Which statement is always true according to Valence Shell Electron Pair Repulsion (VSEPR) theory?

• A. Electron pairs around a central atom arrange themselves to minimize repulsion.
• B. Bond angles in molecules are always exactly 90 degrees.
• C. Lone pairs do not affect the shape of a molecule.
• D. All molecules with four electron pairs around the central atom are always tetrahedral.

Answer: (A)

Verified step by step guidance

1. Understand the core principle of VSEPR theory: it states that electron pairs around a central atom repel each other and therefore arrange themselves as far apart as possible to minimize repulsion.
2. Analyze the first statement: 'Electron pairs around a central atom arrange themselves to minimize repulsion.' This aligns directly with the fundamental concept of VSEPR theory.
3. Consider the second statement: 'Bond angles in molecules are always exactly 90 degrees.' This is not always true because bond angles depend on the molecular geometry and can vary widely (e.g., tetrahedral angles are about 109.5 degrees).
4. Evaluate the third statement: 'Lone pairs do not affect the shape of a molecule.' This is false because lone pairs occupy space and influence molecular shape by repelling bonding pairs, often reducing bond angles.
5. Review the fourth statement: 'All molecules with four electron pairs around the central atom are always tetrahedral.' This is incorrect because the presence of lone pairs among the four electron pairs can change the molecular shape (e.g., trigonal pyramidal or bent), even though the electron pair geometry is tetrahedral.