Identify the incorrect formulae among the following:
(A) `CaO_(2)` (B) `Na_(2)O`
(C ) `Mg_(3)S_(2)` (D) `Mg_(2)O`
(E) `CaS`

Options

A. A, B, D
B. A, C, D
C. C, D, E
D. B, C, D

Explanation

To identify the incorrect formula among the given compounds, we need to analyze each formula based on the valency of the elements involved. Let's go through each option step by step.

### Step 1: Analyze `CaO_(2)`
- **Calcium (Ca)** has a valency of +2 (it loses 2 electrons).
- **Oxygen (O)** has a valency of -2 (it gains 2 electrons).
- The correct formula for calcium oxide is `CaO`, not `CaO_(2)`. Therefore, `CaO_(2)` is incorrect.

### Step 2: Analyze `Na_(2)O`
- **Sodium (Na)** has a valency of +1 (it loses 1 electron).
- **Oxygen (O)** has a valency of -2.
- To balance the charges, we need 2 sodium ions for every 1 oxide ion, which gives the correct formula `Na_(2)O`. This formula is correct.

### Step 3: Analyze `Mg_(3)S_(2)`
- **Magnesium (Mg)** has a valency of +2.
- **Sulfur (S)** has a valency of -2.
- The formula `Mg_(3)S_(2)` indicates that 3 magnesium ions (3 x +2 = +6) balance with 2 sulfur ions (2 x -2 = -4). This does not balance out correctly. The correct formula should be `MgS` or `Mg_(1)S_(1)`. Therefore, `Mg_(3)S_(2)` is incorrect.

### Step 4: Analyze `Mg_(2)O`
- **Magnesium (Mg)** has a valency of +2.
- **Oxygen (O)** has a valency of -2.
- The formula `Mg_(2)O` indicates that 2 magnesium ions (2 x +2 = +4) would require 2 oxide ions (2 x -2 = -4) to balance. This gives the correct formula `MgO`. Therefore, `Mg_(2)O` is incorrect.

### Step 5: Analyze `CaS`
- **Calcium (Ca)** has a valency of +2.
- **Sulfur (S)** has a valency of -2.
- The formula `CaS` indicates that one calcium ion balances with one sulfur ion, which is correct. Therefore, `CaS` is correct.

### Conclusion:
The incorrect formulae among the options are:
- (A) `CaO_(2)`
- (C) `Mg_(3)S_(2)`
- (D) `Mg_(2)O`

### Final Answer:
The incorrect formulae are `CaO_(2)`, `Mg_(3)S_(2)`, and `Mg_(2)O`.