Define first ionisation energy - Leaving Cert Chemistry - Question b - 2014

The first ionisation energy of silicon is greater than that of aluminium because of the greater nuclear charge in silicon. As the number of protons in the nucleus increases, it exerts a stronger force on the electrons, making them harder to remove. This results in a higher ionisation energy.

The first ionisation energy of silicon is less than that of carbon due to its larger atomic radius. As silicon has more electron shells than carbon, its outermost electron is farther away from the nucleus. This distance results in reduced electrostatic attraction, thus requiring less energy to remove the outermost electron.