An element A of valency 2 combines with element B to form a compound with formula `A_5 B_2`. The valency of element B is

Explanation

To determine the valency of element B in the compound \( A_5B_2 \), follow these steps:

### Step 1: Understand the given information
- Element A has a valency of 2.
- The compound formed is \( A_5B_2 \).

### Step 2: Write the assumed valency for element B
- Let the valency of element B be \( x \).

### Step 3: Use the crisscross method to balance the charges
- In the compound \( A_5B_2 \), the subscript of A (which is 5) will become the valency of B, and the subscript of B (which is 2) will become the valency of A.
- Therefore, we can set up the relationship as follows:
- The valency of A (2) corresponds to the subscript of B (2).
- The valency of B (x) corresponds to the subscript of A (5).

### Step 4: Set up the equation
- According to the crisscross method, we can write:
\[ 2 \text{ (valency of A)} \rightarrow 5 \text{ (subscript of A)} \]
\[ x \text{ (valency of B)} \rightarrow 2 \text{ (subscript of B)} \]

### Step 5: Solve for the valency of B
- From the crisscross method, we can conclude:
\[ x = 5 \]

### Conclusion
- Therefore, the valency of element B is 5.

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