Ammonium ion is :

Options

A. Lewis acid
B. Lewis base
C. Bronsted acid
D. Bronsted base

Explanation

To determine the nature of the ammonium ion (NH₄⁺), we can analyze its properties based on the definitions of Lewis acids and bases, as well as Brønsted acids and bases. Here is a step-by-step solution:

### Step 1: Identify the structure of the ammonium ion
The ammonium ion (NH₄⁺) consists of one nitrogen atom bonded to four hydrogen atoms. The nitrogen atom has an atomic number of 7, with an electronic configuration of 1s² 2s² 2p³. When it forms the ammonium ion, it loses one electron, resulting in a configuration of 1s² 2s² 2p².

**Hint:** Remember that the ammonium ion is formed by the addition of a proton (H⁺) to ammonia (NH₃).

### Step 2: Analyze the Lewis acid-base theory
- **Lewis Acid:** A Lewis acid is defined as a species that can accept a pair of electrons.
- **Lewis Base:** A Lewis base is defined as a species that can donate a pair of electrons.

In the case of NH₄⁺, nitrogen has no vacant orbitals available to accept electrons since its octet is complete. Therefore, it cannot act as a Lewis acid. Additionally, NH₄⁺ does not have a lone pair of electrons to donate, so it cannot act as a Lewis base either.

**Hint:** Check the electron configuration and bonding to determine if there are available electrons for donation or acceptance.

### Step 3: Analyze the Brønsted acid-base theory
- **Brønsted Acid:** A Brønsted acid is a species that can donate a proton (H⁺).
- **Brønsted Base:** A Brønsted base is a species that can accept a proton (H⁺).

The ammonium ion (NH₄⁺) can donate a proton to form ammonia (NH₃) and a hydrogen ion (H⁺). Therefore, NH₄⁺ acts as a Brønsted acid.

**Hint:** Consider the ability of a species to donate or accept protons to classify it as a Brønsted acid or base.

### Step 4: Conclusion
Based on the analysis:
- NH₄⁺ does not act as a Lewis acid or base.
- NH₄⁺ acts as a Brønsted acid because it can donate a proton.

Thus, the correct classification of the ammonium ion is that it is a **Brønsted acid**.

**Final Answer:** Ammonium ion (NH₄⁺) is a Brønsted acid.