Question

The number of unpaired electrons in $[Co(H_2O)_6]^{2+}$ is ________.

Answer 1

Correct Answer: 3

To determine the number of unpaired electrons in the complex $[Co(H_2O)_6]^{2+}$, we must first consider the properties of the cobalt ion involved. In this complex, the cobalt is in the +2 oxidation state.

Step 1: Determine the electronic configuration of Co

Cobalt (Co), with an atomic number of 27, has the electron configuration: $[\text{Ar}] 3d^7 4s^2$.

Step 2: Determine the electronic configuration of $Co^{2+}$

When Co loses two electrons to form $Co^{2+}$, these electrons are removed from the 4s orbital and one from the 3d orbital. Thus, the electron configuration for $Co^{2+}$ is: $[\text{Ar}] 3d^7$.

Step 3: Analyze the electronic configuration of $3d^7$

The $d$-orbitals can hold a maximum of 10 electrons. In the case of $3d^7$, the electrons will fill according to Hund’s rule: first singly in each orbital before any pairing occurs.

Considering the five $d$-orbitals, the arrangement is as follows (where ↑ represents an unpaired electron and ↑↓ represents a paired electron):

- Orbital 1: ↑↑
- Orbital 2: ↑↑
- Orbital 3: ↑↑
- Orbital 4: ↑
- Orbital 5: vacant

Conclusion

This configuration results in three unpaired electrons.

Hence, the number of unpaired electrons in $[Co(H_2O)_6]^{2+}$ is 3.