Final answer:
The first ionization energy of selenium is influenced primarily by its electron configuration. The presence of electron-electron repulsion in selenium's p orbital partially offsets the increasing nuclear charge, thereby affecting the ionization energy. Overall, the trend across a period shows increasing ionization energy due to a stronger effective nuclear charge.
Step-by-step explanation:
The first ionization energy of selenium changes mainly due to the electron configuration within its atomic structure. Ionization energy is the energy required to remove an electron from an atom, and several factors influence this energy, including nuclear charge and the distribution of electrons among different sub-shells. In selenium, which belongs to group 16 of the periodic table, an added electron occupies the p orbital, leading to electron-electron repulsion. This repulsion causes selenium to have a somewhat lower ionization energy than might otherwise be expected, due to the increased stability achieved by removing an electron and reducing repulsion.
As a general trend, across a period, atomic size decreases and ionization energy increases because the effective nuclear charge on the valence electrons strengthens, causing a stronger attraction between the nucleus and the electrons, which increases the ionization energy. However, exceptions occur because of specific electron configurations and the relative ease of removing electrons that experience less nuclear attraction or experience substantial electron-electron repulsion, as in the case of selenium.