Any species capable of accepting a lone pair of electrons acts as a lewis acid, where as a species capable of donating a proton acts as a Bronstead acid.
Which of the following is not a lewis acid ?

Options

A. `Mg^(2)+`
B. `SiF_(4)`
C. `SO_(2)`
D. `NH_(4)^(+)`

Answer

To determine which of the following species is not a Lewis acid, we need to analyze each option based on the definition of a Lewis acid. A Lewis acid is any species that can accept a lone pair of electrons.

### Step-by-Step Solution:

1. **Analyze Mg²⁺:**
- Magnesium in its ionic form (Mg²⁺) has lost two electrons. The electron configuration of Mg is [Ne] 3s², and for Mg²⁺, it is [Ne]. This means that the 3s orbital is empty, allowing Mg²⁺ to accept a lone pair of electrons.
- **Conclusion:** Mg²⁺ is a Lewis acid.

2. **Analyze SiF₄:**
- Silicon tetrafluoride (SiF₄) has a silicon atom bonded to four fluorine atoms. Silicon has an empty 3d orbital, which allows it to accept a lone pair of electrons.
- **Conclusion:** SiF₄ is a Lewis acid.

3. **Analyze SO₂:**
- Sulfur dioxide (SO₂) can accept a lone pair of electrons due to the presence of vacant d orbitals. When SO₂ accepts a lone pair, it can form SO₃, indicating that it can act as a Lewis acid.
- **Conclusion:** SO₂ is a Lewis acid.

4. **Analyze NH₄⁺:**
- Ammonium ion (NH₄⁺) is formed when ammonia (NH₃) donates a proton (H⁺). In NH₄⁺, nitrogen has already donated its lone pair to bond with four hydrogens, meaning it cannot accept any more lone pairs. Therefore, NH₄⁺ cannot act as a Lewis acid.
- **Conclusion:** NH₄⁺ is not a Lewis acid.

### Final Answer:
The species that is not a Lewis acid is **NH₄⁺**.