(a) Draw a dot diagram for \(\mathrm{NH}_{3}\) and one for \(\mathrm{PH}_{3}\). (b) Is either molecule polar? (The electronegativities of \(\mathrm{N}, \mathrm{P}\), and \(\mathrm{H}\) are \(3.0,2.1\), and 2.1, respectively.) (c) In which substance are the London forces stronger? Explain. (d) Basing your answer solely on the London forces in the two substances, which substance would you expect to have the higher boiling point? Explain.

To draw the dot diagrams, we need to consider the valence electrons of the atoms involved in the molecules (Nitrogen, Phosphorus, and Hydrogen). For NH3: 1. Nitrogen (N) has 5 valence electrons. 2. Hydrogen (H) has 1 valence electron. 3. The molecule has three Hydrogen atoms and one Nitrogen atom, so the total count of valence electrons becomes 5 + 3*(1) = 8. Place the Nitrogen atom in the center and surround it with three Hydrogen atoms. Then, distribute the valence electrons in such a way that hydrogen completes 1 valence electron and nitrogen completes 8, which is the number of electrons required for a stable shell structure. For PH3: 1. Phosphorus (P) has 5 valence electrons. 2. Hydrogen (H) has 1 valence electron. 3. Repeat the process stated for NH3 but replace Nitrogen with Phosphorus as the central atom.

Polarity depends on the difference in electronegativities between bonded atoms. Calculate the electronegativity difference between Nitrogen and Hydrogen and between Phosphorus and Hydrogen, using the electronegativity values provided (N: 3.0, P: 2.1, H: 2.1). For NH3: Electronegativity difference between N and H: \(|3.0 - 2.1| = 0.9\). For PH3: Electronegativity difference between P and H: \(|2.1 - 2.1| = 0\). In general, a difference greater than 0.5 is considered sufficient for a bond to be considered polar. Thus, NH3 is a polar molecule, whereas PH3 is nonpolar.

London forces are temporary attractive forces that result from the random distribution of electrons in molecules. Larger molecules with more electrons usually experience stronger London forces. NH3 has 10 electrons: 7 from Nitrogen and 3 from Hydrogens. PH3 has 14 electrons: 5 from Phosphorus and 3 from Hydrogens. Since PH3 has more electrons, the London forces are stronger in PH3 than in NH3.

As mentioned earlier, London forces are generally stronger in larger molecules with more electrons. Substances with stronger London forces usually have higher boiling points because more energy is required to overcome these forces. Since PH3 has stronger London forces than NH3, we would expect PH3 to have a higher boiling point due to London forces alone.