Atomic numbers of Cr and Fe and 24 and 26 respectively, Which of the following is paramagnetic with the spin of electrons ?

Options

A. `[Fe(CN)-6]^(4-)`
B. `[Cr(CO)_6]`
C. `[Cr(NH_3)_6]^(3+)`
D. `[Fe(CO)_5]`

Answer

To determine which of the given elements (Cr or Fe) is paramagnetic based on their electron configurations and oxidation states, we can follow these steps:

### Step 1: Identify the Atomic Numbers
- Chromium (Cr) has an atomic number of 24.
- Iron (Fe) has an atomic number of 26.

### Step 2: Write the Electron Configurations
- The electron configuration for Chromium (Cr) is:
\[ \text{Cr: } [Ar] 3d^5 4s^1 \]
- The electron configuration for Iron (Fe) is:
\[ \text{Fe: } [Ar] 3d^6 4s^2 \]

### Step 3: Determine the Oxidation States
- For the question, we need to consider the oxidation states:
- For Iron (Fe), we will consider the +2 oxidation state (Fe²⁺).
- For Chromium (Cr), we will consider the +3 oxidation state (Cr³⁺).

### Step 4: Adjust the Electron Configurations for Oxidation States
- For Fe²⁺ (Iron in +2 state):
- Remove 2 electrons from the outermost shell (4s first, then 3d):
\[ \text{Fe}^{2+}: [Ar] 3d^6 \]

- For Cr³⁺ (Chromium in +3 state):
- Remove 3 electrons (1 from 4s and 2 from 3d):
\[ \text{Cr}^{3+}: [Ar] 3d^3 \]

### Step 5: Analyze the Electron Configurations for Unpaired Electrons
- For Fe²⁺ (3d⁶):
- The distribution of electrons in the 3d subshell is:
\[ \uparrow \uparrow \uparrow \uparrow \uparrow \downarrow \]
- There are 4 unpaired electrons in the 3d subshell.

- For Cr³⁺ (3d³):
- The distribution of electrons in the 3d subshell is:
\[ \uparrow \uparrow \uparrow \]
- There are 3 unpaired electrons in the 3d subshell.

### Step 6: Conclusion on Paramagnetism
- A substance is paramagnetic if it has unpaired electrons.
- Both Fe²⁺ and Cr³⁺ have unpaired electrons:
- Fe²⁺ has 4 unpaired electrons.
- Cr³⁺ has 3 unpaired electrons.

Thus, both Cr³⁺ and Fe²⁺ are paramagnetic.

### Final Answer
Both Chromium (Cr) in +3 oxidation state and Iron (Fe) in +2 oxidation state are paramagnetic.