Molecular Geometry, Hybridization, and Polarity of HCOOH Lewis Structure

HCOOH is also known as formic acid or methanoic acid (IUPAC name). It is the first member of the carboxylic acid family and an organic chemical. Carboxylic acids are organic molecules having the formula RCOOH, where R might be an alkyl group or a H (first member) (higher members).

In the past, formic acid was separated from the distillation of ant bodies, and it is now made industrially from methanol.

Formic acid has a molar mass of 46.03 g/mol and a boiling point of 100.8 Degrees, which is approximately identical to that of a water molecule.

Formic acid is a white liquid with a strong, pervasive stench. In water and polar solvents, it is very soluble. Both in the vapour phase and in hydrocarbons, it occurs as a hydrogen-bonded dimer.

We’ll look at formic acid chemical bonding by drawing its Lewis structure, understanding its molecular geometry, and looking at hybridization. After that, we’ll discuss formic acid’s polar character.

Let’s start with Formic acid’s Lewis Structure, HCOOH.

Lewis Structure of HCOOH

Lewis structures, also known as electron dot structures, are two-dimensional diagrams that depict the bonding electron pairs between atoms in a molecule, as well as lone pairs of electrons on an atom if they exist. Valence electrons, which are found in an atom’s outermost shell, are responsible for bonding and nonbonding.

Except for hydrogen and helium, an atom tends to connect with other atoms in such a way that every atom has eight electrons in its valence shell, according to the Lewis rule or octet rule.

The steps in drawing a Lewis structure are as follows:

Step 1: Write down the electrical configuration of the atom and count the total number of valence electrons in the molecule.

Carbon, hydrogen, and oxygen atoms have the electronic configurations [He] 2s22p2, 1s1, and [He] 2s22p4, respectively. As a result, C, H, and O have valence electrons of 4, 1, and 6, respectively.

Formic acid is made up of two hydrogen atoms and two oxygen atoms. As a result, formic acid has a total valence electron count of 4 + (1 *2) + (6 *2) = 18 electrons.

Step 2: As a centre atom, choose the least electronegative atom with the most group valence.

The greatest number of bonds that an atom can establish with other atoms is known as group valence. C, H, and O have group valances of 4, 1, and 2, respectively.

As a result, the carbon atom will play the role of a centre atom. The carbon atom in formic acid’s skeletal structure will be surrounded by H, O, and OH groups.

Step 3: Arrange all of the valence electrons in the molecule’s skeletal structure.

There are 18 valence electrons in the Lewis structure of formic acid that must be organised. First and foremost, because it is a core atom, begin with the carbon atom. According to the octet rule, the carbon atom will be surrounded by eight electrons.

It will finish the duplet of hydrogen that is directly linked to the carbon atom. The oxygen atom that is linked to the carbon atom now shares four electrons or two electron bond pairs with the carbon atom. To complete its octet, it will have two lone pairs of electrons (four electrons).

The remaining six electrons will be distributed among the carbon atom’s OH group. As a result, formic acid’s probable Lewis structure is:

A single bond will be formed by two bonding electrons, whereas a double bond will be formed by four bonding electrons. As a result, the Lewis structure of formic acid is as follows:

A simple representation of the molecule is the Lewis structure. It provides no information on the molecule’s shape or the atom’s hybridization in the molecule. The valence shell electron pair repulsion (VSEPR) hypothesis and the valance bond theory are required for this (VBT).

Molecular Geometry of HCOOH

The VSEPR theory can predict the molecular geometry or form. It addresses the valence shell electron repulsions between bonding and nonbonding (lone pair) electrons.

The carbon atom is a central atom in the Lewis structure of formic acid, with three bond pairs and no lone pair of electrons. The double bond is handled as one bond pair in VSEPR theory for predicting the shape of the molecule.

As a result, the following table may easily predict the structure of formic acid.

General formulaNumber of bond pairsMolecular shape/geometry
AX33Trigonal planar
AX55Trigonal bipyramidal

Because it has two lone pairs and two bond pairs, formic acid will have a trigonal planar geometry around the carbon atom and a tetrahedral geometry around the oxygen atom.

