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Video transcript

- [Instructor] In a previous video, we've introduced ourselves to the idea of an orbital, that 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. And an orbital is a description of that, where is it more or less likely to be found. And this diagram shows us the types of orbitals which can be found in the various subshells which 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 could fit two electrons, the p subshell can fit six electrons, the d subshell can fit 10 electrons, and the f subshell can fit 14 electrons, two per orbital. Now the goal of this video is to think about electron configurations for particular atoms. And to help us with that, we will look at a periodic table of elements. And 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, it has an atomic number of one which tells us it has one proton, and 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, and 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 1s1, in the first shell which is made only of an s subshell, it has one electron. Now what happens if we go to helium? Well, 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 will go 1s2, and then the third electron is going to go into the second shell, and the subshell that it'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. So notice, two electrons in the first shell and one electron in the second shell. Now what about beryllium? Well, that's gonna 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 four 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 we're 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 2p1. So we'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 then an 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 where 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 gasses are these Group 8 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, 2s2, and then 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.