Boron cannot expand its covalency beyond four due to _____.

Explanation

To answer the question, "Boron cannot expand its covalency beyond four due to ____," we will break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Covalency**: Covalency refers to the number of bonds an atom can form by sharing its electrons with other atoms. In general, elements can expand their covalency by utilizing available orbitals to accommodate more electron pairs.

2. **Electronic Configuration of Boron**: The electronic configuration of boron is \(1s^2 2s^2 2p^1\). This means that in its valence shell (the outermost shell), boron has:
- 1 orbital from the 2s subshell
- 3 orbitals from the 2p subshell
- Totaling to 4 orbitals available for bonding.

3. **Ground State vs Excited State**: In the ground state, boron has its electrons in the lowest energy levels. When we consider the excited state, boron can promote one of its 2s electrons to the 2p subshell, allowing it to form a maximum of four bonds (or covalency of 4).

4. **Absence of Vacant d Orbitals**: Unlike heavier elements in group 13 (like aluminum or gallium), boron does not have access to d orbitals. The presence of vacant d orbitals in these heavier elements allows them to expand their covalency beyond four, as they can accommodate additional electron pairs in these orbitals.

5. **Conclusion**: Therefore, the reason boron cannot expand its covalency beyond four is due to the **absence of vacant d orbitals** that would allow it to form more than four bonds.

### Final Answer:
Boron cannot expand its covalency beyond four due to the absence of vacant d orbitals.