Which of the following is not a correct statement?1. The electron deficient molecules can act as Lewis acids.2. The canonical structures have no real existance.3. Every AB5 molecule does in fact have square pyramidal structure.4. Multiple bonds are always shorter than corresponding single bonds.

To determine which statement is not correct, let's analyze each option one by one:

1. The statement "The electron deficient molecules can act as Lewis acids" is correct. Electron deficient molecules have a shortage of electrons and can accept electron pairs from other molecules, making them Lewis acids.

2. The statement "The canonical structures have no real existence" is also correct. Canonical structures are hypothetical representations of molecules that differ only in the placement of electrons. They are used to describe the resonance of a molecule, but no single canonical structure accurately represents the molecule's true structure.

3. The statement "Every AB5 molecule does in fact have a square pyramidal structure" is not correct. AB5 molecules can have various molecular geometries, including trigonal bipyramidal and square pyramidal. The specific geometry depends on the arrangement of the atoms and lone pairs around the central atom.

4. The statement "Multiple bonds are always shorter than corresponding single bonds" is generally correct. Multiple bonds, such as double or triple bonds, involve the sharing of more electrons between atoms compared to single bonds. This increased electron density leads to a stronger bond and a shorter bond length.

Therefore, the correct answer is option 3: "Every AB5 molecule does in fact have a square pyramidal structure."