00:01
So we have to write the electron configurations for the ions and determine which one has a noble gas configuration.
00:08
The first ion is strontium 2 plus.
00:12
Strontium right on the periodic table is right there.
00:17
So if we take a look at the periodic table, there are some trends that we can notice.
00:21
Right away, the first two columns, you see that they're separated from the rest of the periodic table.
00:28
They're grouped together.
00:29
This is known as the s block, meaning electrons, the outer most shell of these elements will have electrons in the s configuration.
00:44
We can also notice over here on the far right is another block of elements that are grouped together.
00:53
This is known as the p block, meaning electrons in the outermost shell of these specific elements will have a p configuration.
01:07
The lanthanide in the actinide series, this is known as the f block.
01:14
And the transition metals in the middle are known as the d block.
01:24
So if we look at strontium, strontium is in.
01:28
The s block.
01:31
So the next thing we're going to do is we're going to write a configuration.
01:35
And it's going to go 1s2, 2s2, 2p6.
01:43
Now we're going to stop here for a second.
01:45
And we're going to take a look at the number of electrons that strontium has.
01:50
I covered it up.
01:51
So i'm just going to erase that top little line so we can see.
01:55
We can see that strontium has 38 electrons.
02:01
So we have to make sure that our configuration ends with 38 electrons.
02:09
So we have 1s2, 2s2, 2p6.
02:16
So now we can go down to 3s2, and then we go 3p6, and then we jump back down, we have 4s2, and then we have the d block, and then we also have the p block.
02:38
So the d block is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
02:45
So this would become 3d10.
02:49
And then we have 4p6.
02:52
So now we're here.
02:54
And then we have over here, we have 5s2.
03:01
But this is where we take a look at this 2 plus.
03:05
This 2 plus means that it lost two electrons.
03:09
So the number of electrons becomes 36 electrons.
03:16
So let's count up our number of electrons and see how many that we have.
03:21
We have 2, 4, 10, 12, 18, 20, 30, 36, 38.
03:33
So you always remove the outermost electrons.
03:36
Since we have to lose 2, these will go away.
03:40
And the electron configuration stops at 4p6.
03:45
Well, if we take a look at 4p6, that electron configuration is the same as krypton.
03:54
So you can say that strontium 2 plus has the same configuration as krypton.
04:03
And this one is a noble gas.
04:10
Let's take a look next at selenium, and this is going to be 2 minus.
04:17
So let's erase our krypton and our strontium.
04:23
So selenium right here is clearly in the p block.
04:30
It has 34 electrons.
04:34
This 2 minus means it's gained two electrons.
04:39
So it now has 36 electrons.
04:43
So 36 electrons is the same as krypton, which is our noble gas...