Problem 80 - Chemistry: The Central Science

Question

Which of the following molecules or ions contain polar bonds: (a) \(\mathrm{P}_{4}\), (b) \(\mathrm{H}_{2} \mathrm{~S}\), (c) \(\mathrm{NO}_{2}^{-}\), (d) \(\mathrm{S}_{2}{\underline{\phantom{xx}}}^{2-}\) ?

Step-by-Step Solution

Answer

Among the given molecules and ions, \(\mathrm{H}_{2} \mathrm{~S}\) and \(\mathrm{NO}_{2}^{-}\) contain polar bonds due to the significant difference in electronegativity between their bonding atoms.

Step 1: Determine the polarity of P4

In \(\mathrm{P}_{4}\), phosphorus (P) atoms form bonds with other phosphorus atoms. Since electronegativity is the same for each phosphorus atom, there's no difference in electronegativity and therefore, it doesn't contain polar bonds.

Step 2: Determine the polarity of H2S

In \(\mathrm{H}_{2} \mathrm{~S}\), hydrogen (H) is bonded to sulfur (S). Hydrogen has an electronegativity of approximately 2.1, while sulfur has an electronegativity of 2.5. Since the difference in electronegativity values is 0.4, which is significant, the bonds between hydrogen and sulfur are polar.

Step 3: Determine the polarity of NO2-

In \(\mathrm{NO}_{2}^{-}\) (nitrite ion), there is one nitrogen (N) atom bonded to two oxygen (O) atoms. Nitrogen has an electronegativity of 3.0, and oxygen has an electronegativity of 3.5. The difference in electronegativity is 0.5, which indicates that the bonds between nitrogen and oxygen are polar.

Step 4: Determine the polarity of S2^(2-)

In \(\mathrm{S}_{2}{\underline{\phantom{xx}}}^{2-}\) (disulfide ion), the negatively charged ion consists two sulfur (S) atoms bonded together. Since both atoms are the same and have the same electronegativity, there is no difference in electronegativity, so it doesn't contain polar bonds.

Step 5: Conclusion

Among the given molecules and ions, \(\mathrm{H}_{2} \mathrm{~S}\) and \(\mathrm{NO}_{2}^{-}\) contain polar bonds due to the significant difference in electronegativity between their bonding atoms.

Key Concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. When two atoms form a bond, their respective electronegativities determine how the electrons are distributed.

• If two atoms have similar electronegativities, they share electrons more equally.
• When there's a significant difference, the electron pair is drawn more towards the atom with the higher electronegativity.

For instance, in hydrogen sulfide (H₂S), sulfur has a higher electronegativity (2.5) compared to hydrogen (2.1), leading to a polar bond. Understanding electronegativity helps predict if a bond will be polar or non-polar.
This has implications for the properties of molecules, such as solubility and reactivity.

Polar Bonds

Polar bonds occur when there's an uneven distribution of electrons between two bonded atoms. This happens due to a difference in electronegativities between the atoms.

• A common measure to identify polar bonds is an electronegativity difference greater than 0.4.
• In polar bonds, one atom becomes slightly negative, while the other becomes slightly positive.

Take nitrite ion (NO₂^-) as an example. Oxygen is more electronegative than nitrogen, creating polar bonds. Understanding polar bonds is crucial since they influence the molecular behavior and interactions such as hydrogen bonding and dipole-dipole attractions.

Molecular Polarity

Molecular polarity refers to the uneven distribution of charge across a whole molecule, not just individual bonds. For a molecule to be polar, it needs both polar bonds and an asymmetrical shape.
Consider water (H₂O), where the polar bonds along with the bent shape create a polar molecule. However, in symmetrical arrangements like \( ext{S}_2^{2-}\), even if there were polar bonds (which there are not), the symmetry could result in a non-polar molecule.

• Molecular polarity affects properties such as boiling and melting points.
• It plays a crucial role in the behavior of molecules in a chemical reaction.

By understanding the concept of molecular polarity, one can predict how molecules will interact in different environments.