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Groups of the periodic table | Chemistry of life | Biology | Khan Academy

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- Let's talk a little bit about groups of the periodic table. Now, a very simple way to think about groups is that they just are the columns of the periodic table, and the standard convention is to number them. This is the first column, so that's group one, second column, third group, fourth, fifth, sixth, seventh, eighth, group nine, group 10, 11, 12, 13, 14 15, 16, 17, and 18. As some of ya'll might be thinking, what about these F block elements over here? If we were to properly do the periodic table we would shift all of these that everything from the D block and P block all are right words and make room for these F block elements, but the convention is is that we don't number them. But what's interesting? Why do we go to the trouble about calling one of these columns, about calling these columns a group? This is what's interesting about the periodic table is that all of the elements in a column, for the most part, and there's tons of exceptions, but for the most part the elements in the column have very, very, very similar properties. That's because the elements in a column, or the elements in a group tend to have the same number of electrons in their outermost shell. They tend to have the same number of valence electrons. And valence electrons are electrons in the outermost shell they tend to coincide, although there's a slightly different variation. The valence electrons, these are the electrons that are going to react, which tend to be the outermost shell electrons, but there are exceptions to that. There's actually a lot of interesting exceptions that happen in the transition metals in the D block. But we're not gonna go into those details. Let's just think a little about some of the groups that you will hear about and why they react in very similar ways. If we go with group one, group one ... And hydrogen is a little bit of a strange character because hydrogen isn't trying to get to eight valence electrons. Hydrogen in that first shell just wants to get to two valence electrons like helium has. Hydrogen is kind of ... It doesn't share as much in common with everything else in group one as you might expect for, say, all of the things in group two. Group one, if you put hydrogen aside, these are referred to as the alkali metals. And hydrogen is not considered an alkali metal. These right over here are the alkali. Alkali metals. Now why do all of these have very similar reactions? Why do they have very similar properties? Well, to think about that you just have to think about their electron configurations. For example, the electron configuration for lithium is going to be the same as the electron configuration of helium, of helium. Then you're going to go to your second shell, 2s1. It has one valence electron. It has one electron in its outermost shell. What about sodium? Well, sodium is going to have the same electron configuration as neon. Then it's going to go 3s1. Once again, it has one valence electron, one electron in its outermost shell. All of these elements in orange right over here, they have one valence electron and they're trying to get to the octet rule, this kind of stable nirvana for atoms. You could imagine is that they're very reactive and when they react they tend to lose this electron in their outermost shell. That is the case. These alkali metals are very, very reactive. Actually they have very similar properties. They're shiny and soft. Because they're so reactive it's hard to find them where they haven't reacted with other things. Let's keep looking at the other groups. If we move one over to the right this group two right over here, these are called the alkaline earth metals. Alkaline, alkaline earth metals. Once again, they have very similar ... They have very similar properties and that's because they have two valence electrons, two electrons in their outermost shell. Also for them, not as quite as reactive as the alkaline metals. Let me write this out, alkaline earth metals. But for them it's easier to lose two electrons than to try to gain six to get to eight. And so these tend to also be reasonably reactive and they react by losing those two outer electrons. Now something interesting happens as you go to the D block. We studied this when we looked at electron configurations, but if you look at the electron configuration for say scandium right over here, the electron, let me do it in magenta, the electron configuration for scandium, so scandium, scandium's electron configuration is going to be the same as argon. It's going to be argon. Then you're going to fill it in we're in the one, two, three, fourth period. It's going to be 4s2. Then we start filling the D block. These are the D block elements here. You have to remember, the D block you backfill. In the D block, this is going to be now 3s1. How many electrons does it have in its outermost shell? Once again its outermost shell is its fourth shell, is its fourth shell. These are, you could argue, higher energy electrons that fills this ... These are filled before that, and there are exceptions to this especially that we see a lot in the D block. This is what's, I guess you could say to some degree, is defining its reactivity. Although in the transition metals, the D block elements, I'm sorry, I made a little mistake there. This is 4s2 3d1. Let me emphasize that. We're backfilling the D block. But these, their outermost electrons are in ... They still have two of those outermost electrons. There, once again, are exceptions in these transition metals right here that for the most part are going in backfilling that D block. Once you've kind of backfilled those D blocks then you come over here and you start filling the P block. For example, if you look at the electron configuration for, let's say carbon, carbon is going to have the same electron configuration as helium, as helium. Then you're going to fill your S block, 2s2, and then 2p one two. So 2p2. How many valence electrons does it have? Well, in its second shell, its outermost shell, it has two plus two. It has four valence electrons. That's going to be true for the things in this group. And because of that, carbon has similar bonding behavior to silicone, to the other things in its group. We could keep going on, for example, oxygen and sulfur. These would both want to take two electrons from someone else because they have six valence electrons and they want to get to eight. They have similar bonding behavior. You go to this yellow group right over here. These are the halogens. There's special name for them. These are the halogens. These are highly reactive because they have seven valence electrons. They would love nothing more than to get one more valence electron. They love to react. In fact, they especially love to react with the alkali metals over here. Then finally you get to kind of your atomic nirvana in the noble gases here. The noble gases, that's the other name for the group, 18 elements, noble gases. They all have the very similar property of not being reactive. Why don't they react? Because they have eight valence electrons. They have filled their outermost shell. They don't find the need. They're noble. They're kind of above the fray. They don't find the need to have to react with anyone else.

Video Details

Duration: 8 minutes and 20 seconds
Language: English
Genre: None
Views: 8
Posted by: jake.goldwasser on Nov 19, 2015

What is a group in the periodic table, and what do elements in a group have in common?
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