Phosphorus has a higher ionization energy than silicon because: 1. the outer orbitals of phosphorus are located closer to the nucleus than the outer orbitals of silicon. 2. phosphorus has a lower ionization energy than silicon. 3. the outer orbitals of phosphorus are located farther from the nucleus than the outer orbitals of silicon. 4. the effective nuclear charge of phosphorus is less than that of silicon. 5. the effective nuclear charge of silicon is less than that of phosphorus.

Option (1) is the correct answer.

Explanation:

Ionization energy is defined as the energy required to remove the most loosely bound electron from a neutral gaseous atom.

With increase in atomic size of the atom, there will be less force of attraction between the nucleus and the valence electrons of the atom. Hence, with lesser amount of energy the valence electrons can be removed.

When we move across a period then due to increase in force of attraction the size of atom decreases.

Thus, we can conclude that Phosphorus has a higher ionization energy than silicon because the outer orbitals of phosphorus are located closer to the nucleus than the outer orbitals of silicon.