Lewis structure of CH2O

The Lewis structure of CH₂O shows the distribution of electrons and bonding in formaldehyde, a compound consisting of one carbon atom, two hydrogen atoms, and one oxygen atom.^[1][2][3][4] In this structure, the carbon atom is central, forming two single bonds with the hydrogen atoms and a double bond with the oxygen atom. The oxygen atom has two lone pairs of electrons, while the central carbon atom and the hydrogen atoms have no lone pairs.^[5][6] The carbon and oxygen atoms satisfy the octet rule, while the hydrogen atoms adhere to the duet rule. The formal charges are zero for all atoms, indicating that the structure is stable.^{[7][8][9][10]}

The accurate Lewis structure of CH₂O can be drawn in five simple steps. First, calculate the total number of valence electrons and select the central atom, which is the least electronegative among the atoms in CH₂O.^[11][12] Use this information to create a rough sketch of the molecule. Next, form the initial chemical bonds between the atoms in the sketch by placing one electron pair for each bond. Then, determine the total number of lone pairs and distribute them on the atoms, starting with the outer atoms and moving to the central atom to ensure proper distribution. After that, check whether each atom adheres to the octet rule, or the duet rule in the case of hydrogen. If any atom does not have a complete octet, adjust the lone pairs as needed. Finally, calculate the formal charge for each atom, indicate these charges on the atoms, and verify whether the structure is stable.

Alternative method: CH₂O Lewis structure

Steps

Draw skeleton

In this step, first calculate the total number of valence electrons. And then, decide the central atom.

• Let’s calculate the total number of valence electrons

We know that… carbon is a group 14 element, hydrogen is a group 1 element, and oxygen is a group 16 element. Hence, carbon has four valence electrons, hydrogen has one valence electron, and oxygen has six valence electrons.

Now CH₂O has one carbon atom, two hydrogen atoms, and one oxygen atom.

So the total number of valence electrons = valence electrons of carbon atom + (valence electrons of hydrogen atom × 2) + valence electrons of oxygen atom

Therefore, the total number of valence electrons = 4 + 2 + 6 = 12

• Now decide the central atom

We can not assume hydrogen as the central atom, because the central atom is bonded with at least two other atoms. And hydrogen has only one electron in its last shell, so it can not make more than one bond.

Therefore, choose the central atom from carbon and oxygen.

The atom with the least electronegative value is placed at the center. By looking at the periodic table, we get the electronegativity values for carbon and oxygen as follows:

Electronegativity value of carbon = 2.55
Electronegativity value of oxygen = 3.44

Obviously, carbon is less electronegative than oxygen. Hence, assume that carbon is the central atom.

So now, put carbon in the center and hydrogens and oxygen on either side. And draw the rough skeleton structure for the Lewis structure of CH₂O something like this:

Also read: How to draw Lewis structure of O₂ (5 steps)

Show chemical bond

Place two electrons between the atoms to show a chemical bond. Since carbon is surrounded by two hydrogens and oxygen, use six electrons to show three chemical bonds as follows:

Mark lone pairs

As calculated earlier, we have a total of 12 valence electrons. And in the above structure, we have already used six valence electrons. Hence, six valence electrons are remaining.

Two valence electrons represent one lone pair. So six valence electrons = three lone pairs.

Note that both (carbon and oxygen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.

Also, make sure that you start marking these lone pairs on outside atoms first. And then, on the central atom.

The outside atoms are hydrogens and oxygen. But hydrogen can not keep more than 2 electrons in its last shell. Hence, don’t mark the lone pairs on hydrogen.

So oxygen will get three lone pairs. And the central atom (carbon) will not get any lone pair, because all three lone pairs are used.

Now draw the Lewis structure of CH₂O something like this:

In the above structure, you can see that the octet is completed on outside atoms. But, the central atom (carbon) doesn’t form an octet.

So in the next step, we have to complete the octet on the central atom.

Also read: How to draw Lewis structure of CO (5 steps)

Complete octet on central atom

Remember that carbon is a period 2 element, so it can not keep more than 8 electrons in its last shell.

Now carbon already has six valence electrons. Hence, carbon needs two more valence electrons to complete its octet.

So convert one lone pair from the oxygen atom to make a new bond with the carbon atom. And then, the Lewis structure of CH₂O looks something like this:

In the above structure, you can see that the octet is completed on the central atom (carbon), and also on the outside atoms. Therefore, the octet rule is satisfied.

After completing the octet, one last thing we need to do is, calculate the formal charge and check the stability of the above structure.

Also read: How to draw Lewis structure of C₂H₄ (5 steps)

Calculate formal charge and check stability

The following formula is used to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

Collect the data from the above structure and then, write it down below as follows:

• For carbon atom

Valence electrons = 4
Nonbonding electrons = 0
Bonding electrons = 8

Formal charge = 4 – 0 – ½ (8) = 0

• For each hydrogen atom

Valence electrons = 1
Nonbonding electrons = 0
Bonding electrons = 2

Formal charge = 1 – 0 – ½ (2) = 0

• For oxygen atom

Valence electrons = 6
Nonbonding electrons = 4
Bonding electrons = 4

Formal charge = 6 – 4 – ½ (4) = 0

Mention the formal charges of atoms on the structure. So the Lewis structure of CH₂O looks something like this:

In the above structure, you can see that the formal charges of atoms are closer to zero. Therefore, this is the stable Lewis structure of CH₂O.

And each horizontal line drawn in the above structure represents a pair of bonding valence electrons.

Next: Lewis structure of O₂

Related

• Lewis structure of O₂
• Lewis structure of CO
• Lewis structure of C₂H₄
• Lewis structure of BF₃
• Lewis structure of SO₃

References