Electron Configurations

Introduction to using the periodic table to determine electron configurations

In the last couple of videos

we figured out the electron configurations for atoms

that only had electrons in the S and P subshells.

and so we have this obvious problem

we also have the D subshell

which we'll talk about here with these bizarre shapes

And eventually we even get into the F subshells

which are these really kinda exotic looking shapes

and these shapes

they're interesting to look at and think about.

but they're not as important

for actually figuring out the configuration.

so the question arises

what happens when we start

going to the D and F subshells.

The general way to think about it

is the energy shell you're in

is equivelant to the period,

we are in the periodic table

so if we were to do... so just it all fits on one page...

the periods would appear here on the left

but then I wouldn't be able to finish the whole table

So this is period one

let me write this in a darker color.

Period one, two, three, four, five, six

I think I barely am fitting on the page.

So each row is a period

and then for the porposes of

figuring out the electron configuration

we did this in the last video

we want to put helium

let me just copy and paste exactly helium

we want to put Helium in the S block

so we want to put Helium right there

the reason why, just in case you are curios of

why Helium is put there in the periodic table

is because it has very similar properties

to the other elements in this columns or this group

each column is called a group

and we'll talk about valence electrons

and why that leads to different properties

but for electron configuration purposes

we can put it in the S block

and that's not too hard to remember

beacuse it's just one element

and it kind of makes sense，1s1, 1s2, etc.

All you do is draw blocks around them

so, see, this is - I said multiple times already that

this is - this right here is the S block

this over here on the right is the P block

that's the P block

And then this in the middle right here is the D block.

this is the D block.

and so if you want to figure out

the electron configuration of any atom

you just have to... the way you think about it..

they fill in this order, but when you say..

Let's say this, this,

you know when you go from Calcium:

Clacium would have filled out the 4s2 right?

4s1, 4s2, so if I just do it's 4th energy shell

it looks like this.

Calcium is: 4s2..

and the you start filling the D block, right?

Oh, what did I say? I wanted to do... so that's Calcium

instead I wanted to write

the electron configuration for iron

which is in the D block

so turns out

and this is kind of an artifact

and I'll do a more detailed video on this in the future

that it actually goes and backfills the 3rd energy shell

because all of the sudden

the D orbitals can kind of fit in

the gaps of the 3rd energy shell

so what you do is you go one energy shell above it

So whatever period you are in in the d block

you go one minus that.

Sorry, you go that period minus one

to figure out what energy shell the D block is filling

so iron has 1, 2, 3, 4, 5, 6 elements in the D block.

so it's going to have d6,

but it's not going to do 4d6. It's going to be 3d6

And I figured that out because

it's in the fourth period and I subtracted one from that.

so this is kind of the

highest energy 8 electrons in iron.

4s2, 3d6.

If I said what are the electrons

that are in the outer-most energy shell

I would say that there are two electrons

in the outer-most energy shell for iron.

But if I were to say

which energy shell has the highest energy electrons?

It would be these.

Let me actually do the

whole electron configuration for...

let me pick up another one...

let me take, this is Copper right here.

Let me do Copper.

So the most, the highest, energy electrons

it has are going to be 1, 2, 3, 4, 5, 6, 7, 8, 9.

Actually let me not do copper

because copper does something

very interesting in real life

it's one of the few things

that kind of is a special case,

so let me do a different one, let me do ...

let me do the whole thing for iron.

Sorry to be waffling around so much.

If you want to do the entire

electron configuration for iron

it would be 1s2. That‘s the first energy shell

now the second,

let me do that in magenta, right there.

1s2, and then in orange you have 2s2

and then you have 6 in the P section right there

so 2p6.

Now we are in the third energy shell.

The third energy shell

let me go switch to this bluish color.

So then I fill up 3s2, remember this is the S block,

then I fill out 3p6.

fill out those right there: 1, 2, 3, 4, 5, 6.

now I'm going to add these electrons, then I add...

let me pick a nice green.

Then I go to 4s2.

So it's 4s2.

And now this was the interesting thing,

you know, the D block is interesting

Now I fill out another D block,

or my first D block: 1, 2, 3, 4, 5, 6

but it won't be in the 4th energy shell.

It will be in the 4th minus one energy shell:

it will be in the 3rd energy shell

so this will go to 3d6

just like we did at the beginning of the video

and so the 3rd energy shell.

So I would actually write it here.

I could write it, if I wanted to write.. 3d6.

So if I wanted to write things in order of

which energy shell they are

I could have wirtten it this way

If I wanted to write it in order of

the highest energy electron...

remember the shells are kind of

the best way to visualize

how far away we are from the nucleus,

so, in this case,

these higher energy electorns

are going to be further in the nucleus

even though they take..

it's a higher energy state to be in.

If I did it in terms of energy state

I could rearrange these two

but in most of chemistry

what matters is what's in the outter shell

so it's interesting that

although with filled outer 4s2 here

and then we kept adding more and more electrons,

those electrons where just filling a lower energy shell,

so if we.. in this atom, in case of Iron,

when we talk about the electrons

in this outer energy shell

and thoses are called the Valence Electrons

and these are the ones that react.

These are called Valence...

let me use a better color...

Valence Electrons.

This iron has 2 valence electrons,

because the outer shell is 4s2.

Even though it had these...

even after filling 4s2 it had 6 more electrons.

but those kind of backfilled the third energy shell

so that's one way, and than you might say:

oh well what happens when we go to the F shell?

Or the F block?

So that's these down here..

In a lot of periodic tables

you see these Lanthanoids and Actinoids down here

and they are supposed to fill in the gap right here

and that might be a little hard to visualize

and I'll show you why they do this.

You could just as easily made a periodic table

that looks like this.

where you insert them in,

you push everything to the right

and you insert these in

but obviously this kind of periodic table

is a lot harder to fit in

you could have done the same thing

with the D block actually

This one is the S block, this is the F block

and this is the D block

and then this is the P block right here.

when we are dealing with the F block,

so let's say we wanted to figure out..

I don't even know what element this is...

the electron configuration for this atomic symbol La

So it first it's filling out...

this last incremental electron fills the F block.

So it's F - let me do it in a lower case.

It has one in the F orbital,

and this is the 6th period

but with the F block you subtract two.

You subtract 2 from it so it'll be 4f1, and then 6s2.

The S block you just look at the period, 6s2

And then if you were to keep going back

you would then go to 5p6

so then it would be 5p6

and then it would fill out these 10

in the D block right there that are in the 5th period

but remember you subtract one from the D block

so it'd be 4d10

and then it's 5s2

and you just keep going back that way,

and it seems complicated at first but just remember

when you are in the S or the P block

you just look at the period you are in

but then when you start filling the D block

it fills in a subshell that's one lower

and when you start fillng the F block

which are these really large elements

you start fillng a subshell that is two lower

so maybe in the next video

I'll do a couple of these electron configurations

because I think I'm already out of time

and I'll actually show another way to figure this out

that's often covered in some chemistry classes.

See you soon.