Which of the following is a Lewis acid ?

Options

A. `AlCl_(3)`
B. `MgCl_(2)`
C. `CaCl_(2)`
D. `BaCl_(2)`

Explanation

To determine which of the following compounds is a Lewis acid among AlCl3, NdCl2, CaCl2, and BaCl2, we can follow these steps:

### Step 1: Understand the definition of a Lewis acid
A Lewis acid is defined as a substance that can accept an electron pair to complete its octet. This typically occurs when the central atom has an incomplete octet and can stabilize itself by accepting electrons.

### Step 2: Analyze the compounds
- **AlCl3 (Aluminum Chloride)**: Aluminum has an atomic number of 13, with an electron configuration of 2, 8, 3. In AlCl3, aluminum forms three bonds with chlorine atoms, but it still has an incomplete octet (only 6 electrons around it). Therefore, it can accept electrons to complete its octet.

- **NdCl2 (Neodymium Chloride)**: Neodymium is a lanthanide and typically forms ionic compounds. It has a complete octet in its ionic state and does not act as a Lewis acid.

- **CaCl2 (Calcium Chloride)**: Calcium has an atomic number of 20, with an electron configuration of 2, 8, 8, 2. In CaCl2, calcium loses two electrons and achieves a stable octet. It does not have an incomplete octet and thus does not act as a Lewis acid.

- **BaCl2 (Barium Chloride)**: Barium has an atomic number of 56, with an electron configuration of 2, 8, 18, 18, 8, 2. Similar to calcium, barium loses two electrons in BaCl2 and achieves a stable octet. It also does not act as a Lewis acid.

### Step 3: Conclusion
Among the given compounds, only AlCl3 has an incomplete octet and can accept electrons to stabilize itself. Therefore, **AlCl3 is the Lewis acid**.

### Final Answer:
**AlCl3 is the Lewis acid.**

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