Which one of the following is a different pair?

Options

A. Li, Na
B. Be, Ba
C. N, As
D. O, At

Answer

To solve the question of identifying the different pair among the given options, we will follow these steps:

### Step 1: Identify the Groups and Periods
First, we need to understand the periodic table and identify which elements belong to the same group or period.

### Step 2: Analyze Each Pair
We will analyze each pair of elements provided in the question to determine if they belong to the same group or period.

1. **Lithium (Li) and Sodium (Na)**:
- Lithium is in Group 1 (Alkali metals).
- Sodium is also in Group 1.
- **Conclusion**: Same group.

2. **Beryllium (Be) and Barium (Ba)**:
- Beryllium is in Group 2 (Alkaline earth metals).
- Barium is also in Group 2.
- **Conclusion**: Same group.

3. **Nitrogen (N) and Arsenic (As)**:
- Nitrogen is in Group 15 (Pnictogens).
- Arsenic is also in Group 15.
- **Conclusion**: Same group.

4. **Oxygen (O) and Chlorine (Cl)**:
- Oxygen is in Group 16 (Chalcogens).
- Chlorine is in Group 17 (Halogens).
- **Conclusion**: Different groups.

### Step 3: Identify the Different Pair
From the analysis, we can see that:
- Lithium and Sodium are the same group.
- Beryllium and Barium are the same group.
- Nitrogen and Arsenic are the same group.
- Oxygen and Chlorine are from different groups.

Thus, the different pair is **Oxygen and Chlorine**.

### Final Answer
The answer is **Oxygen (O) and Chlorine (Cl)**, which is the different pair.

---