Which of the following is not correct

Options

A. A sigma bond is weaker than `pi` bond
B. A sigma bond is stronger than `pi` bond
C. A double bond is stronger than a single bond
D. A double bond is shorter than a single bond

Answer

To determine which statement is not correct among the given options regarding chemical bonding, we will analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** Sigma bond is weaker than pi bond.

**Explanation:**
- A sigma bond is formed by the head-on (axial) overlap of atomic orbitals, which results in a stronger bond due to the direct overlap.
- A pi bond, on the other hand, is formed by the sidewise overlap of p-orbitals, which is generally weaker than a sigma bond.
- Therefore, this statement is **not correct** because a sigma bond is actually stronger than a pi bond.

### Step 2: Analyze the second statement
**Statement:** Sigma bond is stronger than pi bond.

**Explanation:**
- As explained in the first statement, sigma bonds are indeed stronger than pi bonds due to the nature of their overlap.
- This statement is **correct**.

### Step 3: Analyze the third statement
**Statement:** A double bond is stronger than a single bond.

**Explanation:**
- A double bond consists of one sigma bond and one pi bond. The presence of the additional pi bond makes double bonds stronger than single bonds.
- This statement is also **correct**.

### Step 4: Analyze the fourth statement
**Statement:** A double bond is shorter than a single bond.

**Explanation:**
- Double bonds are indeed shorter than single bonds due to the increased electron density between the two bonded atoms, which pulls them closer together.
- This statement is **correct**.

### Conclusion
After analyzing all the statements, we find that the first statement is the only one that is not correct.

**Final Answer:** The statement that is not correct is: "Sigma bond is weaker than pi bond."

---