Here's why:
* Ionization energy is the energy required to remove one electron from an atom in its gaseous state.
* Factors affecting ionization energy:
* Nuclear charge: A higher nuclear charge (more protons) attracts electrons more strongly, making it harder to remove them (higher ionization energy).
* Electron shielding: Electrons in inner shells shield outer electrons from the nucleus, reducing the attraction.
* Electron configuration: Electrons in higher energy levels are further from the nucleus and easier to remove.
Let's analyze the elements:
* Boron (B): Has a smaller atomic radius and a higher effective nuclear charge compared to the other three elements. This makes it harder to remove an electron.
* Carbon (C): Similar to Boron, it has a relatively high ionization energy.
* Silicon (Si): Larger atomic radius and slightly less effective nuclear charge than Boron and Carbon.
* Aluminum (Al): Largest atomic radius out of the four elements. The outermost electron in Aluminum is further from the nucleus and shielded by more inner electrons. This makes it the easiest to remove, resulting in the lowest ionization energy.
Therefore, Aluminum has the smallest first ionization energy.
