The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a carbon-12 atom taken as 12 units. The molecular mass of a substance indicates the number of times one molecule of a substance is heavier than `(1)/(12)` th of C-12 atom. It is equal to the sum of atomic masses of all the atoms present in a molecule. Depending on the number of atoms of same or different elements present in the molecule, it can be monoatomic, diatomic, triatomic, tetraatomic or a poly-atomic molecule.
Carbon dioxide, hydrogen sulphide, calcium chloride and sodium oxide are examples of

Options

A. monoatomic
B. diatomic
C. triatomic
D. tetra-atomic

Explanation

To solve the question regarding the molecular mass and atomicity of the given substances, we will analyze each substance step by step.

### Step 1: Identify the Molecular Formulas
We start by identifying the molecular formulas of the substances mentioned: carbon dioxide, hydrogen sulfide, calcium chloride, and sodium oxide.

1. **Carbon Dioxide**: The molecular formula is CO₂, which consists of 1 carbon atom and 2 oxygen atoms.
2. **Hydrogen Sulfide**: The molecular formula is H₂S, which consists of 2 hydrogen atoms and 1 sulfur atom.
3. **Calcium Chloride**: The molecular formula is CaCl₂, which consists of 1 calcium atom and 2 chlorine atoms.
4. **Sodium Oxide**: The molecular formula is Na₂O, which consists of 2 sodium atoms and 1 oxygen atom.

### Step 2: Determine the Atomicity
Next, we determine the atomicity of each substance. Atomicity refers to the number of atoms in a molecule.

1. **Carbon Dioxide (CO₂)**:
- Total atoms = 1 (C) + 2 (O) = 3 atoms (Triatomic)

2. **Hydrogen Sulfide (H₂S)**:
- Total atoms = 2 (H) + 1 (S) = 3 atoms (Triatomic)

3. **Calcium Chloride (CaCl₂)**:
- Total atoms = 1 (Ca) + 2 (Cl) = 3 atoms (Triatomic)

4. **Sodium Oxide (Na₂O)**:
- Total atoms = 2 (Na) + 1 (O) = 3 atoms (Triatomic)

### Step 3: Classify the Molecules
Based on the atomicity calculated, we can classify the molecules:

- **Carbon Dioxide**: Triatomic
- **Hydrogen Sulfide**: Triatomic
- **Calcium Chloride**: Triatomic
- **Sodium Oxide**: Triatomic

### Final Summary
All four substances are triatomic molecules, as each contains three atoms in total.