Which of the following are Lewis acids ?
(i)`H_2O` , (ii)`BF_3`, (iii) `H^+` , (iv) `NH_4^+`

Answer

To determine which of the given species are Lewis acids, we need to understand the definition of a Lewis acid. A Lewis acid is a substance that can accept an electron pair.

Let's analyze each option step by step:

### Step 1: Analyze `H2O`
- **Structure**: Water has two lone pairs of electrons on the oxygen atom.
- **Electron Acceptance**: For a species to be a Lewis acid, it must have a vacant orbital to accept electrons. Since water has a complete octet (8 electrons around oxygen) and does not have a vacant orbital, it cannot accept electrons.
- **Conclusion**: `H2O` is **not** a Lewis acid.

### Step 2: Analyze `BF3`
- **Structure**: Boron trifluoride has boron at the center with three fluorine atoms bonded to it.
- **Electron Acceptance**: Boron has only 6 electrons in its valence shell (it is electron-deficient) and does not complete its octet. Therefore, it has a vacant orbital and can accept an electron pair.
- **Conclusion**: `BF3` is a Lewis acid.

### Step 3: Analyze `H+`
- **Structure**: The hydrogen ion has lost its electron, resulting in no electrons.
- **Electron Acceptance**: Since `H+` has no electrons, it has a vacant orbital and can accept an electron pair.
- **Conclusion**: `H+` is a Lewis acid.

### Step 4: Analyze `NH4+`
- **Structure**: Ammonium ion has a nitrogen atom bonded to four hydrogen atoms and carries a positive charge.
- **Electron Acceptance**: The nitrogen in `NH4+` has a complete octet (8 electrons) and does not have any vacant orbitals to accept additional electrons.
- **Conclusion**: `NH4+` is **not** a Lewis acid.

### Final Conclusion:
The Lewis acids among the given options are `BF3` and `H+`.

### Summary:
- **Lewis Acids**: `BF3`, `H+`
- **Not Lewis Acids**: `H2O`, `NH4+`

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