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Electron configurations for the first period

Video transcript

let's look at how to write electron configurations for the first period so here's the first period in the periodic table and we have only two elements to worry about we have hydrogen and then over here we have helium so let's start with hydrogen atomic number of one so if this atomic number of one that means there's one proton for hydrogen in a neutral atom the number of protons is equal to the number of electrons so there's one proton there must be one electron so our goal is to write an electron configuration for that one electron of hydrogen and we're going to use the Alpha principle alpha is German for building up because as you write electron configurations you're thinking about the best way to build up an atom so you think about where to put your electrons here we have only one electron to worry about so where's the best place to put the one electron for hydrogen well we want to put that electron as close to the nucleus as possible in order to maximize the attractive force between the positive charge and the negative charge so therefore the electron goes into the lowest energy level possible and that's when n is equal to one so we talked about quantum numbers earlier if n is equal to one there's only one allowed value for L and that's equal to zero if L is equal to zero there's only one allowed value for ml right so the magnetic quantum number that's equal to zero so L is equal to zero tells us we're talking about an S orbital and this tells us how many orientations only one values only one orientation for an S orbital an S orbital is shaped like a sphere alright so in this sphere and this three-dimensional volume here this is the most likely place the most likely region we're going to find this one electron and so the electron for hydrogen is going to go into an S orbital an S orbital in the first energy level so let's go ahead and write the electron configuration we write the electron configuration as 1s one let's talk about what those mean here so this this first one right this is talking about the energy level right the shell and is equal to 1/s says the electron for hydrogen goes into an S orbital and this this superscript one here this is telling us how many electrons are in that orbital and here of course we're talking about only one electron so one s one means one electron in an S orbital in the first energy level there's another way to write an electron configuration or to draw one out it's called orbital notation so you draw a line here which represents an orbital we're talking about an S orbital in the first energy level so we could label this orbital as being the 1s orbital and we put the one electron of hydrogen into that 1s orbital let's say the the electron enters the orbital spin up so this arrow pointing up is is representing one electron with an up spin so the fourth quantum number M s we could say that's positive one half spin so here are two ways to write the electron configuration one s 1 and or we could draw orbital orbital notation like that for hydrogen alright and so we're done with hydrogen's one electron let's move on to helium now let's so two electrons to worry about and so atomic number of two so two protons and two electrons so two electrons to worry about we're still in the first shell we're still in the first energy level so n is equal to 1 if n is equal to one L must be equal to 0 ml must be equal to 0 and so we're still talking about an S orbital in the first energy level right so we're still talking about an S orbital in the first energy level so for helium right an S orbital in the first energy level like that alright let's let's think about orbital notation for for helium here so we have two electrons so an S orbital in the first energy level so we could draw the first electron for helium as spin up alright and the second electron for helium we we would have to do that spin it down so we have to pair the spins one spin up and one spin down so why why do we have to do that so let me go ahead and write I'm going to write I'm going to write negative 1/2 here for the spin the reason we have to pair this because of the Pauli exclusion principle which says that no two electrons in an atom can have the same four quantum numbers so this first electron that we put in the 1s orbital alright so this one right here spin up that would be that would be these same four quantum numbers as that would be these four quantum numbers up here so instead of rewriting them all just I'll just Circle Circle them for hydrogen alright and so for the second electron here the one that I put in the orbital spin down right that can't have the same same set of quantum numbers so n is equal to n is equal to one L is equal to 0 ml is equal zero all those all those have to be the same but the last one here right is different that's why we make it negative one half so it's spin down and so the two electrons in in helium have a different set of four quantum numbers right they differ by the last quantum number and so that's the idea of the Pauli exclusion principle and as a consequence the Pollack's goes in print as a consequence of the Pauli exclusion principle and orbital can contain a maximum of two electrons because you've exhausted all of the possible combinations of quantum numbers we've used them up completely and so the 1s orbital is completely full so we could we could also write the electron configuration for helium right as 1 s 2 and once again what that means is we're talking about an S orbital S orbital in the first energy level and there are two electrons in that S orbital so one s 2 is the electron configuration for helium and since we have two electrons in the 1s orbital we can't fit in anymore electrons and so the first the first shell is closed we have a closed shell there are no more orbitals in the first energy level if you want to add another electron you have to move on to the next shell and so that takes us into the second period on the periodic table