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 AB5 molecule does infact have square pyramid structure
D. Multiple bonds are always shorter than corresponding single bonds

Answer

To determine which of the following statements is not correct, we will analyze each option step by step.

### Step 1: Analyze Option 1
**Statement:** Electron deficient molecules can act as Lewis acids.

- **Explanation:** Lewis acids are defined as species that can accept an electron pair. Electron deficient molecules, by definition, lack sufficient electrons and are therefore inclined to accept electrons. A classic example is BCl3, where boron has only three electrons in its valence shell and can accept electron pairs from Lewis bases.

- **Conclusion:** This statement is correct.

### Step 2: Analyze Option 2
**Statement:** Canonical structures have no real existence.

- **Explanation:** Canonical structures (or resonance structures) are theoretical constructs used to represent the delocalization of electrons in a molecule. They do not exist as individual entities but rather represent different ways of arranging electrons in a molecule. The actual structure of the molecule is a hybrid of these resonance forms.

- **Conclusion:** This statement is correct.

### Step 3: Analyze Option 3
**Statement:** Every AB5 type molecule does in fact have a square pyramidal structure.

- **Explanation:** AB5 type molecules can have different geometries depending on the presence of lone pairs. For example, if there are five bond pairs and no lone pairs, the geometry is trigonal bipyramidal. However, if one of the bond pairs is replaced by a lone pair, the structure becomes square pyramidal. Therefore, not all AB5 molecules will have a square pyramidal structure.

- **Conclusion:** This statement is not correct.

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

- **Explanation:** Generally, multiple bonds (double or triple bonds) are shorter than single bonds due to increased electron density between the two nuclei, which pulls them closer together. This is a well-established principle in chemistry.

- **Conclusion:** This statement is correct.

### Final Conclusion
The statement that is not correct is **Option 3**: "Every AB5 type molecule does in fact have a square pyramidal structure."

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