Using the Modern periodic table, predict the formules as compounds which might be formed by the following pairs of elements : (i) Boron & Oxygen (ii) Calcium & Fluorine.

Answer

To predict the formulas of compounds formed by the pairs of elements Boron & Oxygen and Calcium & Fluorine using the modern periodic table, we follow these steps:

### Step 1: Identify the Group and Valency of Each Element

1. **Boron (B)**:
- Boron is located in Group 13 of the periodic table.
- The valency of Boron is +3.

2. **Oxygen (O)**:
- Oxygen is located in Group 16 of the periodic table.
- The valency of Oxygen is -2.

### Step 2: Determine the Compound Formula for Boron and Oxygen

3. **Crisscross Method**:
- We take the valency of Boron (+3) and place it as a subscript for Oxygen.
- We take the valency of Oxygen (-2) and place it as a subscript for Boron.
- This gives us B₂O₃ as the formula for the compound formed between Boron and Oxygen.

### Step 3: Identify the Group and Valency of the Second Pair of Elements

4. **Calcium (Ca)**:
- Calcium is located in Group 2 of the periodic table.
- The valency of Calcium is +2.

5. **Fluorine (F)**:
- Fluorine is located in Group 17 of the periodic table.
- The valency of Fluorine is -1.

### Step 4: Determine the Compound Formula for Calcium and Fluorine

6. **Crisscross Method**:
- We take the valency of Calcium (+2) and place it as a subscript for Fluorine.
- We take the valency of Fluorine (-1) and place it as a subscript for Calcium.
- This gives us CaF₂ as the formula for the compound formed between Calcium and Fluorine.

### Final Answer
- The compound formed by Boron and Oxygen is **B₂O₃**.
- The compound formed by Calcium and Fluorine is **CaF₂**.

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