To reduce bond pair-bond pair repulsions around the carbon atom, the trigonal planar geometry of formic acid around the carbon atom should result in a bond angle (H-C-O or O-C-O) of 120°.

The actual bond angle is slightly different than 120° because the double bond pair and single bond pair repel each other more strongly. As a result, the bond angles between H-C=O and O=C-O are larger than 120°.

As a tetrahedral geometry, the bond angle around the oxygen atom, i.e. C-O-H, should be 109.5 °. The C-O-H bond angle, on the other hand, is 106 degrees to reduce repulsion between two lone pairs on the oxygen atom.

The following figure depicts the bond lengths as calculated by an x-ray diffractometer:

The valence bond theory (VBT) and steric number can be used to determine the hybridization of the carbon and oxygen atoms in formic acid.

Hybridization of HCOOH

Hybridization is the process of combining atomic orbitals to generate equivalent-energy hybrid orbitals. The total number of hybrid orbitals equals the total number of atomic orbitals. To establish a covalent bond, the resultant hybrid orbital overlaps with the hybrid orbitals of other atoms or with atomic orbitals.

In formic acid, the hybridization of the carbon atom can be calculated in the following way:

The electrical configuration of the carbon atom in its ground state is [He] 2s22p2. One of the electrons from the 2s orbital will excite to the carbon atom’s 2p orbital, resulting in the [He] 2s12p3 excited state configuration.

Because the carbon atom forms three sigma bonds with other atoms, one of the carbon atom’s two 2s orbitals and two 2p orbitals will mix to form three sp2 hybrid orbitals, while one of the p orbitals will stay unhybridized, forming a pi bond with the oxygen atom. The sigma bonds are represented by the orbital diagram of formic acid, which is seen below.

The carbon atom is sp2 hybridised, and one of the oxygen atoms is also sp2 hybridised, whilst another oxygen atom coupled to hydrogen and carbon atom is sp3 hybridised, as can be seen in the orbital diagram.

The steric number or the methods described above can also be used to determine the hybridization and shape.

The sum of the number of atoms linked to a certain atom and the number of lone pairs of electrons at the same atom is the steric number.

Hybridization will be sp, sp2, and sp3, respectively, if the steric number is 2, 3, or 4. The steric number has been used to explain the hybridization of distinct atoms in formic acid in the table below.

Polarity of HCOOH

The polarity of a molecule is determined by its net dipole moment and charge distribution. The hydrogen atom, carbon atom, and oxygen atom have electronegativity values of 2.5, 2.2, and 3.5, respectively.

In formic acid, the carbon atom is the most important. The C-H bond electronegativity difference is 2.5 – 2.2 = 0.3, whereas the C-O and O-H bonds have electronegativity differences of 1.0 and 1.3, respectively.

As a result, the C-H bond has a minor polarity. Polar covalent bonds include the O-H and C-O bonds. With a partial positive charge on carbon and a partial negative charge on oxygen, it forms a dipole in C-O bonds. In the same way, the O-H bond is dipole. The polar character of formic acid is the result of this.

Because the charges in formic acid are not equally distributed, it is classified as a polar molecule. Because of its polar nature, it is extremely soluble in water and most polar solvents.


Formic acid is a pungent-smelling chemical substance. It is the first carboxylic acid in the family.

We’ve looked at the fundamental features of formic acid in terms of bonding.

The carbon atom is a central atom in formic acid. In the Lewis structure of formic acid, all atoms except hydrogen follow the octet rule. Trigonal planar and sp2 hybridization are the molecular geometry and hybridization of the centre carbon atom, respectively. Because formic acid is a polar molecule, it can be dissolved in polar liquids.

Please feel free to ask any questions you have about formic acid’s bonding properties.

Thank you very much.

Good luck with your studies.

Read more: Structure, Geometry, Hybridization, and Polarity in OCS Lewis

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.


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