`AlCl_3` is a Lewis acid.

Answer

**Step-by-Step Solution:**

1. **Definition of Lewis Acid**: A Lewis acid is defined as a substance that can accept an electron pair from a Lewis base. This means that Lewis acids are electron pair acceptors.

2. **Structure of AlCl₃**: Aluminum chloride (AlCl₃) consists of aluminum (Al) and chlorine (Cl) atoms. In this compound, aluminum is the central atom surrounded by three chlorine atoms.

3. **Vacant Orbital in Aluminum**: Aluminum has a vacant p-orbital. This vacant orbital allows aluminum to accept an electron pair, which is a key characteristic of Lewis acids.

4. **Electrophilic Nature of AlCl₃**: The presence of chlorine atoms, which are highly electronegative, pulls electron density away from the aluminum atom. This makes the aluminum atom electron-deficient and enhances its ability to act as an electrophile.

5. **Conclusion**: Due to the presence of a vacant orbital and the electron-withdrawing effect of the chlorine atoms, AlCl₃ can accept electron pairs, confirming that it is indeed a Lewis acid.

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