(a) Explain why BrF4- is square planar, whereas BF4- is tetrahedral.

Key Concepts

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used to predict the geometry of molecular structures based on the repulsion between electron pairs around a central atom. According to this theory, electron pairs will arrange themselves to minimize repulsion, leading to specific shapes depending on the number of bonding and lone pairs.

Lone Pairs vs. Bonding Pairs

In molecular geometry, lone pairs of electrons occupy more space than bonding pairs because they are localized on a single atom. This difference in spatial requirements affects the overall shape of the molecule. For BrF4-, the presence of two lone pairs leads to a square planar arrangement, while BF4- has no lone pairs, resulting in a tetrahedral shape.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding pairs of electrons. In BF4-, the boron atom undergoes sp3 hybridization, leading to a tetrahedral geometry. In contrast, BrF4- involves dsp2 hybridization due to the presence of lone pairs, resulting in a square planar structure.