As a general rule, MXnolecules (where M represents a central...

As a general rule, MX_nolecules (where M represents a central atom and X represents terminal atoms; $n=2$ 5) are polar if there is one or more lone pairs of electrons on $\mathrm{M} . \mathrm{NH}_{3}(\mathrm{M}=\mathrm{N}, \mathrm{X}=\mathrm{H}, \mathrm{n}=3$ ) is an example. There are two molecular structures with lone pairs that are exceptions to this rule. What are they?

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Key Concepts

VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the shape of a molecule based on the repulsions between all electron pairs, both bonding and nonbonding. It explains why, in some cases, even when lone pairs are present, the geometric arrangement of atoms can be highly symmetric, leading to exceptions in the usual correlation between lone pairs and molecular polarity.

Lone Pair Effects on Polarity

Usually, a lone pair on the central atom distorts the symmetry of a molecule, leading to an uneven distribution of charge and a net dipole moment. However, there are exceptions where the overall molecular symmetry cancels the individual dipoles, resulting in a nonpolar molecule despite the presence of lone pairs.

Molecular Geometry and Symmetry

The three?dimensional arrangement of bonding and nonbonding electron pairs (as described by VSEPR theory) determines whether the dipole moments cancel out. In certain geometries—such as linear or square planar—the spatial distribution of ligands and lone pairs can be arranged so that the molecule is symmetric overall, thus producing no net dipole moment.

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