Question

Is it possible for a molecule to be nonpolar even though it contains polar covalent bonds? Explain.

Short answer

Yes, a molecule can be nonpolar even if it contains polar covalent bonds, provided that the molecule has a symmetrical shape causing the dipole moments to cancel out, as in the case of carbon dioxide (CO2).

Step-by-step solution

Understanding Molecular Polarity

Molecular polarity is determined not just by the presence of polar covalent bonds, but also by the shape of the molecule and the orientation of those bonds. For a molecule to be nonpolar, the dipole moments of its polar covalent bonds must cancel out due to the molecule's symmetry.

Analyzing Geometric Configuration

To evaluate if a molecule with polar covalent bonds can be nonpolar, one must consider the molecule’s geometry. If the molecule has a symmetrical geometric configuration, such as linear, tetrahedral, or trigonal planar, it is possible for the polarities of the individual bonds to cancel each other out.

Examining Molecule Examples

Take carbon dioxide (CO2) as an example. It contains two polar covalent bonds between the carbon and the oxygen atoms. However, CO2 is linear, and the two dipole moments are equal in magnitude but opposite in direction, therefore they cancel each other out, making CO2 a nonpolar molecule.

Key concepts

Polar Covalent Bonds

When we speak of polar covalent bonds, we're referring to a type of chemical bond where electrons are shared unequally between two atoms. This unequal sharing results from one atom having a stronger pull on the electrons—known as electronegativity—than the other atom involved in the bond. For instance, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms, pulling the shared electrons closer to itself.

These differences in electronegativity give rise to partial charges, with the more electronegative atom gaining a partial negative charge, denoted as δ-, and the less electronegative atom acquiring a partial positive charge, δ+. The result is a polar molecule with a positive end and a negative end—reminiscent of a magnet with its north and south poles. It's important to understand that not every molecule with polar covalent bonds is polar; the overall molecular geometry can influence the polarity as a whole.

Molecular Geometry

Delving into molecular geometry, we find it's a significant factor in determining molecular polarity. The three-dimensional arrangement of atoms within a molecule can lead to an equal or unequal distribution of charges, depending on the symmetry. Symmetrical molecules, like methane (CH₄), exhibit a uniform distribution of charges where the effect of one polar bond is balanced by others placed opposite to it.

However, asymmetrical molecules such as ammonia (NH₃) have an unequal charge distribution, making them polar. Understanding molecular geometry is essential because it can affect not only a molecule's reactivity but also its physical properties like boiling and melting points. To predict molecular geometry, scientists often use the VSEPR theory (Valence Shell Electron Pair Repulsion theory), which posits that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion.

Dipole Moments

Diving deeper into molecular characteristics, we encounter the concept of dipole moments. A dipole moment is a vector quantity that measures the extent of the charge separation in a molecule, essentially quantifying polarity. Higher dipole moments reflect greater polarity. These dipole moments occur due to differences in electronegativity as discussed, and they have both a magnitude and a direction—from the δ+ (positive) to the δ- (negative) end.

In symmetrical molecules, even though dipole moments might be present in the bonds, they cancel each other out, leading to a net dipole moment of zero; thus, the molecule is nonpolar. This is the case with carbon dioxide (CO₂), mentioned in the exercise solution. On the other hand, in a molecule like water, the V-shaped geometry and the two hydrogen atoms at an angle result in an overall dipole moment, reinforcing water's status as a polar molecule.