The geometry of ammonia molecule can be best described as :

Options

A. Nitrogen at one vetex of a regular tetrahedron, the other three vertices being occupied by three hydrogens
B. Nitrogen at the centre of the tetrahedron, three of the vertices being occupied by three hydrogens
C. Nitrogen at the centre of an equilateral triangle, three corners being occupied by three hydrogens
D. Nitrogen at the junction of a T, three open ends being occupied by three hydrogens

Explanation

To determine the geometry of the ammonia (NH₃) molecule, we can follow these steps:

### Step 1: Identify the central atom and its valence electrons
- The central atom in ammonia is nitrogen (N).
- Nitrogen has 5 valence electrons.

### Step 2: Count the number of attached atoms
- Ammonia has three hydrogen (H) atoms attached to the nitrogen atom.

### Step 3: Use the VSEPR theory to calculate the steric number
- The steric number (SN) can be calculated using the formula:
\[ \text{Steric Number} = \frac{1}{2} \left( V + M + C - A \right) \]
where:
- \( V \) = number of valence electrons of the central atom (5 for nitrogen)
- \( M \) = number of monovalent atoms attached (3 for hydrogen)
- \( C \) = charge of the anion (0 for neutral molecules)
- \( A \) = charge of the cation (0 for neutral molecules)

Plugging in the values:
\[ \text{Steric Number} = \frac{1}{2} \left( 5 + 3 + 0 - 0 \right) = \frac{8}{2} = 4 \]

### Step 4: Determine hybridization
- A steric number of 4 indicates that the nitrogen atom is \( sp^3 \) hybridized.

### Step 5: Identify bond pairs and lone pairs
- In ammonia, there are 3 bond pairs (N-H bonds) and 1 lone pair on the nitrogen atom.

### Step 6: Determine the molecular geometry
- The presence of one lone pair and three bond pairs leads to a tetrahedral electron geometry. However, the molecular geometry is described as trigonal pyramidal due to the lone pair.

### Step 7: Analyze the options given in the question
- The options provided were:
1. Nitrogen at one vertex of a regular tetrahedron.
2. Nitrogen at the center of a tetrahedron, with three vertices occupied by hydrogen.
3. Nitrogen at the center of an equilateral triangle.
4. Nitrogen at the junction of a T with three hydrogen atoms.

- The correct description of the ammonia molecule's geometry is option 2: Nitrogen is at the center of the tetrahedron, and the three vertices are occupied by the three hydrogen atoms.

### Conclusion
The geometry of the ammonia molecule can be best described as nitrogen at the center of a tetrahedron, with three hydrogen atoms occupying the vertices.

---