Electronic configuration of a transition element X in +3 oxidation state is [Ar] $3d^{5}.$ What is its atomic number?

Question

Accepted Answer

To determine the atomic number of the transition element X with the electronic configuration [Ar] $3d^{5}$ in its +3 oxidation state, we need to understand the electronic configuration of transition elements and how oxidation states affect them.

1. **Explanation for Option 3 (Correct Answer):**
   - The given electronic configuration in the +3 oxidation state is [Ar] $3d^{5}$. This means that in its neutral state, the element X would have lost 3 electrons to achieve this configuration.
   - Transition elements typically lose electrons from the 4s orbital before the 3d orbital when they form positive ions. Therefore, the neutral atom of element X would have the configuration [Ar] $3d^{6} 4s^{2}$.
   - Adding the electrons back to the configuration for the neutral atom gives us [Ar] $3d^{6} 4s^{2}$, which corresponds to the element iron (Fe) with an atomic number of 26.
   - Therefore, the atomic number of element X is 26, which matches Option 3.

2. **Explanation for why other options are incorrect:**

- **Option 1 (Atomic number 20):**
     - An atomic number of 20 corresponds to calcium (Ca), which is not a transition element and does not have a partially filled d-subshell in its ground state. Calcium has the configuration [Ar] $4s^{2}$, and in a +3 oxidation state, it would not have a $3d^{5}$ configuration.

- **Option 2 (Atomic number 23):**
     - An atomic number of 23 corresponds to vanadium (V), which in its neutral state has the configuration [Ar] $3d^{3} 4s^{2}$. In a +3 oxidation state, vanadium would have the configuration [Ar] $3d^{2}$, not $3d^{5}$.

- **Option 4 (Atomic number 27):**
     - An atomic number of 27 corresponds to cobalt (Co), which in its neutral state has the configuration [Ar] $3d^{7} 4s^{2}$. In a +3 oxidation state, cobalt would have the configuration [Ar] $3d^{6}$, not $3d^{5}$.

In conclusion, the only element that fits the given electronic configuration in the +3 oxidation state is iron (Fe) with an atomic number of 26, making Option 3 the correct answer.