Question

The hardest lanthanide element is

• A. Sm
• B. La
• C. Gd
• D. Dy
• E. Yb

Hint:

Half-filled configurations (like 4f$^7$) give extra stability → higher hardness.

Answer 1

The Correct Option is C

Concept: Hardness in lanthanides depends on:
• Number of unpaired electrons
• Strength of metallic bonding
• Lanthanide contraction

Step 1: Electronic configuration.
Gd: \[ [Xe]\,4f^7 5d^1 6s^2 \]

Step 2: Special stability.

• Half-filled 4f subshell (4f$^7$)
• Maximum exchange energy
• Strong metallic bonding

Step 3: Effect.

• Strong bonding → higher hardness
• Greater resistance to deformation

Step 4: Comparison.
Other lanthanides either:
• Have fewer unpaired electrons
• Or weaker bonding

Step 5: Final answer. \[ \boxed{Gd} \]