Electron configurations with the periodic table | Chemistry | Khan Academy

- [Presenter] Let's explore
electronic configurations.

It's basically arranging electrons

of different elements in
various shells and subshells.

Let me quickly show you some examples.

Yes, this will look overwhelming,

but for now, focus on these numbers.

One, two, three, four, five, six.

These numbers represent the
shells, the energy shells,

and if you look at the
letters like s, p, d,

and they can be f as well,
that represents the subshells.

So look, we've arranged the electrons

of various elements in
their shells and subshells.

That's what electronic
configuration is all about.

But the big question
is how do you write it?

I mean, I used to find it super hard.

Well, by the end of this video,

we would've learned a cool way

to actually write the
electronic configuration

of any element just by
looking at the periodic table.

So if you're excited, let's begin.

So this is the periodic table
where we arrange the elements

according to their atomic numbers.

So hydrogen is one, then helium is two,

three, four, five, six,
seven, eight, nine, 10,

and then 11 to 12, just like
reading a book, all right?

Now, we need to familiarize ourselves

with two important terms.

The first one is periods,

period represents the horizontal rows.

So we call this as the
first period, second period,

and so on and so forth
until the seventh period.

I'm sure you must be wondering,
what about these elements?

We'll get to them in a second.

But the second one are
the vertical groups.

So the vertical columns are called groups.

So this is group one
element, group two element,

and so on and so forth,
up till group 18 elements.

Okay, what about these elements over here?

Why are they separated out?

Well, if you look at the elements
over here, if we zoom in,

you can see, this one is
57 and the next one is 72,

which means there's a big gap.

That gap is filled by these elements.

So these elements on the top
over here belong to period six,

and the elements over here
belong to period seven

in between these two.

Now, if we had to write
them out all over here,

our periodic table would become too wide

and it'll be hard to fit in a page

and that's why we are writing
it out separately over here.

Don't worry too much about
them because for now,

what's important for us is to think about

how do the periods and the
groups help us figure out

the electronic configuration.

So here's the thing, the
period represents the highest

energy shell or the valence
shell of the elements.

For example, hydrogen and
helium are in period one.

That means the first shell
is their valency shell.

That's the highest energy shell.

If you look at sodium, for
example, it's in period three.

That means the highest energy
shell is the third shell

and so on and so forth.

So let's quickly look at some examples.

If you look at the electronic
configuration of beryllium,

don't focus on anything else,

just look at the highest shell.

It's two because it's
in the second period.

Look at nitrogen, highest shell, two,

it's in the second period.

Look at sodium, as we
said, three, third period.

Look at sulfur, don't
look at anything else,

but the highest shell is three.

It's in the third period.
Okay, what about the groups?

What do they tell us?

Well, they tell us the subshell

in which the outermost electrons are.

For example, if you look at group one

and group two elements,
the outermost electrons

of those elements will
be in the S subshell.

So we call them the S block elements.

If you look at group 13 to group 18,

the outermost electrons of these elements

will be in P subshell, so
we call them as P block.

Similarly, over here,
group three to group 12,

their outmost electrons
will be in the D subshell.

So we call them as D block elements.

We also call them transition elements

because their properties
are in between there

of S block and P block.

So sort of like transitioning
from here to there.

But anyways, finally, if you
look at the elements over here,

their outermost electrons
are in the F subshell,

and therefore, we call
them F block elements.

And again, these are also called
inner transition elements.

We'll not get into the technical reason

behind why it's called so,

but for our purposes, we
can think that they are

at least on the period table,

literally inner transition elements.

Anyways, again, going
back to the same example

that we saw earlier this time,

focus on the electrons in
the outermost subshell.

Which subshell is that
for these two, look,

it's S subshell because
they belong to S block.

And for these two, it's look, P subshell,

because they belong to the P block.

Why is it two s and two p?

Oh, that's because they
belong to the second period.

So the valency shell is two.

And why is it 3s and 3p?

Oh, that's because they
belong to the third period.

So their valency shell is three.
It all makes sense, right?

Of course, the final
question for us would be

how do we figure out
the number of electrons

in that outer most subshell?

Like how do we know it's
two, one, three, four?

Well, for that, you can
just count how many squares

to the right that particular
square sits in that block.

Okay, here's what I mean.

If you look at beryllium,
it sits one, two,

it's a second square,

and therefore, there'll be two electrons.

If you look at sodium, it's
the first square in this block,

and therefore, there's only one electron.

Look here, Nitrogen sits on one, two,

three squares to the right,
and therefore, three electrons.

And sulfur, one, two, three,
four squares to the right,

and therefore, there are four electrons.

So look, if I know the
location of the elements

in the periodic table, I know
exactly what the last term is.

The period tells me, the valence shell,

the blocks tell me which subshell

the outermost electrons belong to.

And by counting the number
of squares in that block

to the right, we get
the number of electrons

in that subshell.

And of course, the same thing applies

to D block and F block as well.

But now, the final question is

how do we write the entire
electronic configuration?

For that, we'll just write
down all the subshells

on the periodic table itself.

So this makes sense, right?

This is the 2s, 2p, 3s,
3p, and so on and so forth.

I'm not writing down the D
block and the F block right now

because there's a small caveat over there.

So we'll tackle them a little later.

And you can see, helium is an exception.

It actually belongs to S block,
but it's written over here

because it's a noble gas.

So it has similar properties
of all the other noble gases

which come over here.

That's why it's written over
here. So it's an exception.

It's not a P block,
and that's why it's 1s.

But from here, you have 2p,
3p and all of that, okay?

So first let's get some
practice for light elements.

So let's start with hydrogen,
the simplest element,

which has only one electron, okay?

The way to do it is you just locate it

on the periodic table.

