H2O (water) is a polar molecule, while CO2 (carbon dioxide) is non-polar, primarily due to their molecular geometries and the resulting net effect of their individual bond polarities.
While both molecules contain polar bonds, their overall shapes dictate whether these bond dipoles cancel each other out or contribute to a net dipole moment. A polar molecule is defined by having a non-zero net dipole moment.
Understanding Polarity: Bonds vs. Molecules
It's crucial to distinguish between bond polarity and molecular polarity:
- Bond Polarity: This arises from the difference in electronegativity between two bonded atoms. When electrons are unequally shared, one atom develops a slight negative charge (δ-) and the other a slight positive charge (δ+), creating a dipole. Both CO2 and H2O molecules contain polar bonds.
- Molecular Polarity: This refers to the overall polarity of the entire molecule. It depends on both the polarity of individual bonds and the molecule's three-dimensional shape, which determines how these individual bond dipoles are oriented and if they cancel each other out.
The Case of CO2 (Carbon Dioxide)
Carbon dioxide (CO2) is a non-polar molecule, despite having two polar carbon-oxygen (C=O) bonds.
- Bond Polarity: Oxygen is more electronegative than carbon, so each C=O bond is polar, with electron density pulled towards the oxygen atoms.
- Molecular Geometry: CO2 has a linear molecular geometry. The central carbon atom is bonded to two oxygen atoms in a straight line (180° bond angle).
- Dipole Cancellation: Because the two polar C=O bonds are identical in strength and pull in opposite directions, their individual dipole moments perfectly cancel each other out. This symmetrical arrangement results in a zero net dipole moment, making CO2 a non-polar molecule.
The Case of H2O (Water)
Water (H2O) is a polar molecule.
- Bond Polarity: Oxygen is significantly more electronegative than hydrogen, making each oxygen-hydrogen (O-H) bond highly polar, with electron density shifted towards the oxygen atom.
- Molecular Geometry: H2O has a bent molecular geometry (approximately 104.5° bond angle). This bent shape is due to the two lone pairs of electrons on the oxygen atom, which exert repulsion and push the O-H bonds closer together.
- Net Dipole Moment: Due to its bent shape, the individual O-H bond dipoles do not cancel out. Instead, they add up vectorially, resulting in a significant net dipole moment pointing towards the oxygen atom. This non-zero net dipole moment makes H2O a polar molecule.
Summary Comparison: CO2 vs. H2O
The following table summarizes the key differences contributing to their polarity:
| Feature | Carbon Dioxide (CO2) | Water (H2O) |
|---|---|---|
| Bond Polarity | Has two polar C=O bonds | Has two polar O-H bonds |
| Molecular Geometry | Linear | Bent |
| Symmetry | Symmetrical | Asymmetrical |
| Cancellation of Dipoles | Yes, dipoles cancel out | No, dipoles do not cancel out |
| Net Dipole Moment | Zero (μ = 0 D) | Non-zero (μ ≠ 0 D) |
| Overall Polarity | Non-polar | Polar |
Understanding molecular geometry is fundamental to predicting a molecule's polarity, which in turn influences its physical and chemical properties, such as solubility, boiling point, and intermolecular forces. For more information on molecular shapes, explore resources on VSEPR theory.
