Question

What factors determine whether a molecule is polar or nonpolar? (Chapter 8\()\)

Short answer

A molecule's polarity is determined by two factors: electronegativity differences and molecular geometry. Electronegativity is the ability of an atom to attract electrons in a bond, and a difference greater than 0.4 typically results in a polar bond. Molecular geometry affects the polarity of the molecule; a symmetrical shape results in a nonpolar molecule, while an asymmetrical shape makes the molecule polar. To determine the overall molecular polarity, both electronegativity differences and molecular geometry need to be evaluated.

Step-by-step solution

Electronegativity Differences

Electronegativity is the ability of an atom to attract electrons in a bond. In a molecule, if there are atoms with different electronegativities, the electrons will not be shared equally between them. This results in a polar bond, which has a positive and negative end. To determine whether a bond is polar or not, one can compare the electronegativity values of the atoms involved. A difference in electronegativity greater than 0.4 usually results in a polar bond.

Molecular Geometry

The arrangement of atoms in space contributes to the polarity of the molecule. In a molecule composed of polar bonds, the molecular geometry can either reinforce or cancel out the individual bond polarities. If the molecule has a symmetrical shape (e.g., linear, trigonal planar, tetrahedral, octahedral), the bond polarities will cancel each other out, resulting in a nonpolar molecule. If the molecule has an asymmetrical shape (e.g., bent, trigonal pyramidal), the bond polarities will not cancel each other out, making the molecule polar.

Overall Molecular Polarity

To determine whether a molecule is polar or nonpolar, one needs to evaluate both the electronegativity differences and the molecular geometry. A molecule with symmetrical shape and zero net bond polarity is nonpolar. A molecule with asymmetrical shape and a nonzero net bond polarity is polar.

Example: Carbon Dioxide (CO2)

Let's analyze the polarity of carbon dioxide (CO2). It has two polar bonds between carbon and oxygen, with oxygen being more electronegative than carbon. However, the linear geometry of the CO2 molecule ensures that the polarities of the two C=O bonds cancel each other out. As a result, CO2 is a nonpolar molecule.

Example: Water (H2O)

Now let's analyze the polarity of water (H2O). It has two polar bonds between oxygen and hydrogen, with oxygen being more electronegative than hydrogen. Since water has a bent molecular shape, the bond polarities do not cancel out, making H2O a polar molecule.

Key concepts

Electronegativity Differences

Electronegativity is a crucial concept in understanding molecular polarity. It refers to an atom's ability to attract and hold onto electrons within a chemical bond. This property varies amongst different elements. When atoms with dissimilar electronegativities form a bond, electrons tend to favor the atom with the higher electronegativity. This results in a polar bond, characterized by a region of partial negative charge (near the more electronegative atom) and a region of partial positive charge.
Differences in electronegativity are measured numerically, and a difference greater than 0.4 typically indicates a polar bond. For instance, in a water molecule (H₂O), oxygen is significantly more electronegative than hydrogen, which leads to a separation of charges and hence, polar bonds.
Understanding electronegativity differences is the first step in determining if a molecule might be polar but not the sole criterion.

Molecular Geometry

The shape of a molecule is another key factor affecting its polarity, known as molecular geometry. Even if a molecule contains polar bonds, the overall geometry might cause these bonds to cancel out, leading to a nonpolar molecule.
To better understand this, consider the VSEPR theory (Valence Shell Electron Pair Repulsion Theory), which helps predict the 3D arrangement of atoms within a molecule. **Symmetrical geometries** (like linear, trigonal planar, and tetrahedral structures) generally lead to nonpolar molecules because the dipoles cancel each other out.
On the other hand, **asymmetrical geometries** (such as bent and trigonal pyramidal shapes) often prevent the cancellation of dipoles. Therefore, molecules with these shapes tend to be polar. A perfect example is water (H₂O), which has a bent shape, causing the polar bonds' dipoles to add up rather than cancel, resulting in a polar molecule.

Polar and Nonpolar Molecules

The polarity of a molecule arises from both electronegativity differences and its molecular geometry. A molecule becomes **polar** when it has polar bonds with an overall asymmetrical shape, ensuring that the partial charges do not offset each other. This results in a molecule having distinct positive and negative poles.
Conversely, a molecule is **nonpolar** if it has either nonpolar bonds or polar bonds that are arranged symmetrically, causing the charges to cancel out.
Here are key points to consider:

• Molecules like **carbon dioxide** (CO₂) are nonpolar, despite having polar bonds, due to their symmetrical linear shape.
• Molecules like **water** (H₂O) are polar because they have an asymmetrical shape that doesn't cancel out the polar bonds' effects.

Understanding the delicate interplay between electronegativity and molecular geometry will allow you to predict and explain the behavior of molecules concerning their interaction with electric fields and solubility in various solvents.