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 and how they relate to bond strength.

### 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 is influenced by the type of overlap:
- **pp overlap** (2p-2p) is stronger than
- **sp overlap** (2s-2p) which is stronger than
- **ss overlap** (2s-2s).

2. **Principal Quantum Number**:
The bond strength is inversely proportional to the principal quantum number (n). A lower principal quantum number indicates a stronger bond. Therefore, bonds involving orbitals with lower principal quantum numbers will generally be stronger.

3. **Analyzing the Options**:
- **I. 2s-2s**: This is an ss overlap.
- **II. 2s-2p**: This is an sp overlap.
- **III. 2p-2p**: This is a pp overlap.
- **IV. 3s-3s**: This is an ss overlap, but with a higher principal quantum number (n=3).

4. **Comparing the Overlaps**:
- The bond strength order based on overlap type is:
- 2p-2p (III) > 2s-2p (II) > 2s-2s (I).
- The bond strength for 3s-3s (IV) will be the weakest due to the higher principal quantum number.

5. **Final Order of Bond Strength**:
Based on the above analysis, the correct order of sigma bond strength is:
- III (2p-2p) > II (2s-2p) > I (2s-2s) > IV (3s-3s).

Thus, the correct order of sigma bond strength is **3 > 2 > 1 > 4**.