Final answer:
Phosphorus has a higher ionization energy than silicon because it is further to the right in the same period and has a more stable electron configuration.
Step-by-step explanation:
The element with the higher ionization energy between silicon (Si) and phosphorus (P) is phosphorus. Ionization energy tends to increase across a period from left to right due to the increasing positive charge of the nucleus, which more strongly attracts electrons and requires more energy to remove an electron. Both silicon and phosphorus are found in the same period of the periodic table, but phosphorus is to the right of silicon, indicating that it has a higher ionization energy. Additionally, phosphorus has a half-full p subshell, which is a particularly stable configuration, making its electrons more difficult to remove.