Final answer:
Phosphorus has a lower electron affinity than silicon because it has a higher ionization energy.
Step-by-step explanation:
The electron affinity of phosphorus is lower than silicon because phosphorus has a higher ionization energy than silicon. The electron affinity is the energy change that occurs when an atom gains an electron. Higher ionization energy means it is more difficult to remove an electron from an atom. Since phosphorus has a higher ionization energy, it is less likely to gain an electron and therefore has a lower electron affinity than silicon.