Name: Worked example: Lewis diagram of formaldehyde (CH₂O) | AP Chemistry | Khan Academy
Uploaded: 2020-03-27T17:39:52.000Z
Duration: 6 min 8 s
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- [Instructor] What we're gonna do in this video

to construct the Lewis diagram for formaldehyde.

Formaldehyde has one carbon, two hydrogens,

Try to construct a valid Lewis structure,

Now the first step, and we saw this in a previous video,

we want to think about all of the valence electrons

is so that while we're trying to create this structure,

we are making use of all of the valence electrons.

And to figure out how many total valence electrons we have,

we can look at a periodic table of elements.

it's in that second row, in that second period,

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

And then, oxygen, it also is in the second period,

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

And so the total valence electrons in this molecule

are gonna be four plus two, which is six, plus six,

Now the next step is to try to draw a structure.

draw single bonds, I'll say, single bonds.

what do we think is going to be our central atom?

And the rule of thumb is the least electronegative atom,

is a good candidate for our central atom.

So between carbon and oxygen, we know that oxygen

is one of the most electronegative atoms,

well one of the most electronegative elements

And so carbon is a good candidate for the central atom.

and then let's put these other atoms around it.

So, let's put our oxygen right over there,

So I've just used two, four, six valence electrons.

I am now left with six valence electrons,

allocate the remaining valence electrons,

for atoms that are not hydrogen, and then for hydrogen,

trying to get it to have two valence electrons.

All right, so let's start with this oxygen.

This oxygen already has these two electrons

So in order to get to the octet rule, it needs six more.

Well I've just used up the remaining six valence electrons.

So I don't really have any more to play with,

but let's see how the other atoms are feeling.

So hydrogen here, it's able to share these two electrons

that are in this covalent bond, so it's feeling good.

It can kind of pretend that it has a full outer shell,

that first shell, that's filled with two electrons.

are feeling like they have a full outer shell.

So, let's focus on the central atom, central atom,

or do we need to do something interesting here?

it's able to have two, four, six electrons

hanging around it, but it would love to have eight.

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Worked example: Lewis diagram of formaldehyde (CH₂O) | AP Chemistry | Khan Academy

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