Concept: This question tests fundamental concepts in chemical bonding, molecular geometry, bond length, and Lewis acid-base theory.

Why (A) is correct:

The statement "Every $A{B}_5$ molecule does in fact have square pyramidal structure" is incorrect. While some $A{B}_5$ molecules can have a square pyramidal structure (e.g., $Br{F}_5$ due to the presence of a lone pair on the central atom), the most common and stable geometry for an $A{B}_5$ molecule with no lone pairs on the central atom is trigonal bipyramidal (e.g., $PC{l}_5$). The geometry depends on the number of lone pairs on the central atom as per VSEPR theory.

Option Analysis:

• A) Every $A{B}_5$ molecule does in fact have square pyramidal structure. This is incorrect. $A{B}_5$ molecules can have trigonal bipyramidal geometry (e.g., $PC{l}_5$) or square pyramidal geometry (e.g., $Br{F}_5$), depending on the presence of lone pairs.
• B) Multiple bonds are always shorter than corresponding single bonds. This is a correct statement. For example, C=C bond is shorter than C-C bond, and C≡C bond is shorter than C=C bond. Increased electron density between atoms pulls them closer.
• C) The electron deficient molecules act as Lewis acids. This is a correct statement. Lewis acids are electron pair acceptors. Electron-deficient molecules (like $B{F}_3$ or $AlC{l}_3$) have incomplete octets and can accept electron pairs.
• D) The canonical structures have no real existence. This is a correct statement. Canonical structures (or resonance structures) are hypothetical representations used to describe the delocalization of electrons in a molecule. The actual molecule is a resonance hybrid, which is an average of all canonical structures and does not oscillate between them.

Correct Answer: (A)