CHO2- Lewis structure

CHO₂^– (formate) has one carbon atom, one hydrogen atom, and two oxygen atoms.

In the CHO₂^– Lewis structure, there are two single bonds and one double bond around the carbon atom, with one hydrogen atom and two oxygen atoms attached to it. The oxygen atom with a single bond has three lone pairs, and the oxygen atom with a double bond has two lone pairs.

Also, there is a negative (-1) charge on the oxygen atom with a single bond.

Alternative method: Lewis structure of CHO₂^–

Rough sketch

• First, determine the total number of valence electrons

In the periodic table, carbon lies in group 14, hydrogen lies in group 1, and oxygen lies in group 16.

Hence, carbon has four valence electrons, hydrogen has one valence electron, and oxygen has six valence electrons.

Since CHO₂^– has one carbon atom, one hydrogen atom, and two oxygen atoms, so…

Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of one hydrogen atom = 1 × 1 = 1
Valence electrons of two oxygen atoms = 6 × 2 = 12

Now the CHO₂^– has a negative (-1) charge, so we have to add one more electron.

So the total valence electrons = 4 + 1 + 12 + 1 = 18

Learn how to find: Carbon valence electrons, Hydrogen valence electrons, and Oxygen valence electrons

• Second, find the total electron pairs

We have a total of 18 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 18 ÷ 2 = 9

• Third, determine the central atom

Here hydrogen can not be 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.

Now we have to choose the central atom from carbon and oxygen. Place the least electronegative atom at the center.

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

Therefore, place carbon in the center and hydrogen and oxygen on either side.

• And finally, draw the rough sketch

Lone pair

Here, we have a total of 9 electron pairs. And three bonds are already marked. So we have to only mark the remaining six electron pairs as lone pairs on the sketch.

Also remember 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.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogen and oxygens. But no need to mark on hydrogen, because hydrogen already has two electrons.

So for each oxygen, there are three lone pairs, and for carbon, there is zero lone pair because all six electron pairs are over.

Mark the lone pairs on the sketch as follows:

Formal charge

Use the following formula to calculate the formal charges on atoms:

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

For carbon atom, formal charge = 4 – 0 – ½ (6) = +1

For hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

For each oxygen atom, formal charge = 6 – 6 – ½ (2) = -1

Here, both carbon and oxygen atoms have charges, so mark them on the sketch as follows:

The above structure is not a stable Lewis structure because both carbon and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

Convert a lone pair of the oxygen atom to make a new C — O bond with the carbon atom as follows:

Final structure

The final structure of CHO₂^– features a central carbon atom connected to a hydrogen atom and two oxygen atoms. Within this configuration, the carbon atom forms a single bond with the hydrogen and a single bond with one oxygen, while establishing a double bond with the second oxygen to satisfy its octet. The double-bonded oxygen retains two lone pairs, and the single-bonded oxygen atom fulfills its octet by maintaining three lone pairs. This specific layout is the most stable because it allows for resonance, where the double bond character is shared equally between the two oxygen atoms. Therefore, this electronic distribution serves as the definitive and most accurate Lewis representation of the formate ion.

To complete the representation, draw square brackets around the entire Lewis structure and place a “-” or “-1” sign as a superscript outside the upper right bracket. This notation signifies that the negative charge is a property of the whole ion.

Next: SeF₄ Lewis structure

External video

• HCO2- Lewis Structure: How to Draw the Lewis Structure for HCO2- – YouTube • Wayne Breslyn