Which of the following is not correct statement ?

Options

A. Multiple bonds are always shorter than corresponding single bonds
B. The electron-deficient molecules can act as lewis acids
C. The canonical structures have no real existence
D. Every `AB_(5)` molecules does in fact have square pyramid structrue

Answer

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

1. **Statement 1**: Multiple bonds are always shorter than the corresponding single bonds.
- **Analysis**: This statement is true. Multiple bonds (double or triple bonds) involve more electron sharing between two atoms compared to single bonds. This increased electron density between the nuclei pulls the atoms closer together, resulting in shorter bond lengths.

2. **Statement 2**: Electron deficient molecules can act as Lewis acids.
- **Analysis**: This statement is also true. Lewis acids are defined as species that can accept an electron pair. Electron deficient molecules, which lack a complete octet, have a tendency to accept electrons, thus acting as Lewis acids.

3. **Statement 3**: Canonical structures have no real existence.
- **Analysis**: This statement is true as well. Canonical structures, or resonance structures, are theoretical constructs used to represent the delocalization of electrons in molecules. They do not exist as individual entities; rather, the actual molecule is a hybrid of these structures.

4. **Statement 4**: Every AB5 molecule does in fact have a square pyramidal structure.
- **Analysis**: This statement is false. AB5 molecules can have different geometries depending on the presence of lone pairs. For example, if there are no lone pairs on the central atom, the geometry is trigonal bipyramidal, not square pyramidal. Therefore, not every AB5 molecule has a square pyramidal structure.

**Conclusion**: The incorrect statement is **Statement 4**.