Formal charge | Molecular and ionic compound structure and properties | AP Chemistry | Khan Academy

- [Instructor] In this video, we're going

to introduce ourselves to
the idea of formal charge,

and as we will see, it
is a tool that we can use

as chemists to analyze molecules.

It is not the charge on
the molecule as a whole,

it's actually a number
that we can calculate

for each of the individual
atoms in a molecule,

and as we'll see in future videos,

it'll help us think about
which resonance structures,

which configurations of a
molecule will contribute most

to a resonance hybrid.

So before going too deep into that,

let's just give ourselves a
definition for formal charge,

and then as practice,
we're going to calculate

the formal charge on the various atoms

in each of these resonance
structures for nitrous acid.

These are both legitimate Lewis diagrams.

They're both legitimate
resonance structures

for nitrous acid, but we'll think about

which one contributes more
to the resonance hybrid

based on formal charge.

So the definition of formal charge,

and we're going to do this
for each atom in our molecule,

for each atom, we're going
to calculate the number

of valence electrons in free,

in free neutral,

neutral atom,

atom.

From that, we are going
to subtract the number

of valence electrons allocated,

allocated to bonded,

bonded atom.

And so you're next question is,

what does is mean to be allocated?

Well, I will break up this
definition a little bit.

So if we want to think
about the valence electrons

that are allocated to a bonded atom,

these are going to be the
number of lone pair electrons,

number of lone pair electrons

plus one half of the
number of shared electrons.

So lets try and make sense of this

by applying this
definition of formal charge

to the constituents of nitrous acid.

So let's start with
this hydrogen over here.

So what's the number of valence electrons

in a free, neutral atom of hydrogen?

Well we've seen this multiple times,

you could look at this on the
periodic table of elements,

free neutral hydrogen
has one valence electron.

Now how many valence electrons

are allocated to the bonded atom?

Well one way to think about it is,

draw a circle around that
atom in the molecule,

and you want to capture
all of the lone pairs,

and you want to capture,
you can think of it

as half the bond, you
could say for each bond,

it's going to be one electron

'cause it's half of the shared electrons,

each bond is two shared electrons,

but you're gonna say half of those,

and then you have no lone pairs over here,

so the number of valence electrons
allocated to bonded atom,

in the case of hydrogen here, is one,

and so we are dealing with a formal charge

of zero for this hydrogen.

Now what about this oxygen here?

Well we do the same exercise,

I like to draw a little
bit of a circle around it.

And so the number of valence electrons

in a free, neutral oxygen
we've seen multiple times,

that is six, and then from that,

we're going to subtract the
number of valence electrons

allocated to the bonded atom.

So the bonded atom has
two lone pair electrons,

and then it gets half
of the shared electrons,

so half of the shared electrons
would be one from this bond,

one from that bond,
and one from that bond.

So you add them all together,
two, three, four, five.

So six minus five is
equal to positive one,

and so the formal charge
on this oxygen atom,

in this configuration of
nitrous acid is positive one.

Now what about the nitrogen?

Well we'll do a similar exercise there.

A free neutral nitrogen
has five valence electrons,

we've seen that multiple times,

you can look at that from the
periodic table of elements,

and then from that,
we're going to subtract

the number of valence electrons

allocated to the bonded to nitrogen,

well we see one, two, three,

and then two more lone pair
electrons, so that is five,

and so you have zero formal charge there.

And then let's look at this last oxygen.

So this last oxygen, a free neutral oxygen

has six valence electrons,

from that, we're going
to subtract the number

of valence electrons
allocated to the bonded atom,

so two, four, six lone pair electrons,

plus half of this bond, so that's seven

allocated valence electrons,

six minus seven equals negative one.

So this oxygen has a formal
charge of negative one,

and I really want to remind you,

we're not talking about the
charge of the entire molecule,

formal charge is really
a mathematical tool

we use to analyze this configuration,

but one way you can kind
of conceptualize it is,

in this configuration,
this oxygen on average

has one more electron hanging around it,

one more valence electron
hanging around it

than a free neutral oxygen would.

This oxygen has one less valence
electron hanging around it

than a neutral free oxygen would.

Now let's look at this
configuration down here,

well this hydrogen is
identical to this hydrogen,

it has no lone pair electrons

and it just has one
covalent bond to an oxygen,

so we would do the same analysis

to get that its formal charge is a zero,

but now let's think about
this oxygen right over here.

A free neutral oxygen has
six valence electrons,

the number of valence
electrons allocated to this one

is two, four, five, and six,

so six minus six is
zero, no formal charge,

and we go to this nitrogen.

Free nitrogen has five valence electrons,

this nitrogen has two, three, four,

five valence electrons allocated to it,

so minus five, it has zero formal charge.

And then last but not least,
this oxygen right over here.

A free neutral oxygen has
six valence electrons,

this one has two, four,
five, six valence electrons

allocated to the bonded atom,

and so minus six is equal to zero.

And so what we see is
this first configuration,

or you could say this
first resonance structure

for nitrous acid had some formal charge,

it had a plus one on this oxygen

and minus one on this oxygen,

while the one down here
had no formal charge,

everything had a formal charge of zero,

and as we'll see in future videos,

the closer the individual atom
formal charges are to zero,

the more likely that that structure,

that resonance structure,

will contribute more to
the resonance hybrid,

but we'll talk about that
more in future videos,

the whole pint of this one

is just to get comfortable
calculating formal charge

for the individual atoms in a molecule.