Molecular Geometry Of CO2: How Lewis Structure Predicts Shape Of Molecules

The molecular geometry of a compound provides insight into its physical properties, chemical properties, and reactivity. To help you understand how and why carbon dioxide (CO₂) behaves the way it does, I have put together this article on its molecular geometry.

The molecular geometry of CO₂ explains other properties like its polarity. It covers the arrangement of atoms, electron pairs, and bond angles.

However, to fully understand the shape, bond length, and bond angles of a molecule, you must first understand its hybridization and Lewis structure.

Although the Lewis structure does not determine molecular geometry, it can help predict the structure of the molecule. Continue reading to understand better.

Properties of CO₂

• Carbon dioxide, CO₂ is a molecule with one carbon atom and two oxygen atoms. It has a molar mass of 44 g/mol
• At room temperature, it is a colorless and odorless gas. However, at high concentrations, the gas has a sharp and acidic smell
• It is a non-flammable gas, a slightly toxic gas that is denser than air and can displace oxygen. It has a density of 1.977 g/mol
• CO₂ has a melting point -68.08°F (-56.6°C) and a boiling point of -109.3°F (-78.5°C)
• CO₂ is soluble in water. It dissolves in water to form carbonic acid. But its solubility decreases with increasing temperature
• It turns lime water milky and blue litmus red

Lewis structure of CO₂

The Lewis dot structure of CO₂ shows the distribution of electrons around the molecule and will help predict its shape. Here’s how to draw the Lewis dot structure of CO₂:

• The carbon atom will be the center of the structure because it has the least electronegativity, and the oxygen atoms will be on the sides
• Afterward, use the valence electrons to distribute the bonds. The carbon atom has 4 valence electrons, whereas each oxygen atom has 6 valence electrons
• Write out the atoms using their symbols (C & O). Use dots to represent the valence electrons on each atom. Put four dots around the carbon atom and six dots around each oxygen atom
• The octet rule still stands even in molecules. But, with this arrangement, carbon does not obey the octet rule. Offset this by taking one lone pair of electrons from each oxygen atom to form a covalent bond
• By now, all atoms should have eight electrons in their outermost shells
• Then, check for the stability of each atom by calculating its formal charge. The lesser the formal charge, the more stable the atom
• The physical representation of the Lewis dot structure of CO₂ is:

What is the molecular geometry of CO₂?

CO₂ has a molecular geometry. According to the VSEPR theory, the shape of a molecule can be predicted by considering the electron pair repulsion in the valence shell.

From the Lewis structure of CO₂, there are two sigma bonds and no lone pair of electrons. The bonding pair of electrons around the central carbon atom will repel each other and cause the oxygen atoms to push far to the ends of the molecule.

This repulsion gives the CO₂ molecule its linear molecular geometry and shape. It maintains this shape to keep the repulsion minimal. As a linear molecule, CO₂ has a bond angle of 180° and each C=O bond has a bond length of 116.3 pm.

What is the hybridization of CO₂?

Following hybridization rules, the hybridization of the entire molecule is determined by the hybridization of the central atom. The central atom here is carbon and it has a sp hybridisation which becomes the hybridisation of the whole CO₂ molecule.

In the ground state, carbon has an electronic configuration of 1s²2s²2p². In its excited state, the electronic configuration becomes 1s²2s¹2p³. The three p-orbitals are 2p_x¹2p_y¹2p_z¹, each having one electron.

The 2s and 2p_x orbitals hybridize to form the sp hybrid orbital and the 2p_y and 2p_z orbitals are left unhybridized.

On the other hand, each oxygen atom has an electronic configuration of 1s²2s²2p⁴. When excited, it becomes 1s²2s²2p³3s¹. It forms three sp² hybrid orbitals with its 2s orbital and two of the three 2p orbitals.

Each of the two of the sp² hybrid orbitals of the oxygen atoms has a lone pair of electrons which are the unbonded pairs seen on the oxygen atoms.

The third sp² hybrid orbital of both oxygen atoms forms a sigma bond with the central carbon atom while the unhybridized pz orbitals of the oxygen atoms form pi bonds with the p orbital of the carbon atom.

What is the electron geometry of CO₂?

CO₂ has a linear electron geometry. The electron geometry is the same as the molecular geometry because there are no lone pair-bond pair repulsions that can alter the linear shape of the molecule.

Is CO₂ a polar or non-polar molecule?

CO₂ is a non-polar molecule against the polarity of the C=O bonds in the molecule. Three factors contribute to the lack of polarity of CO₂. They are electronegativity, dipole moment, and molecular geometry.

According to the electronegativity guidelines for determining polarity, a bond is polar if the electronegativity difference is greater than 0.4 and non-polar if the difference is less than 0.4.

In the CO₂ molecule, oxygen (3.44) is more electronegative than carbon (2.55). They have an electronegativity difference of 0.89 which justifies the polarity of the C=O bonds. Each of these bonds has dipole moments because of their polarity.

However, the molecular geometry of the overall molecule comes in and neutralizes the polarity of the C=O bonds. The symmetrical arrangement of the atoms makes the dipole moments on both sides cancel out each other.

This gives the overall molecule zero net dipole moment and makes it non-polar.

FAQs

Why is CO₂ linear and H₂O is bent?

The presence of a lone pair of electrons in H₂O and its absence in CO₂ is a major reason for their different shapes despite their seemingly similar appearance.

According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the shape of a molecule is determined by the repulsion between the electron pair in the valence shell of the central atom.

The bent shape of the H₂O molecule is due to the lone pair electrons, which have a stronger repulsion on the oxygen atom. The lone pair repulsion is higher than the bond pair repulsion. As a result, it pulls the oxygen atom and changes it to a V-shaped or bent structure.

On the other hand, the CO₂ does not have any lone pair electrons and the repulsion between the bonds in the molecule is equal. This explains its linear structure.

Does CO₂ have coordinate covalent bonds?

No, CO₂ does not have coordinate covalent bonds because none of the atoms donate two electrons to the shared pair. Coordinate covalent bonds are a type of covalent bond in which only one atom contributes the two electrons in the shared pair that makes the bond.

Does CO₂ obey the octet rule?

CO₂ obeys the octet rule. The central carbon atom and the two oxygen atoms have eight electrons surrounding each of them. From the Lewis structure, each of the oxygen atoms shares four electrons with the central carbon and has two lone pairs of electrons.

Conclusion

After calculating the total number of valence electrons around the CO₂ molecule, it is easier to distribute them around the carbon and oxygen atoms and get an appropriate Lewis structure. The Lewis structure of CO₂ shows it has a linear molecular geometry with a 180° bond angle.

The C=O bonds in the carbon dioxide molecule are polar. However, the overall molecule lacks polarity because the dipole moments from the polar bonds cancel out each other due to the symmetrical arrangement of the atoms.

Here’s how molecular geometry and electronegativity explain the lack of polarity of methane, CH₄.

Thanks for reading.