Which of the following is not a correct statement ?

Options

A. The electron deficient molecules can act as Lewis acids
B. The canonical structures have no real existence
C. Every `AB_5` molecule does infact have square pyramid structure
D. Multiple bonds are always shorter than corresponding single bond

Explanation

To determine which statement is not correct, we will analyze each of the four statements provided in the question.

### Step 1: Analyze Statement A
**Statement A:** "Electron deficient molecules can act as a Lewis acid."

- **Explanation:** Electron deficient molecules have less than an octet of electrons and can accept a pair of electrons. For example, H⁺ is an electron deficient species that can accept a lone pair of electrons, acting as a Lewis acid.
- **Conclusion:** This statement is **correct**.

### Step 2: Analyze Statement B
**Statement B:** "The canonical structure have no real existence."

- **Explanation:** Canonical structures, or resonance structures, represent different ways of arranging electrons in a molecule. They do not exist as distinct entities but rather contribute to the overall hybrid structure of the molecule. Therefore, this statement is true.
- **Conclusion:** This statement is **correct**.

### Step 3: Analyze Statement C
**Statement C:** "Every AB5 molecule does in fact have square pyramidal structure."

- **Explanation:** AB5 molecules typically exhibit a trigonal bipyramidal geometry due to sp³d hybridization. A square pyramidal structure is not the typical geometry for AB5 molecules; hence, this statement is incorrect.
- **Conclusion:** This statement is **incorrect**.

### Step 4: Analyze Statement D
**Statement D:** "Multiple bonds are always shorter than corresponding single bonds."

- **Explanation:** Multiple bonds (double or triple) involve more electron sharing between atoms, which leads to shorter bond lengths compared to single bonds. This statement is true.
- **Conclusion:** This statement is **correct**.

### Final Conclusion
After analyzing all four statements, we find that **Statement C** is the one that is not correct. It incorrectly states that every AB5 molecule has a square pyramidal structure when they actually have a trigonal bipyramidal structure.

### Summary of the Correct Answer:
The statement that is not correct is: **"Every AB5 molecule does in fact have square pyramidal structure."**

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