Which of the following overlaps of atomic orbitals does not form `pi`-bond if z-axis is the internuclear axis?

Options

A. s - orbital and s - orbital
B. s - orbital and `p_z` orbital
C. `p_z` - orbital and `p_z` orbital
D. `p_y` - orbital and `p_y` - orbital

Answer

To solve the question of which overlaps of atomic orbitals do not form a pi bond when the z-axis is the internuclear axis, we need to analyze the types of overlaps that can occur between different atomic orbitals.

### Step-by-Step Solution:

1. **Understanding Sigma and Pi Bonds**:
- A **sigma bond** is formed by the head-on (end-to-end) overlap of atomic orbitals along the internuclear axis.
- A **pi bond** is formed by the side-to-side (lateral) overlap of atomic orbitals.

2. **Analyzing Each Option**:
- **Option 1: s orbital and s orbital**:
- The overlap of two s orbitals is spherical and occurs head-on. Therefore, this overlap forms a sigma bond.

- **Option 2: s orbital and pz orbital**:
- The s orbital overlaps with the pz orbital along the z-axis (internuclear axis). This overlap is also head-on, resulting in a sigma bond.

- **Option 3: pz orbital and pz orbital**:
- The overlap of two pz orbitals occurs along the z-axis. This is again a head-on overlap, forming a sigma bond.

- **Option 4: py orbital and py orbital**:
- The overlap of two py orbitals occurs laterally (side-to-side) when the z-axis is the internuclear axis. This results in a pi bond.

3. **Conclusion**:
- The overlaps that do not form pi bonds are:
- Option 1 (s and s)
- Option 2 (s and pz)
- Option 3 (pz and pz)
- Therefore, the correct answer is that options 1, 2, and 3 do not form pi bonds.

### Final Answer:
Options 1, 2, and 3 do not form pi bonds when the z-axis is the internuclear axis.