In which of the following the valency of each of the constituent elements is equal to the total number of atoms in one molecule of the compound?

Options

A. HCl
B. `H_(2)S`
C. CaO
D. `MgCl_(2)`

Explanation

To solve the question, we need to analyze each of the given compounds and determine if the valency of each constituent element is equal to the total number of atoms in one molecule of the compound.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are HCl, H2S, CaO, and MgCl2.

2. **Analyze HCl**:
- **Number of Atoms**: HCl consists of 1 Hydrogen (H) atom and 1 Chlorine (Cl) atom. Thus, the total number of atoms = 1 + 1 = 2.
- **Valency**: The valency of Hydrogen is 1, and the valency of Chlorine is also 1.
- **Comparison**: The total number of atoms (2) does not equal the valency of both elements (1 + 1 = 2), but since we need each valency to match the total number of atoms, this does not satisfy the condition.

3. **Analyze H2S**:
- **Number of Atoms**: H2S consists of 2 Hydrogen (H) atoms and 1 Sulfur (S) atom. Thus, the total number of atoms = 2 + 1 = 3.
- **Valency**: The valency of Hydrogen is 1, and the valency of Sulfur is 2.
- **Comparison**: The total number of atoms (3) does not equal the sum of the valencies (1 + 2 = 3), and again, this does not satisfy the condition.

4. **Analyze CaO**:
- **Number of Atoms**: CaO consists of 1 Calcium (Ca) atom and 1 Oxygen (O) atom. Thus, the total number of atoms = 1 + 1 = 2.
- **Valency**: The valency of Calcium is 2, and the valency of Oxygen is 2.
- **Comparison**: The total number of atoms (2) does not equal the valency of both elements (2 + 2 = 4), so this does not satisfy the condition.

5. **Analyze MgCl2**:
- **Number of Atoms**: MgCl2 consists of 1 Magnesium (Mg) atom and 2 Chlorine (Cl) atoms. Thus, the total number of atoms = 1 + 2 = 3.
- **Valency**: The valency of Magnesium is 2, and the valency of Chlorine is 1.
- **Comparison**: The total number of atoms (3) does not equal the sum of the valencies (2 + 1 = 3), and thus this does not satisfy the condition.

6. **Conclusion**: After analyzing all the compounds, we find that none of the compounds meet the criteria where the valency of each constituent element is equal to the total number of atoms in one molecule of the compound.

### Final Answer:
None of the given compounds satisfy the condition.