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AP.Chem:
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Video transcript

in this video we're gonna get even more appreciation for why the periodic table of elements is so useful and in particular we're going to focus on groups of the periodic table of elements when we talk about a group we're just talking about a column and as we'll see even though the elements in a given column might have very different atomic numbers they all have similar properties and the reason why they all have similar properties is in most cases they have the same number of valence electrons remember valence electrons are the reactive electrons the ones that might interact with other things and because elements with similar valence electrons will have similar reactivities they will form similar ions similar ions and they will have similar roles similar roles in ionic compounds ionic compounds now for the sake of this video I'm going to focus most on the extremes of the periodic table the groups at the left and the right because those are the closest to having a full outer shell either by losing electrons or by gaining electrons so just to remind ourselves what does it mean to have a full full outer shell well in general people will refer to the octet rule for our second third fourth fifth and on and on shells you're full when you have eight electrons eight electrons the major exception to the octet rule is the first shell whereas it is full with two electrons so helium either even though it only has two electrons is very very very stable and the major data point that we have around this octet rule are the group eighteen elements right over here also known as the noble gases there none of the noble gases because they're very unreactive they're very content they don't want to mess around with anyone else and that's because all of the noble gases have full outer shells helium's outer shell is the first shell and it's full neons outer shells the second shell it's full art guns outer shell is the third shell and it's full and so on and so forth now if we go one group to the left of the noble gasses we get to the halogens now the halogens have seven valence electrons so you could imagine there are only one electron away from having an electron configuration like the noble gas to the right of each of them so these halogens right over here these really like to attract electrons to form a negative ion or an anion so you will often times see fluorine as a fluoride anion so it has a negative one charge or you'll also see chlorine with a negative one charge as the chloride anion and I could go on and on you'll often see iodine gained an electron and have a negative one charge if you go one step to the left the oxygen group oxygen sulfur and on and on these elements have six valence electrons so still easier for them to have a full outer shell by gaining two electrons than by losing six electrons so these elements also like to attract electrons so you can see oxygen as an oxide anion has gained two electrons it's swiped it from somebody else sulfur as a sulfide anion now if you go to the other extreme of the periodic table if you look at Group one elements they have one valence electron and especially the ones that you look in you see in red here which are known as the alkali metals it's much easier for them to lose an electron to have a full outer shell then for them to gain seven electrons the reason why hydrogen is a bit of an exception is it doesn't have to gain seven electrons to have a full outer shell it has to gain one so hydrogen could lose one and essentially have no electrons or it could gain one electron and it would have a full outer shell like helium but when we think about ionic compounds these alkali metals are really some of the most interesting participants because as you can imagine for them to get stable they they want to give away an electron so you're very likely to see them having given away an electron and having a positive one charge so you'll oftentimes see a lithium ion with a positive one charge a sodium ion with positive one charge a potassium ion with a positive one charge and that's in general true of all of these group one elements now what about these group two elements also known as the alkaline earth metals well once again it's easier for them to lose two electrons than for them to gain six to have a full outer shell so you will typically see beryllium having a positive two charge it would have lost those two electrons magnesium as having a positive two charge calcium as having a positive two charge now given that how would you expect things on the left and things on the right to form ionic compounds so you might guess if you have an alkali metal in the presence of a halogen things could get very reactive in fact things will get very reactive because these want to give away electron these want to take electron and that's what'll happen the electrons will leave the group one element and then they will go to the halogen and in the process it might release a lot of energy but what you would be left with is an ionic compound for example lithium loses a electron and has a positive one charge that positive ion would be very attracted to a chlorine anion that has just gained that an electron maybe it's the same electron or it swipe that electron from another lithium atom and so these two things would be attracted and they could form lithium chloride and all of these alkali metals could play that same role in this ionic compound as lithium so it's also typical to see sodium chloride that is table salt it's also typical to see potassium chloride so on and so forth and on the other hand fluorine or bromine or iodine can play a similar role as chlorine so you could see something like sodium iodide or potassium iodide once again the alkali metal would have lost an electron the halogen would have gained an electron and then they are attracted to each other in forming these ionic compounds what kind of ionic compounds might be formed with these Group two elements let's take calcium for example it's not unreasonable for calcium to lose two electrons to have a stable outer shell to have an electron configuration like argon so if it loses two electrons it has a positive two charge and you can imagine those two electrons get lost to two different iodine atoms so each of them have a negative one charge so times two and then what type of ionic compound could they form well you could have one calcium and then two iodine's so calcium iodide is actually an ionic compound you would see and has a neutral charge overall because calcium has a positive two charge and each of the iodine's have a negative one charge but then you have two of them so it is neutral overall what might calcium do with oxygen well calcium likes to lose two electrons oxygen likes to gain two electrons so you could see something like calcium oxide so I will leave you there the big picture here is the column in which an element is tells you a lot about its reactivity because it tells you in general how many valence electrons it has and atoms are most stable when they have a full outer shell and so that helps you predict hey is it easier for them to lose electrons and form a positive ion or gain electrons and form a negative ion and then from that you can make predictions as to the types of ionic compounds that could be formed with the different elements
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