Question
The correct trend in the first ionization enthalpies of the elements in the 3^ rd period of periodic table is :
JEE Main
Chemistry
Classification of Elements and Periodicity in Properties
2026
JEE Main 2026 (23 January Shift 1)
JEE Main Chemistry Question (2026) — Solution
Options
- A. Si < S < Al < P < Cl
- B. Al < S < P < Si < Cl
- C. Al < Si < S < P < Cl
- D. S < Si < Al < P < Cl
Answer
C. Al < Si < S < P < Cl
Step-by-step solution
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
Related: Chemistry — Classification of Elements and Periodicity in Properties · All PYQ Banks