An element X belongs to group 17 and third period of the periodic table
Give the formula of the compound formed when it combines with an element Y having valency three?

Explanation

To solve the problem, we need to determine the formula of the compound formed when element X (which belongs to group 17 and the third period of the periodic table) combines with element Y (which has a valency of 3).

### Step-by-Step Solution:

1. **Identify Element X**:
- Element X belongs to group 17 and the third period of the periodic table.
- The elements in group 17 are known as halogens. The halogens in the third period include chlorine (Cl).
- Therefore, element X is chlorine (Cl).
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