Electronegativity

Voiceover: What I want to
talk about in this video

are the notions of Electronegativity,

electro, negati, negativity,

and a closely, and a closely related

idea of Electron Affinity,
electron affinity.

And they're so closely
related that in general,

if something has a high electronegativity,

they have a high electron affinity,

but what does this mean?

Well, electron affinity
is how much does that atom

attract electrons, how much
does it like electrons?

Does it want, does it
maybe want more electrons?

Electronegativity is a
little bit more specific.

It's when that atom is
part of a covalent bond,

when it is sharing
electrons with another atom,

how likely is it or how badly does it want

to hog the electrons
in that covalent bond?

Now what do I mean by hogging electrons?

So let me make, let me write this down.

So how badly wants to hog,

and this is an informal
definition clearly,

hog electrons, keep the electrons,

to spend more of their time closer to them

then to the other party
in the covalent bond.

And this is how, how
much they like electrons,

or how much affinity they
have towards electrons.

So how much they want electrons.

And you can see that these are very,

these are very related notions.

This is within the context
of a covalent bond,

how much electron affinity is there?

Well this, you can think of it
as a slightly broader notion,

but these two trends go absolutely
in line with each other.

And to think about, to just think about

electronegativity makes it
a little bit more tangible.

Let's think about one of the most famous

sets of covalent bonds,

and that's what you see
in a water molecule.

Water, as you probably know, is H two O,

you have an oxygen atom,

and you have two hydrogens.

Each of the hydrogen's
have one valence electron,

and the oxygen has, we see
here, at it's outermost shell,

it has one, two, three, four,
five, six valence electrons.

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

And so you can imagine,
hydrogen would be happy

if it was able to somehow
pretend like it had another

electron then it would have
an electron configuration

a stable, first shell that
only requires two electrons,

the rest of them require eight,

hydrogen would feel, hey
I'm stable like helium

if it could get another electron.

And oxygen would feel,
hey I'm stable like neon

if I could get two more electrons.

And so what happens is they
share each other's electrons.

This, this electron can
be shared in conjunction

with this electron for this hydrogen.

So that hydrogen can kind
of feel like it's using

both and it gets more stable,

it stabilizes the outer shell,

or it stabilizes the hydrogen.

And likewise, that electron could be,

can be shared with the hydrogen,

and the hydrogen can kind
of feel more like helium.

And then this oxygen can feel like

it's a quid pro quo,

it's getting something in
exchange for something else.

It's getting the electron, an electron,

it's sharing an electron
from each of these hydrogens,

and so it can feel like
it's, that it stabilizes it,

similar to a, similar to a neon.

But when you have these covalent bonds,

only in the case where they are equally

electronegative would you have a case

where maybe they're sharing,

and even there what happens

in the rest of the molecule might matter,

but when you have something like this,

where you have oxygen and hydrogen,

they don't have the
same electronegativity.

Oxygen likes to hog electrons
more than hydrogen does.

And so these electrons are not gonna spend

an even amount of time.

Here I did it kind of just drawing these,

you know, these valence
electrons as these dots.

But as we know, the electrons are in this

kind of blur around, around the,

around the actual nuclei,

around the atoms that make up the atoms.

And so, in this type of a covalent bond,

the electrons, the two electrons
that this bond represents,

are going to spend more
time around the oxygen

then they are going to
spend around the hydrogen.

And these, these two
electrons are gonna spend

more time around the oxygen,

then are going to spend
around the hydrogen.

And we know that because
oxygen is more electronegative,

and we'll talk about
the trends in a second.

This is a really important
idea in chemistry,

and especially later on as
you study organic chemistry.

Because, because we know that

oxygen is more electronegative,

and the electrons spend more time

around oxygen then around hydrogen,

it creates a partial
negative charge on this side,

and partial positive charges
on this side right over here,

which is why water has many of
the properties that it does,

and we go into much more in
depth in that in other videos.

