Considering x-axis as the internuclear axis, which out of the following will not form a sigma bond?

Options

A. 1s and 1s
B. 1s and `2p_x`
C. `2p_y` and `2p_y`
D. 1s and 2s

Answer

To determine which of the given options will not form a sigma bond, we need to analyze the types of orbital overlaps that occur in each case. A sigma bond is formed by the head-on overlap of atomic orbitals along the internuclear axis.

### Step-by-Step Solution:

1. **Understanding Sigma Bonds**:
- Sigma bonds are formed by the head-on (axial) overlap of orbitals. This can occur between two s orbitals, an s and a p orbital, or two p orbitals aligned along the internuclear axis.

2. **Analyzing Each Option**:
- **Option 1: 1s and 1s**:
- Both are s orbitals. They can overlap head-on along the x-axis, forming a sigma bond.
- **Conclusion**: This option forms a sigma bond.

- **Option 2: 1s and 2px**:
- The 1s orbital can overlap with the 2px orbital. The 2px orbital has a lobe that can align with the s orbital along the x-axis, allowing for head-on overlap.
- **Conclusion**: This option forms a sigma bond.

- **Option 3: 2py and 2py**:
- The 2py orbitals are oriented perpendicular to the x-axis. When aligned this way, they cannot overlap head-on; instead, they will overlap sidewise.
- **Conclusion**: This option does NOT form a sigma bond; it will form a pi bond instead.

- **Option 4: 1s and 2s**:
- The 1s and 2s orbitals can overlap head-on along the x-axis as both are s orbitals.
- **Conclusion**: This option forms a sigma bond.

3. **Final Answer**:
- The option that will not form a sigma bond is **Option 3: 2py and 2py**.