Question

Consider the two molecules \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\). They are both made of the same elements, and yet only one is polar. Why is this so?

Short answer

In conclusion, the CO molecule is polar due to its linear geometry and bond dipole not canceling out, whereas the CO2 molecule is nonpolar because its linear geometry results in equal and opposite bond dipoles that cancel each other out.

Step-by-step solution

Determine the Electronegativity Difference

The first step is to check the difference in electronegativity between the two elements (C and O) present in both molecules. Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. A bond can be considered polar if there is a significant difference in electronegativity between the atoms involved. In this case, the electronegativity value of Carbon (C) is 2.55, and Oxygen (O) has a value of 3.44. The electronegativity difference between C and O is \(3.44 - 2.55 = 0.89\) Since 0.89 is greater than 0.5, the bond between C and O can be considered polar.

Analyze Molecular Geometry

Next, we need to consider the molecular geometry of the given molecules to determine their overall polarity. For CO molecule: The molecule CO has two atoms bonding with each other. So it's a linear molecule, and the bond dipole doesn't cancel out. This makes the CO molecule polar. For CO2 molecule: The CO2 molecule consists of one central carbon atom bonded to two oxygen atoms. The molecule has a linear geometry, as the oxygen atoms are at 180 degrees from each other. Oxygen is more electronegative than carbon, leading to a bond dipole for each CO bond.

Check for Cancellation of Bond Dipoles

In the CO2 molecule, since the bond dipoles are equal in magnitude and opposite in direction, they cancel each other out, making the molecule nonpolar. In conclusion, CO is a polar molecule due to its bond dipole not being canceled out in its linear geometry, while CO2 is nonpolar because its linear geometry cancels out the bond dipoles.

Key concepts

Electronegativity Difference

Electronegativity difference plays a crucial role in determining molecular polarity. Electronegativity is how strongly an atom attracts electrons in a bond. When two atoms form a bond, the difference in their electronegativity values will dictate if the bond is nonpolar, polar, or ionic.

• A small difference (less than 0.5) usually indicates a nonpolar covalent bond.
• A moderate difference (0.5 to 1.7) suggests a polar covalent bond.
• A large difference (greater than 1.7) is typically indicative of an ionic bond.

For carbon monoxide (CO), the electronegativity difference between carbon (2.55) and oxygen (3.44) is 0.89, implying that the bond is polar. Both carbon and oxygen are holding onto electrons strongly, but since oxygen is more electronegative, it pulls the shared electrons closer, creating a partial negative charge on oxygen and a partial positive charge on carbon.
In carbon dioxide (CO₂), each C–O bond has the same electronegativity difference, but this bond alone doesn't determine the polarity of the whole molecule, which leads us to consider molecular geometry.

Molecular Geometry

Molecular geometry is the 3D arrangement of atoms in a molecule and heavily influences molecular polarity. The shape or structure of a molecule can cause dipoles to cancel each other out or compound to create an overall polar molecule.

• The CO molecule consists of two atoms, carbon and oxygen, arranged linearly. This lack of additional atoms means there is no opportunity for dipole cancellation.
• The CO₂ molecule has a more complex molecular geometry, though it, too, is a linear molecule. Here, the carbon atom is in the center, bonded to two oxygen atoms positioned 180 degrees apart.

In CO, the asymmetrical pull towards the oxygen atom creates a net dipole moment that is not canceled, making CO polar. Conversely, in CO₂, even though each carbon-oxygen bond is polar, its linear geometry allows the dipoles to be exactly opposite and of equal magnitude, hence they cancel each other out. Therefore, CO₂ is nonpolar.

Bond Dipole Cancellation

Bond dipole cancellation is a phenomenon where multiple bond dipoles within a molecule negate each other due to molecular symmetry, leading to a nonpolar overall structure.

• In linear molecules like CO₂, where the carbon atom is in the center bonded to two oxygen atoms, the equal and opposite bond dipoles are perfectly positioned to cancel each other out. This results in no net dipole moment and a nonpolar molecule overall.
• In contrast, in CO, there is no other bond dipole to cancel the effect of the polar C–O bond. As a result, the CO molecule remains polar.

Thus, understanding the arrangement of atoms and the symmetry in a molecule is key to predicting whether bond dipoles will cancel. CO lacks the symmetry required for cancellation, making it polar, while CO₂ achieves this due to its linear and symmetrical structure.