Why does I2 have a higher boiling point than F2? And why does NH3 have a higher boiling point than PH3?
NH3 has a higher b.p. than PH3 because of hydrogen bonding. The "old" answer for the higher b.p. I2 over F2 was higher molar mass; the "new" answer is increased "intermolecular" (London) forces.
To understand why I2 has a higher boiling point than F2, and why NH3 has a higher boiling point than PH3, we need to consider the intermolecular forces present in these molecules.
Boiling point is essentially a measure of the strength of the forces holding the molecules together in the liquid phase. The stronger the intermolecular forces, the higher the boiling point.
In the case of I2 versus F2, both molecules are diatomic, consisting of two atoms. However, the difference in electronegativity between iodine (I) and fluorine (F) is significant. Fluorine is highly electronegative, meaning it strongly attracts electrons towards itself. On the other hand, iodine is less electronegative.
The difference in electronegativity leads to a significant polarity difference between I2 and F2. Fluorine is so electronegative that it attracts electron density towards itself, creating a highly polar molecule. As a result, the positive end of one F2 molecule is strongly attracted to the negative end of another.
In contrast, I2 has a much smaller electronegativity difference, resulting in a nonpolar molecule. Without a significant polarity difference, the intermolecular forces in I2 are weaker. Consequently, a higher temperature is needed to break these forces, resulting in a higher boiling point compared to F2.
Moving on to NH3 versus PH3, both molecules are comprised of a central atom bonded to three hydrogen atoms. However, the difference lies in the central atom – nitrogen (N) versus phosphorus (P).
Nitrogen is more electronegative than phosphorus, so the N-H bond in NH3 is more polar than the P-H bond in PH3. This polarity difference creates stronger dipole-dipole interactions in NH3 compared to PH3. Dipole-dipole interactions are intermolecular forces that occur between polar molecules.
Therefore, NH3 has stronger intermolecular forces due to its more significant polarity, resulting in a higher boiling point compared to PH3.