The knowledge of valencies of various radicals helps us to write the formulae of chemical compounds. The total positive charge on positive ions ( cations ) is equal to the total negative charge on negative ions ( anions) in a molecule. Therefore, in writing the formula of a compound, the positive and negative ions are adjusted in such a way that the total number of positive charges of positive ions ( cations ) becomes equal to the total number of negative charges of negative ions ( anions) . There is another simple method for writing the formulae of ionic compounds. In this method, the valencies ( or positive or negative charges ) of the ions can be 'crossed over' to give subscripts. The purpose of crossing over of charges is to find the number of ions required to equalise the number of positive and negative charges.
The formula of a compound is `X_(3)Y`. The valencies of elements X and Y will be respectively.

Options

A. 1 and 3
B. 3 and 1
C. 2 and 3
D. 3 and 2

Explanation

To determine the valencies of elements X and Y in the compound represented by the formula \(X_3Y\), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Formula**: The given formula is \(X_3Y\). This indicates that there are 3 atoms of element X and 1 atom of element Y in the compound.

2. **Identify the Charges**: In ionic compounds, the total positive charge from cations must equal the total negative charge from anions. Here, we can assume that element X is a cation and element Y is an anion.

3. **Assign the Valency**:
- The subscript of X in the formula \(X_3Y\) is 3, which suggests that the valency of Y must balance the charge contributed by the 3 X ions.
- The subscript of Y is 1, indicating that the valency of X must balance the charge contributed by the 1 Y ion.

4. **Apply the Crisscross Method**:
- According to the crisscross method, the valency of Y (which is 1) becomes the subscript for X, and the valency of X (which is 3) becomes the subscript for Y.
- Therefore, the valency of X is 1 and the valency of Y is 3.

5. **Conclusion**:
- The valencies of elements X and Y are respectively 1 and 3.

### Final Answer:
- Valency of X = 1
- Valency of Y = 3