Which of the following reactions involves the combination of two elements?

Options

A. `CaO+CO_(2)toCaCO_(3)`
B. `2Na+1/2O_(2)toNa_(2)O`
C. `SO_(2)+1/2O_(2)toSO_(3)`
D. `NH_(3)+HCl toNH_(4)Cl`

Explanation

To determine which of the given reactions involves the combination of two elements, we need to analyze each option based on the definitions of elements and compounds.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Element**: A pure substance that consists of only one type of atom. Examples include Sodium (Na) and Oxygen (O).
- **Compound**: A substance formed when two or more different types of atoms combine chemically. Examples include Calcium Oxide (CaO) and Carbon Dioxide (CO2).

2. **Analyze Each Reaction**:
- **Option 1: CaO and CO2**:
- CaO (Calcium Oxide) is a compound made from Calcium (Ca) and Oxygen (O).
- CO2 (Carbon Dioxide) is a compound made from Carbon (C) and Oxygen (O).
- Since both are compounds, this option does not involve the combination of two elements.

- **Option 2: Sodium + ½ Oxygen → Na2O**:
- Sodium (Na) is an element.
- Oxygen (O) is also an element.
- Here, Sodium and Oxygen are combining to form a compound (Na2O), but they are both elements before the reaction. Thus, this reaction involves the combination of two elements.

- **Option 3: SO2**:
- SO2 (Sulfur Dioxide) is a compound made from Sulfur (S) and Oxygen (O).
- This option does not involve the combination of two elements since it is a compound.

- **Option 4: Ammonia (NH3) and HCl**:
- Ammonia (NH3) is a compound made from Nitrogen (N) and Hydrogen (H).
- HCl (Hydrochloric Acid) is a compound made from Hydrogen (H) and Chlorine (Cl).
- Both are compounds, so this option does not involve the combination of two elements.

3. **Conclusion**:
- The only reaction that involves the combination of two elements is **Option 2: Sodium + ½ Oxygen → Na2O**.

### Final Answer:
The reaction that involves the combination of two elements is **Sodium + ½ Oxygen → Na2O**.