00:02
All right, hello.
00:03
So in this problem, we are being asked to give a proof of the rule that for main group elements, the number of valence electrons equals the group number.
00:14
So just to clarify what that means, we have a periodic table over here.
00:18
And so what the question stem is basically telling us is that for these main group elements, so anything that's kind of here, so 1a, 2a, and then 3 through 8 here, so those are the main groups.
00:32
The number of valence electrons is equal to the group number.
00:37
And just by looking at this, that might make sense immediately for the most part, with the exception of, let's say, helium here, all of the noble gases will have eight valence electrons because we know that that completes the full octet shell.
00:51
So, you know, and we also know that the electron configurations are given here too at the bottom, but we can see that as we move away to the left from the valence electrons, sorry, from the noble gases, we start to lose one valence electron with each element, and that's what each of these blocks represents.
01:11
So we've been asked to prove this for the following elements over here, and we'll do that using electron configurations.
01:19
So let's start with aluminum.
01:21
So aluminum is located right here in group 3a, and so that should tell you that it has three valence electrons.
01:31
But let's try to do this using the electron configuration, right? so we know that it is in the third period here, and we know that it's in the p block over here.
01:42
So that means that it's going to have electrons in its p orbital.
01:46
And you can kind of see that as you go, so it would be 1s2 on top over here, that's already going to be complete.
01:54
Then you have the 2s2, the second shell and the s -sub shell that's filled, and then 2p6.
01:59
So you have six electrons over here.
02:01
We come over here, that's 3s2, and then we start counting.
02:04
So we can see that because it's the first element here in this p block, it's only going to have one in the 3p shell.
02:11
And that's kind of, you can kind of see it written here, but we obviously don't want to just take our word for we want to see if we can prove it ourselves.
02:18
So based on that, we see the electron configuration for aluminum is 1s2, 2s2, 2p6, 3s2, 3s2, 3p1.
02:30
And based on this, the way we figure out the number of valence electrons is, we look at the biggest principal number.
02:36
So that's three for aluminum.
02:39
And so we then count the number of valence electrons that are there, and that will tell us how much we have.
02:44
This changes a little bit for when we get into some elements that are kind of in larger periods.
02:53
But what's important to keep in mind is that balance electrons are always only going to be whatever's in the furthest.
03:00
S or p for these elements that are in the main group.
03:05
So anything that's in the s block will have, if it's valence electrons only being in the s subshell, anything that's over here, you will ignore basically what's in the d.
03:14
That will not count for the balance electrons, and we'll see that in a minute.
03:18
But essentially, it's always important to remember that valence electrons, for anything that's in your p block, will always be in both the s and the p of the largest shell.
03:31
Okay, so using that logic, we see, you know, there are two s electrons and one p electron in the third main level here, and therefore we have three valance electrons.
03:46
Okay, next for strontium.
03:48
We know that strontium is here.
03:49
Strontium is a group two.
03:51
It's an alkaline earth metal.
03:53
And we know generally those tend to have two electrons because they lose two electrons, and that will give them the nearest normal gas configuration.
04:02
And so we can see that strontium is going to be in the 1, 2, 3, 4, 5.
04:09
It's going to be in the 5th main shell.
04:12
And because it is in the s block, it's the second one over.
04:16
It's going to have 1 -2 as electrons.
04:19
And therefore, its electron configuration is going to be 5s2.
04:24
And of course, i didn't write everything else before that.
04:26
But remember, we can write electron configurations by taking from the next.
04:31
Nearest noble gas.
04:32
So for strontium, that would be krypton.
04:34
So this would we can write this as krypton plus five as two.
04:38
I just didn't want to write everything else out, but everything else is present.
04:42
So because we see that five is the highest energy level, we have only two electrons there.
04:46
So we have two valence electrons here.
04:47
And we also know that because it's an alkaline earth metal, we know alkaline earth metals tend to lose two electrons.
04:52
And therefore we have two valence electrons here.
04:54
And so far, we're seeing that pattern being met of whenever we're in that, that number group, we have that many valence electrons for these main group elements.
05:04
All right, next step is potassium.
05:07
So potassium is an alkali metal.
05:10
It's right here.
05:11
So because of that, it is in the first group.
05:14
It's only going to have one valence electron.
05:15
And once again, we can prove that using electron configurations.
05:19
We see argon is the nearest noble gas.
05:21
So we write that...