Answer:
The first ionization energy refers to the energy required to remove one electron from an atom in the gas phase. In this case, we are comparing the first ionization energies of arsenic (As) and gallium (Ga).
As has a higher first ionization energy than Ga due to its higher effective nuclear charge, smaller atomic radius, and greater electron shielding. These factors contribute to the stronger attraction between the nucleus and the valence electrons in As, making it more difficult to remove an electron from As compared to Ga.