Question

What is effective atomic number of cobalt in \([Co(NH_3)_6]^{3+}\) if \(Co(Z = 27)\)?

• A. 30
• B. 33
• C. 27
• D. 36

Hint:

EAN rule often gives the total electrons around metal. For \([Co(NH_3)_6]^{3+}\), EAN = 36 matches the next noble gas (Kr).

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
Effective atomic number (EAN) = atomic number of metal – oxidation state + 2 × number of ligands (for neutral ligands like NH\(_3\)).

Step 2: Key Formula or Approach:
EAN = Z – (oxidation state) + 2 × (number of monodentate ligands). For NH\(_3\) (neutral), each donates 2 electrons.

Step 3: Detailed Explanation:
Co Z = 27. In \([Co(NH_3)_6]^{3+}\), oxidation state of Co = +3 (since NH\(_3\) neutral, overall +3). Electrons from metal after oxidation: 27 – 3 = 24. Six NH\(_3\) ligands donate 6 × 2 = 12 electrons. Total = 24 + 12 = 36.

Step 4: Final Answer:
EAN = 36, option (D).