Question

The correct trend in the first ionization enthalpies of the elements in the 3rd period of periodic table is :

• A. $ Al
• B. $Si
• C. $S
• D. $Al

Hint:

Whenever you see a Nitrogen vs Oxygen or Phosphorus vs Sulfur comparison in Ionization Energy, the half-filled configuration always wins!

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Ionization enthalpy generally increases across a period. However, exceptions occur due to extra stability of half-filled subshells and penetration effects of s-electrons vs p-electrons. 

Step 2: Key Formula or Approach: 
1. Across 3rd Period: $Na<Mg>Al<Si<P>S<Cl<Ar$. 
2. Exception 1: Mg (\(3s^2\)) $>$ Al (\(3p^1\)) due to fully filled s-subshell and higher penetration. 
3. Exception 2: P (\(3p^3\)) $>$ S (\(3p^4\)) due to extra stability of half-filled p-subshell in Phosphorus. 
Step 3: Detailed Explanation: 
Arranging the given elements based on these rules: - Al (13) is lowest because it's at the start of the p-block and has lower effective nuclear charge than Si. - Si (14) follows Al. - S (16) is lower than P (15) because P has a stable \(3p^3\) half-filled configuration. - Cl (17) is the highest among these as it is furthest to the right. Thus: \(Al<Si<S<P<Cl\). 
Step 4: Final Answer: 
The correct trend is $Al"