Using the VSEPR theory, predict the molecular structure of...

Using the VSEPR theory, predict the molecular structure of each of the following molecules or ions containing multiple bonds.
a. $\mathrm{CO}_{3}^{2-}$
b. HNO $_{3}$ (hydrogen is bonded to oxygen)
c. $\mathrm{NO}_{2}^{-}$
d. $\mathrm{C}_{2} \mathrm{H}_{2}$

Key Concepts

Resonance Structures

Resonance structures are different Lewis structures that represent the same molecule and illustrate the delocalization of electrons over two or more atoms. When applying VSEPR theory, the consideration of resonance is important because it can lead to an averaged or delocalized electron distribution, affecting bond lengths and, sometimes, the interpretation of which atom is central in more complex systems.

Multiple Bonding

In the context of VSEPR theory, multiple bonds such as double or triple bonds are treated as a single electron domain because the additional electron density is confined to the area between the two bonded nuclei. This simplifies the determination of electron domain geometry since a multiple bond, despite having more shared electrons, does not count as extra domains when predicting the overall molecular shape.

Steric Number

The steric number of a central atom is the sum of the number of atoms bonded to it plus the number of lone electron pairs around it. This number is crucial as it determines the electron domain geometry by indicating the number of regions of electron density, which then informs the predicted bond angles and overall shape of the molecule.

Electron Domain Geometry

Electron domain geometry refers to the spatial arrangement of all electron domains—bonding pairs (including those in multiple bonds) and lone pairs—around a central atom. Each electron domain is treated as a region of electron density, and the shapes and angles in the geometry are predicted by considering the repulsion between these regions, regardless of whether bonds are single, double, or triple.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsions between electron pairs around a central atom. The theory assumes that electron pairs, whether involved in bonding or existing as lone pairs, will position themselves as far apart as possible to minimize repulsive interactions, thereby determining the overall molecular shape.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of the atoms within a molecule, which is derived from the electron domain geometry after accounting for the positions of lone pairs. While electron pairs determine the initial geometric framework, the observable shape of the molecule is influenced by the fact that lone pairs take up more space than bonding pairs, thus slightly altering the idealized distances and angles.

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