Which of the following is correct order of `sigma` - bond strength ?
I. 2s-2s
II. 2s-2p
III. 2p-2p
IV. 3s-3s

Options

A. `s-s gt p-p gt p-s`
B. `s-s gt p-s gt p-p`
C. `p-p gt p-s gt s-s`
D. `p-p gt s-s gt p-s`

Answer

To determine the correct order of sigma bond strength among the given options (I. 2s-2s, II. 2s-2p, III. 2p-2p, IV. 3s-3s), we can analyze the nature of the bonds formed by the different types of atomic orbitals involved.

### Step-by-Step Solution:

1. **Understanding Sigma Bonds**:
- Sigma (σ) bonds are formed by the head-on overlap of atomic orbitals. The strength of a sigma bond depends on the type of orbitals overlapping.

2. **Analyzing the Options**:
- **I. 2s-2s**: This bond is formed by the overlap of two s orbitals. S orbitals are spherical and non-directional, which generally results in weaker bonds compared to p orbitals.
- **II. 2s-2p**: This bond is formed by the overlap of one s orbital and one p orbital. The p orbital is directional, leading to a stronger bond than the 2s-2s bond.
- **III. 2p-2p**: This bond is formed by the overlap of two p orbitals. P orbitals are directional and can overlap effectively, resulting in a strong bond.
- **IV. 3s-3s**: This bond is similar to the 2s-2s bond but involves higher energy orbitals (3s). However, the nature of the s orbital remains the same (non-directional), leading to a weaker bond compared to p orbital overlaps.

3. **Comparing Bond Strength**:
- The general order of bond strength based on the types of orbitals involved is:
- 2p-2p (strongest)
- 2s-2p (intermediate strength)
- 2s-2s (weaker)
- 3s-3s (weaker than 2s-2s due to higher energy level)

4. **Final Order**:
- Based on the analysis, the correct order of sigma bond strength from strongest to weakest is:
- III. 2p-2p > II. 2s-2p > I. 2s-2s > IV. 3s-3s

### Conclusion:
The correct order of sigma bond strength is:
**2p-2p > 2s-2p > 2s-2s > 3s-3s**