The number of d-electrons in Fe²⁺ (atomic number Z = 26) is not equal to the number of:

Question

Accepted Answer

We compare the number of electrons in the specific orbitals for each species:

1.  **Analyze Fe²⁺ (Z=26):**
    *   Neutral Fe configuration: $[Ar] 3d^6 4s^2$.
    *   Fe²⁺ configuration (remove 2 electrons from 4s): $[Ar] 3d^6$.
    *   Number of **d-electrons = 6**.

2.  **Evaluate Options:**
    *   **Option A (Mg, Z=12):** Configuration is $1s^2 2s^2 2p^6 3s^2$. Total **s-electrons** = $2(1s) + 2(2s) + 2(3s)$ = **6**. (Equal to Fe²⁺ d-electrons).
    *   **Option B (Cl, Z=17):** Configuration is $1s^2 2s^2 2p^6 3s^2 3p^5$. Total **p-electrons** = $6(2p) + 5(3p)$ = **11**. (**Different** from Fe²⁺ d-electrons).
    *   **Option C (Fe, Z=26):** Configuration is $[Ar] 3d^6 4s^2$. Total **d-electrons** = **6**. (Equal to Fe²⁺ d-electrons).
    *   **Option D (Ne, Z=10):** Configuration is $1s^2 2s^2 2p^6$. Total **p-electrons** = **6**. (Equal to Fe²⁺ d-electrons).

Therefore, the count for Chlorine (11) is the only one that differs.

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