Q. 12 Which among the following has the least second ionization energy?
(A) Al
(B) Si
(C) P
(D) S
Second ionization energy measures the energy needed to remove a second electron from a gaseous +1 ion. For these period 3 elements, Mg requires the lowest energy at 1450.7 kJ/mol due to its electron configuration after the first ionization.
Answer Key
(D) Mg
Mg⁺ (losing an electron from 3s¹) has the lowest second IE at 1450.7 kJ/mol, compared to Al (1816.7 kJ/mol), Si (1577.1 kJ/mol), and P (1907 kJ/mol).
Option Analysis
(A) Al
Al loses its first 3p¹ electron easily (first IE: 577.5 kJ/mol), forming Al⁺ ([Ne] 3s²). The second IE removes from stable full 3s², requiring high energy (1816.7 kJ/mol).
(B) Si
Si⁺ ([Ne] 3s² 3p¹) loses a 3p electron for second IE (1577.1 kJ/mol), higher than Mg but lower than Al due to less stable p orbital vs. full s².
(C) P
P⁺ ([Ne] 3s² 3p²) removes from less stable 3p² (1907 kJ/mol), higher than Si and Al due to increasing effective nuclear charge across the period.
(D) Mg
Mg⁺ ([Ne] 3s¹) removes from half-filled 3s¹ to stable [Ne] configuration (1450.7 kJ/mol), lowest as it avoids breaking a full subshell unlike others.
Introduction to Second Ionization Energy Trends
Second ionization energy trends across period 3 elements like Al, Si, P, and Mg depend on electron configurations post-first ionization and nuclear charge effects. Mg shows the least second ionization energy due to forming a stable noble gas configuration easily.
Detailed Comparison Table
| Element | Configuration (Neutral) | +1 Ion Config. | Second IE (kJ/mol) | Reason for Value |
|---|---|---|---|---|
| Al | [Ne] 3s² 3p¹ | [Ne] 3s² | 1816.7 | Removes from stable 3s² |
| Si | [Ne] 3s² 3p² | [Ne] 3s² 3p¹ | 1577.1 | 3p electron easier than full s² |
| P | [Ne] 3s² 3p³ | [Ne] 3s² 3p² | 1907 | Higher Zeff in p orbitals |
| Mg | [Ne] 3s² | [Ne] 3s¹ | 1450.7 | Lowest; achieves [Ne] stability |
Why Mg Has Least Second Ionization Energy
Mg’s second ionization forms Mg²⁺ ([Ne]), a stable noble gas structure from 3s¹, requiring minimal energy compared to Al’s full 3s² breach or Si/P’s p-orbital removals amid rising nuclear pull. This exception highlights subshell stability over general left-to-right IE increase.
CSIR NET Exam Tips
For competitive exams, memorize period 3 second IE order: Mg < Si < Al < P, focusing on stability jumps like full/half-filled subshells. Practice with values from reliable tables for quick MCQ solving.
