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. `I gt II gt III gt IV`
B. `III gt II gt I gt IV`
C. `IV gt I gt II gt III`
D. `III gt I gt II gt IV`

Answer

To determine the correct order of sigma bond strength among the given options, we need to analyze the types of overlaps involved in each bond type. The bonds mentioned are:

I. 2s-2s
II. 2s-2p
III. 2p-2p
IV. 3s-3s

### 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 extent of overlap between the orbitals involved.

2. **Types of Overlaps**:
- **s-s overlap (2s-2s)**: The overlap between two s orbitals is generally weaker due to their spherical shape and non-directionality.
- **s-p overlap (2s-2p)**: The overlap between an s orbital and a p orbital is stronger than s-s overlap because the p orbital has a directional shape, allowing for better overlap.
- **p-p overlap (2p-2p)**: The overlap between two p orbitals is the strongest among these options as they can align head-on, maximizing the overlap.
- **3s-3s overlap**: Although this is s-s overlap, the principal quantum number is higher (n=3), which means the orbitals are larger and the overlap is weaker compared to 2s-2s.

3. **Ranking the Bond Strengths**:
- **Strongest**: 2p-2p (III)
- **Second**: 2s-2p (II)
- **Third**: 2s-2s (I)
- **Weakest**: 3s-3s (IV)

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

### Conclusion:
The correct answer is option II (2s-2p) as the second strongest bond, but the overall order is as follows:
- III (2p-2p) > II (2s-2p) > I (2s-2s) > IV (3s-3s).