helium.

Remember, the noble gases are these group eight elements right over here. So, it's going to have the electron configuration of helium, which tells us this right over here. And then from that, we're going to also have 2s² 2p². You could just take helium's electron configuration right over here and put it right over here, and you would get exactly what we wrote before.