The general electronic configuration of outer most and penultimate shell is given as `(n-1)s^2(n-1)p^6(n-1)d^x ns^2`. Then for an element with `n = 4` and `x = 7`
The element is isoelectronic with which of the follow ing species? 

Options

A. `Fe^(-)`
B. `N i^(+)`
C. `Cu^(2+)`
D. All of these.

Answer

To solve the problem, we need to determine the electronic configuration of the element based on the provided general electronic configuration and then find which species is isoelectronic with it.

### Step-by-Step Solution:

1. **Identify the given values**:
- We have \( n = 4 \) and \( x = 7 \).

2. **Write the general electronic configuration**:
- The general electronic configuration is given as:
\[ (n-1)s^2(n-1)p^6(n-1)d^x ns^2 \]
- Substituting \( n = 4 \) and \( x = 7 \):
\[ (4-1)s^2(4-1)p^6(4-1)d^7 4s^2 \]
- This simplifies to:
\[ 3s^2 3p^6 3d^7 4s^2 \]

3. **Count the total number of electrons**:
- From the configuration:
- \( 3s^2 \) contributes 2 electrons.
- \( 3p^6 \) contributes 6 electrons.
- \( 3d^7 \) contributes 7 electrons.
- \( 4s^2 \) contributes 2 electrons.
- Total electrons:
\[ 2 + 6 + 7 + 2 = 17 \text{ electrons} \]

4. **Determine the atomic number**:
- The atomic number of the element is equal to the total number of electrons, which is 27.

5. **Identify the element**:
- The element with atomic number 27 is Cobalt (Co).

6. **Find isoelectronic species**:
- Isoelectronic species have the same number of electrons.
- Cobalt (Co) has 27 electrons.
- We can look for ions that also have 27 electrons:
- **Fe²⁺** (Iron ion): Iron (Fe) has an atomic number of 26, and when it loses 2 electrons, it has 26 electrons. However, Fe³⁺ has 25 electrons.
- **Cu²⁺** (Copper ion): Copper (Cu) has an atomic number of 29, and when it loses 2 electrons, it has 27 electrons.
- **Mn⁴⁺** (Manganese ion): Manganese (Mn) has an atomic number of 25, and when it loses 4 electrons, it has 21 electrons.
- **Fe³⁺** (Iron ion): Iron (Fe) has an atomic number of 26, and when it loses 3 electrons, it has 23 electrons.
- The ion that is isoelectronic with Co is **Fe²⁺**.

### Conclusion:
The element with \( n = 4 \) and \( x = 7 \) is Cobalt (Co), and it is isoelectronic with **Fe²⁺**.