Which of the following has the same number of electrons as an oxide ion `( O^(2-) )` ?

Options

A. `K^(+)`
B. `Mg^(2+)`
C. `Cl^(-)`
D. `S^(2-)`

Explanation

To determine which of the following has the same number of electrons as an oxide ion \( O^{2-} \), we need to follow these steps:

### Step 1: Determine the number of electrons in the oxide ion \( O^{2-} \)
- The atomic number of oxygen (O) is 8, which means a neutral oxygen atom has 8 electrons.
- The oxide ion \( O^{2-} \) indicates that the oxygen atom has gained 2 additional electrons.
- Therefore, the total number of electrons in \( O^{2-} \) is:
\[ 8 \text{ (electrons in neutral O)} + 2 \text{ (gained electrons)} = 10 \text{ electrons} \]

### Step 2: Compare with other ions
Now we will compare the number of electrons in other ions to find which one has 10 electrons.

#### A. Potassium ion \( K^{+} \)
- The atomic number of potassium (K) is 19, meaning a neutral potassium atom has 19 electrons.
- The potassium ion \( K^{+} \) has lost 1 electron:
\[ 19 - 1 = 18 \text{ electrons} \]

#### B. Magnesium ion \( Mg^{2+} \)
- The atomic number of magnesium (Mg) is 12, meaning a neutral magnesium atom has 12 electrons.
- The magnesium ion \( Mg^{2+} \) has lost 2 electrons:
\[ 12 - 2 = 10 \text{ electrons} \]

#### C. Chlorine ion \( Cl^{-} \)
- The atomic number of chlorine (Cl) is 17, meaning a neutral chlorine atom has 17 electrons.
- The chlorine ion \( Cl^{-} \) has gained 1 electron:
\[ 17 + 1 = 18 \text{ electrons} \]

#### D. Sulfide ion \( S^{2-} \)
- The atomic number of sulfur (S) is 16, meaning a neutral sulfur atom has 16 electrons.
- The sulfide ion \( S^{2-} \) has gained 2 electrons:
\[ 16 + 2 = 18 \text{ electrons} \]

### Step 3: Conclusion
From the above calculations, we find that:
- \( O^{2-} \) has 10 electrons.
- \( K^{+} \) has 18 electrons.
- \( Mg^{2+} \) has 10 electrons.
- \( Cl^{-} \) has 18 electrons.
- \( S^{2-} \) has 18 electrons.

Thus, the ion that has the same number of electrons as the oxide ion \( O^{2-} \) is the magnesium ion \( Mg^{2+} \).

### Final Answer:
**The ion that has the same number of electrons as an oxide ion \( O^{2-} \) is \( Mg^{2+} \).**

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