First ionization enthalpy generally increases across a period due to increasing nuclear charge and decreasing atomic radius. However, exceptions occur based on electron configuration stability.
For 3rd period elements, the approximate ionization enthalpy values are: Al (578), Si (787), P (1012), S (1000), Cl (1251) kJ/mol.
(1) Al has lower IE than Si because removing 3p¹ is easier than removing one electron from 3p².
(2) S has slightly lower IE than P despite being to the right because P has a half-filled p-orbital (3p³) which is more stable.
(3) Cl has the highest IE as it approaches a filled p-orbital.
The correct order is: Al < Si < S < P < Cl