Question

The number of unpaired electrons in the complex ion \([CoF_6]^{3–}\) is: (Atomic No.of Co = 27) 

• A. 2
• B. 3
• C. 4
• D. Zero

Answer 1

The Correct Option is C

Cobalt has an atomic number of 27. The electron configuration for a neutral cobalt atom (Co) is: 1s² 2^s2 2^p6 3s² 3p⁶ 4s² 3d⁷. 
When cobalt forms the complex \([CoF_6]^{3-}\), it loses three electrons to become \(Co^{3+}\): 1s² 2s² 2^p6 3s² 3p⁶ 4s⁰ 3d⁶. 
Now, you can count the number of unpaired electrons in the 3d orbitals. In this case, there are 4 unpaired electrons. 

So, the correct answer is (C) : 4