And also when you study organic chemistry,

a lot of the likely reactions that are

going to happen can be predicted,

or a lot of the likely molecules that form

can be predicted based
on elecronegativity.

And especially when you start going

into oxidation numbers
and things like that,

electronegativity will tell you a lot.

So now that we know what
electronegativity is,

let's think a little bit about what is,

as we go through, as we start,

as we go through, as
we go through a period,

as say as we start in group one,

and we go to group, and
as we go all the way

all the way to, let's say the halogens,

all the way up to the yellow
column right over here,

what do you think is going to be

the trend for electronegativity?

And once again, one way to think about it

is to think about the extremes.

Think about sodium, and
think about chlorine,

and I encourage you to pause

the video and think about that.

Assuming you've had a go at it,

and it's in some ways the same idea,

or it's a similar idea
as ionization energy.

Something like sodium
has only one electron

in it's outer most shell.

It'd be hard for it to
complete that shell,

and so to get to a stable
state it's much easier

for it to give away that
one electron that it has,

so it can get to a stable
configuration like neon.

So this one really wants
to give away an electron.

And we saw in the video
on ionization energy,

that's why this has a
low ionization energy,

it doesn't take much
energy, in a gaseous state,

to remove an electron from sodium.

But chlorine is the opposite.

It's only one away from
completing it's shell.

The last thing it wants to
do is give away electron,

it wants an electron really,
really, really, really badly

so it can get to a configuration of argon,

so it can complete it's third shell.

So the logic here is
that sodium wouldn't mind

giving away an electron,

while chlorine really
would love an electron.

So chlorine is more
likely to hog electrons,

while sodium is very
unlikely to hog electrons.

So this trend right here,

when you go from the left to the right,

your electronegativity, let me write this,

your getting more electronegative.

More electro, electronegative, as you,

as you go to the right.

Now what do you think
the trend is going to be

as you go down, as you go down in a group?

What do you think the trend
is going to be as you go down?

Well I'll give you a hint.

Think about, think about
atomic radii, and given that,

pause the video and think about

what do you think the trend is?

Are we gonna get more
or less electronegative

as we move down?

So once again I'm assuming
you've given a go at it,

so as we know, from the
video on atomic radii,

our atom is getting larger,
and larger, and larger,

as we add more and more and more shells.

And so cesium has one electron
in it's outer most shell,

in the sixth shell,

while, say, lithium has one electron.

Everything here, all
the group one elements,

have one electron in
it's outer most shell,

but that fifty fifth electron,

that one electron in the
outer most shell in cesium,

is a lot further away then
the outer most electron

in lithium or in hydrogen.

And so because of that, it's, well one,

there's more interference
between that electron and the

nucleus from all the other
electrons in between them,

and also it's just further away,

so it's easier to kind of grab it off.

So cesium is very likely to give up,

it's very likely to give up electrons.

It's much more likely to give
up electrons than hydrogen.

So, as you go down a given group,

you're becoming less, less
electronegative, electronegative.

So what, what are, based on this,

what are going to be
the most electronegative

of all the atoms?

Well they're going to be the ones

that are in the top and the
right of the periodic table,

they're going to be these right over here.

These are going to be
the most electronegative,

Sometimes we don't think as
much about the noble gases

because they aren't, they
aren't really that reactive,

they don't even form covalent bond,

because they're just happy.

While these characters up here,

they sometimes will form covalent bonds,

and when they do, they really
like to hog those electrons.

Now what are the least electronegative,

sometimes called very electropositive?

Well these things down
here in the bottom left.

These, over here, they have only,

you know in the case of cesium,

they have one electron to give away

that would take them to a
stable state like, like xenon,

or in the case of these group two elements

they might have to give away two,

but it's much easier to give away two

then to gain a whole bunch of them.

And they're big, they're big atoms.

So those outer most electrons are getting

less attracted to the positive nucleus.

So the trend in the periodic table

as you go from the bottom left,

to the top right,

you're getting more, more
electro, electronegative.