Question

In each case, which of the following pairs of bonded elements forms the more polar bond? a. \(\mathrm{S}-\mathrm{F}\) or \(\mathrm{S}-\mathrm{Cl}\) b. \(\mathrm{N}-\mathrm{O}\) or \(\mathrm{P}-\mathrm{O}\) c. \(\mathrm{C}-\mathrm{H}\) or \(\mathrm{Si}-\mathrm{H}\)

Short answer

The more polar bonds in each pair are: a. \(\mathrm{S}-\mathrm{F}\) b. \(\mathrm{P}-\mathrm{O}\) c. \(\mathrm{C}-\mathrm{H}\)

Step-by-step solution

Electronegativity values

Consult an electronegativity table to find the values for each element: S = 2.58, F = 3.98, Cl = 3.16, N = 3.04, O = 3.44, P = 2.19, C = 2.55, H = 2.20, Si = 1.90

Calculate electronegativity differences

Determine the electronegativity differences for each bond: a. \(\Delta_{SF} = |3.98-2.58|=1.40\), \(\Delta_{SCl} = |3.16-2.58|=0.58\) b. \(\Delta_{NO} = |3.44-3.04|=0.40\), \(\Delta_{PO} = |3.44-2.19|=1.25\) c. \(\Delta_{CH} = |2.55-2.20|=0.35\), \(\Delta_{SiH} = |2.20-1.90|=0.30\)

Compare electronegativity differences

Compare the differences and choose the bond with the higher value in each pair: a. \(\mathrm{S}-\mathrm{F}\) has a higher electronegativity difference than \(\mathrm{S}-\mathrm{Cl}\) (1.40 > 0.58), so the \(\mathrm{S}-\mathrm{F}\) bond is more polar. b. \(\mathrm{P}-\mathrm{O}\) has a higher electronegativity difference than \(\mathrm{N}-\mathrm{O}\) (1.25 > 0.40), so the \(\mathrm{P}-\mathrm{O}\) bond is more polar. c. \(\mathrm{C}-\mathrm{H}\) has a higher electronegativity difference than \(\mathrm{Si}-\mathrm{H}\) (0.35 > 0.30), so the \(\mathrm{C}-\mathrm{H}\) bond is more polar.

Key concepts

Electronegativity Difference

When we speak of electronegativity difference, we're referring to the contrast in electronegativity values of two atoms bonded together. Electronegativity itself is a measure of an atom's ability to attract and hold onto electrons when it forms a chemical bond. The higher the electronegativity, the greater an atom's pull on the shared electrons.

In chemical bonds, when two atoms have different electronegativity values, the shared electrons are more attracted to the atom with the higher electronegativity. This causes an unequal sharing of electrons, leading to the formation of a polar bond. The greater the difference, the more polar the bond. For instance, in the exercise provided, sulfur (S) has an electronegativity of 2.58 while fluorine (F) is at 3.98, resulting in a significant difference (abla_{SF} = 1.40) indicating a highly polar bond. Comparatively, the difference between sulfur and chlorine (abla_{SCl} = 0.58) is smaller, suggesting a less polar bond.

Chemical Bonding

At the heart of every chemical compound lies chemical bonding, the force that holds atoms together. Bonds can range from ionic, where electrons are transferred from one atom to another, to covalent, where they are shared. Within the covalent category, we encounter two types: nonpolar and polar.

Nonpolar covalent bonds occur between atoms with identical or very similar electronegativity values, leading to an equal sharing of electrons. Conversely, polar covalent bonds, as seen in the textbook exercise, involve an unequal sharing due to a significant electronegativity difference. The result of this inequality is a molecule with a positive end (near the less electronegative atom) and a negative end (near the more electronegative atom), much like a magnet with its north and south poles.

Molecular Polarity

The concept of molecular polarity takes into account not just the polarity of individual bonds but also the shape and orientation of these bonds within the molecule. It's the symmetry, or lack thereof, that ultimately dictates whether a molecule is polar or nonpolar. A molecule with polar bonds can still be nonpolar overall if the structure is symmetrical, causing the polarities to cancel out.

Take, for example, carbon dioxide (CO2), which contains polar C=O bonds. Due to its linear shape, the molecule is nonpolar because the dipole moments oppose each other and cancel out. However, when polar bonds are not symmetrically arranged, as in water (H2O), the uneven distribution of charges leads to a polar molecule. Understanding molecular polarity is crucial because it affects properties like solubility and boiling point, and it also plays a significant role in the biological activity of molecules in the body.