_____phosphorus is reactive because of its highly strained tetrahedral structure.

Explanation

**Step-by-Step Text Solution:**

1. **Identify the Allotropes of Phosphorus:**
Phosphorus exists in several allotropes, including white phosphorus, yellow phosphorus, red phosphorus, and black phosphorus. Among these, white and yellow phosphorus are known for their high reactivity.

2. **Understand the Structure of White/Yellow Phosphorus:**
White phosphorus (P4) has a tetrahedral structure where four phosphorus atoms are arranged at the corners of a tetrahedron. Each phosphorus atom is covalently bonded to three other phosphorus atoms.

3. **Analyze the Bond Angles:**
In white phosphorus, the bond angles between the phosphorus atoms are approximately 60 degrees. This is significantly lower than the ideal tetrahedral angle of 109.5 degrees.

4. **Discuss the Consequences of Low Bond Angles:**
The low bond angle leads to significant angular strain within the molecule. The repulsion between the bonded pairs of electrons increases due to this strain, making the structure unstable.

5. **Conclude on Reactivity:**
Due to the highly strained tetrahedral structure and the resulting instability, white phosphorus is very reactive, especially at room temperature. It can ignite spontaneously in air and is typically stored underwater to prevent it from reacting with oxygen.

6. **Final Answer:**
Therefore, the answer to the question is: **White phosphorus** is reactive because of its highly strained tetrahedral structure.

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