`Be^(2+)` is isoelectronic with which of the following ions ?

Options

A. `H^+`
B. `Li^(+)`
C. `Na^(+)`
D. `Mg^(2+)`.

Answer

To determine which ion is isoelectronic with \( \text{Be}^{2+} \), we need to follow these steps:

### Step 1: Determine the number of electrons in \( \text{Be}^{2+} \)
- Beryllium (Be) has an atomic number of 4, which means it has 4 electrons in its neutral state.
- Since \( \text{Be}^{2+} \) has a \( +2 \) charge, it means that 2 electrons have been removed from the neutral beryllium atom.
- Therefore, the number of electrons in \( \text{Be}^{2+} \) is:
\[ 4 - 2 = 2 \text{ electrons} \]

### Step 2: Check the number of electrons in the given ions
We will check each of the provided ions to see if they have 2 electrons.

1. **For \( \text{H}^{+} \)**:
- Hydrogen has an atomic number of 1, which means it has 1 electron.
- \( \text{H}^{+} \) has lost 1 electron, so it has:
\[ 1 - 1 = 0 \text{ electrons} \]
- Not isoelectronic with \( \text{Be}^{2+} \).

2. **For \( \text{Li}^{+} \)**:
- Lithium has an atomic number of 3, which means it has 3 electrons.
- \( \text{Li}^{+} \) has lost 1 electron, so it has:
\[ 3 - 1 = 2 \text{ electrons} \]
- Isoelectronic with \( \text{Be}^{2+} \).

3. **For \( \text{Na}^{+} \)**:
- Sodium has an atomic number of 11, which means it has 11 electrons.
- \( \text{Na}^{+} \) has lost 1 electron, so it has:
\[ 11 - 1 = 10 \text{ electrons} \]
- Not isoelectronic with \( \text{Be}^{2+} \).

4. **For \( \text{Mg}^{2+} \)**:
- Magnesium has an atomic number of 12, which means it has 12 electrons.
- \( \text{Mg}^{2+} \) has lost 2 electrons, so it has:
\[ 12 - 2 = 10 \text{ electrons} \]
- Not isoelectronic with \( \text{Be}^{2+} \).

### Step 3: Conclusion
From the analysis above, the only ion that is isoelectronic with \( \text{Be}^{2+} \) is \( \text{Li}^{+} \).

### Final Answer:
\( \text{Li}^{+} \) is isoelectronic with \( \text{Be}^{2+} \).

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