Hydrogen is over here, and
therefore, I know it is 1s,

and it's the first square in my S block.

So 1s1, 1s1, and that's it.

We are done.

And you can see, hydrogen has one electron

and there is one electron
over here, perfect.

Let's do a few more. It'll make sense.

We'll get the hang of it, okay?

Helium. Helium is over here.

So helium would be 1s, but
look, it is the second square

to the right in the S block.

This is an exception, right?

It's a second square, one, two,

second square to the right in the S block.

So it'll be 1s2, and that's it.

And you can verify that there
are two electrons in helium.

Okay, let's try lithium.
Lithium is over here.

Now things will get interesting.
So how do we write it?

Well, we always start from here.

You start from here and you
start reading like a book.

So we start with 1s, 1s2.

So we know 1s2 gets completely filled,

and then we come here.

So 1s2, let's do that, 1s2,
and then we come here 2s.

But lithium is the first
square in my S block.

So 2s1, 2s1, and we are done.

And again, we can verify two
plus one, three electrons.

Yeah, lithium has three electrons in them.

Okay, let's try carbon.
Carbon is over here.

Again, all we have to do is locate them

on the periodic table. Okay?

So where do we start?

We always start from here, from the top.

So 1s2, 1s is completely done.

1s2, so we are here.

Then 2s2, 2s also is completely done.

So 2s2, and then we get to 2p.

And notice in the P
block, carbon is one, two,

two squares to the right in the P block.

So it'll be 2p2, that's the last one.

And we are done.

And again, we can check, two
plus two plus two is six.

Carbon has six electrons in them. Okay.

Why don't you try chlorine?
Chlorine is over here.

So let me raise all of these. All right.

Ready to pause the video

and see if we can try it yourself first.

All right, let's do it. We
always start from the top.

So we have 1s2, 1s2 is done.

Then we come to 2s, that's
also done. 2s2, 2s2.

Then we go to 2p, that also gets done.

So 2p, one, two, three,
four, five, six. So 2p6.

Then we come down, 3s,
it also gets done, 3s2.

And then finally, we come to 3p.

3p, one, two, three, four, five.

It's five squares to the right
in the P block. So it's 3p5.

And now we can check, chlorine
has total of 17 electrons.

So let's check that. So
five plus two is seven,

plus six is 13, plus four is 17.

And there you have it. All right.

Now, let's look at what happens
for D block and F block.

All right? You ready?

What do you notice? You
see a 3d over here, not 4d.

This is a 4d, not 5d, it's
one less. Why is that?

That's because the
electrons of the D orbital

are never in the valence shell.

They go into the core.

So the important thing is that
the electrons in the D block

are always one level
below the valence shell.

And that's why this number
is one less than the period.

So that's the major difference.

But the procedure and
everything else stays the same.

So let's take an example.
Let's consider vanadium.

Vanadium lies over here.
Okay, so let me raise this.

And now that I know it that's over here,

everything else, the
procedure stays the same.

So we start from here, we have 1s2, 1s2.

Then we go here, 2s2, 2s2.

Then 2p6, one, two, three,
four, five, six. So 2p6.

Then 3s2, 3s2.

Then 3p6, 3p6, this is
getting exciting, right?

Then 4s2, 4s2.

And then finally, we'll be in 3d.

And it is one, two, three
squares to the right

in the D block, and therefore, it is 3d3.

And if we add them all up,
the vanadium has 23 electrons,

so we'll just check it.

Three plus two is five, plus six is 11,

plus two is 13, plus six
is 19, plus four is 23.

There you have it.

And again, look, electrons
in the D subshell

are one level below the valency.

So it's not 43, even though
it's in the fourth period.

It's not 43, it is 3d3. Okay, you try.

Consider tin, tin lies over here, okay?

And it has 50 electrons.

So why don't you pause and give it a shot.

All right, let's do
this from the top, 1s2.

Let's do it a little quickly now.

Then 2s2, 2p6, 2s2, 2p6.

3s2, 3p6, 3s2, 3p6.

Then 4s2, 4s2.

Then we have 3d,

and one, two, three, four, five,

six, seven, eight, nine, 10, so 3d10.

Then we have 4p6, 4p6.

Then we go to 5s2, 5s2.

Then we have 4d10, 4d10.

And finally, 5p, one, two, 5p2.

If we add these all up, we should get 50.

And if you check that, we'll get 50.

And just like with the D block,

if you go into the F block elements,

then you will see the electrons are not

in the valence shell.

In fact, they are valency shell minus two.

That's why if you look
at the sixth period,

you go into 4f, and for the
seventh period you get 5f.

All right, we are pretty much done

because we can now write
electronic configuration

of any element that you can locate

on the period table, right?

But when you get to heavier elements,

look, the electronic
configuration becomes so huge.

So we invent a shortcut,

and the shortcut is try
to locate a noble gas,

the nearest noble gas that
comes before the element.

So for example, when it comes
to tin, the nearest noble gas

that comes before the element is krypton.

And so now, what we do is we say,

look, the electronic
configuration would be

everything that krypton has.

So we write krypton in the bracket,

which basically means
everything that krypton has

and then some more.

And then you start from
krypton and then you go here.

So we say 5s2, 4d10, and then 5p2.

So 5s2, 4d10 and 5p2.

See, this is the part that
we have written over here.

And all the rest over here,

we say that's basically krypton's
electronic configuration

so we write that in the bracket.

This is a shorthand notation
of what we wrote over here.

So let's consider one last
example, which is scandium,

which comes over here.

So can you try writing with the shortcut?

Okay, so we locate the nearest noble gas

that comes before it,
that is argon in our case.

So we write argon. So we start with argon.

And then after argon we have
4s2, and then we have 3d1.

And there you have it.