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Counting valence electrons for main group elements

Video transcript

now that we've classified our elements into groups on the periodic table let's see how to determine the number of valence electrons and so for this video we're only talking about the valence electrons for elements in the main group so we talked about the main groups you're using the 1 through 8 system for classifying groups so 1 2 3 4 5 6 7 & 8 so we're going to ignore the other way to to number the groups and so therefore we're going to ignore groups 3 through 12 for this video and so we're talk about the main groups the valence electrons are the electrons in the outermost shell or the outermost energy level and so let's say let's see if we can figure out how many valence electrons sodium has so for sodium if I wanted to write an electron configuration for sodium I assumed you already know how to do these so you would say it is 1s2 2s2 2p6 and that takes you all the way over here to neon and then that brings you to the third period or the third energy level and you have one more electron to worry about and so that electron would go into a 3 s orbital so the full electron configuration is 1s2 2s2 2p6 + 3 s 1 when I want to figure out how many valence electrons sodium has right the number of valence electrons would be equal to the number of electrons in the outermost shell the outermost energy level for sodium sodium has the first energy levels second energy level and the third energy level the outermost energy level would of course be the third energy level so if I see how many electrons sodium has in its outermost energy level right it's only 1 this time so that means that sodium has 1 valence electron and that's very convenient because sodium is found in Group one and so we can say that for main groups if you want to figure out how many valence electrons you have right it's just equal to the group number right so the group number is equal to the number of valence electrons and so that makes everything really easy and so if I wanted to represent a neutral atom of sodium with its one valence electron draw sodium here and I could draw one valence electron next to sodium like that alright let's go ahead and write the electron configuration for chlorine next so here's chlorine over here and so if I wanted to write the electron configuration for chlorine it would be 1s2 2s2 2p6 and once again that takes me all the way to neon and so now I'm over here in the third energy level or the third period I can see that I would fill 3 s 2 so 3 s 2 and that puts me into my P orbitals so how many electrons are in my P orbitals 1 2 3 4 5 so I'm in the third energy level on P orbitals and I have five electrons so that would be the electron configuration for chlorine if I want to figure out how many valence electrons chlorine has I have to look for the electrons in the outermost shell or the outermost energy level so I have once again the first energy level the second energy level and the third energy level so I want the total number of electrons in the outermost energy level so how many electrons are in the third energy level well there's two and five for a total of seven so chlorine has seven the health electrons and once again that's very convenient because chlorine is in group seven and so let's go ahead and draw chlorine with its seven valence electrons so here is chlorine alright so one two three four five six and seven like that and so the reason I pick sodium and chlorine is of course because sodium and chlorine will react together to form sodium chloride and let's analyze what happens using our electron configurations and so sodium is going to lose one electron so a neutral atom of sodium has equal numbers of protons and electrons but if sodium loses its one valence electrons so it's going to lose its one valence electron and I can show it's one valence electron actually is moving over here to the chlorine so now when i when i draw a sodium alright i have to represent it as an ion a cation sodium used to have equal numbers of protons and electrons but it just lost one electron therefore it's left with an unbalanced number of protons right so as one more proton than electrons so it gets a plus one charge so na plus is the sodium cation the sodium cation is stable and the reason why has to do with the resulting electron configuration alright so if I look at the resulting electron configuration right let me go ahead and use yellow here it would be 1s2 2s2 2p6 and so the electron configuration for the sodium cation is the same as neon which is a noble gas and we know that noble gases are generally unreactive and then has to do with the fact that their electron configurations are full in their outermost energy level right so so the sodium cation is stable because it has an electron configuration like that of a noble gas so for chlorine if we think about how chlorine reacts chlorine has seven valence electrons and let's find it on our periodic table here so here is chlorine chlorine has seven valence electrons that chlorine gets one more then chlorine would have would have an electron configuration like a noble gas like that if argon so chlorine will gain an electron here so let's let's go ahead and write the new electron configuration if a neutral atom of chlorine picks up an electron right well the electron would add right in here so instead of 3p5 right we would right we would write 3p6 and so the electron configuration for the chloride anion would be 1s2 2s2 2p6 3s2 3p6 let me just go ahead and highlight that 1s2 2s2 2p6 3s2 and then 3p6 let's go ahead and draw it right so we're no longer talking about a neutral chlorine atom here we're talking about a chloride anion that picked up one electron alright so it took that electron from sodium so I'm going to show that electron in red has moved over here to chlorine like that and so chlorine gains an electron so it used to be overall neutral used to have an equal number of positive charges and negative charges but it just added one more electron alright so that gives that gives chlorine a negative charge was now the chloride anion and so you have an ionic bond that form between the sodium cation and the chloride anion here so the attraction of these opposite charges forms an ionic bond and so this is an example of a group one alkali metal reacting with a halogen right so in our video on the periodic table we talked about elements right we talked about these being our alkali metals and since these alkali metals are all in Group one they all have one valence electron and we talked about our halogens over here as also being extremely reactive and the reason they are so reactive is if they add one more electron they have the electron configuration of a noble gas and so drawing the electron configurations thinking about valence electrons and thinking about the resulting electron configurations allows you to figure out how these things react and so that is the reason why we can say that group 1 metals are so reactive and why we can say that group 7 the halogens or 17 are so reactive it's because it's because of this concept of electron configurations and drawing out your valence electrons and so we could we could figure out how many valence electrons something else has right so let's say let's say we were asked to figure out how many valence electrons oxygen has alright so all we would need to do is look at the group number right so this would be oxygen is in group six and so therefore oxygen has six valence electrons and so if you wanted to represent oxygen with its six valence electrons you can go ahead and draw in six valence electrons like that and so it's a very useful thing to think about that if you want to find a number of valence electrons right think about the group number for main group elements