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

Answer: (C)

4

Answer 2

Cobalt has an atomic number of 27. The electron configuration for a neutral cobalt atom (Co) is: 1s2 2s2 2p6 3s2 3p6 4s2 3d7.
When cobalt forms the complex $[CoF_6]^{3-}$, it loses three electrons to become $Co^{3+}$: 1s2 2s2 2p6 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