Here's why:
* Ionization Energy: The amount of energy required to remove one electron from a neutral gaseous atom in its ground state.
* Trends in the Periodic Table: Ionization energy generally increases as you move:
* Across a Period (left to right): This is because the effective nuclear charge increases, pulling electrons closer to the nucleus and making them harder to remove.
* Up a Group (top to bottom): This is because the outermost electrons are further from the nucleus and experience less attraction.
Since fluorine is directly above chlorine in the periodic table (in the same group), it has a higher first ionization energy.
