Chemical formula of a molecular compound represents the actual number of atoms of different elements present in one molecule of the compound. With the help of symbols and valencies or charges, a formula can be made. First write the symbol such as positive ion to the left and negative ion to the right and then criss cross the valencies or charges . Thus in this way we can write a chemical formula.
The valency of A is 2 and B is 1, a chemical formula formed by both of them will be

Options

A. `AB_2`
B. `A_2B`
C. 2AB
D. 2BA

Explanation

To find the chemical formula of a compound formed by elements A and B, with given valencies, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Valencies**:
- The valency of element A is 2.
- The valency of element B is 1.

2. **Write the Symbols**:
- Write the symbol of the positive ion (A) on the left and the negative ion (B) on the right. This gives us:
- A (for element A) and B (for element B).

3. **Criss-Cross the Valencies**:
- Now, we will criss-cross the valencies.
- The valency of A (which is 2) becomes the subscript for B.
- The valency of B (which is 1) becomes the subscript for A.

4. **Form the Chemical Formula**:
- After criss-crossing, we get:
- A becomes A (1) and B becomes B (2).
- Therefore, the chemical formula will be written as AB₂.

5. **Final Result**:
- The chemical formula of the compound formed by A and B is **AB₂**.