Suppose that an ethane molecule, CH3 CH3, is broken into two CH3 molecules in such a way that on

step 1 look at molecular geometry. So when assigning molecular geometries, what we're looking to do is

Suppose that an ethane molecule, CH3 CH3, is broken into two...

Suppose that an ethane molecule, $\mathrm{CH}_{3} \mathrm{CH}_{3},$ is broken into two $\mathrm{CH}_{3}$ molecules in such a way that one $: \mathrm{CH}_{3}$ molecule retains the electron pair that was originally the one making up the $\mathrm{C}-\mathrm{C}$ bond. The other $\mathrm{CH}_{3}$ molecule has two fewer electrons. Imagine that momentarily this $\mathrm{CH}_{3}$ molecule has the geometry it had in the ethane molecule. Describe the electron repulsions present in this molecule, and indicate how they would be expected to rearrange its geometry.

Key Concepts

Electron Deficiency and Charge Effects

When a molecule loses electrons, it becomes electron deficient, often leading to the formation of a positively charged center. This loss modifies the usual electron domain structure by effectively removing electron repulsion from one side of the atom and leaving an imbalance. Such charge effects drive the molecule to adopt a new geometry that can better stabilize the positive charge, typically resulting in a planar structure where the remaining electrons can be delocalized more effectively.

Molecular Hybridization and Geometry Rearrangement

Molecular hybridization involves the mixing of atomic orbitals to form new hybrid orbitals that determine the geometry of a molecule. In species that become electron deficient, the loss of electron density can lead to a change in the effective hybridization state of the central atom. As a result, the molecule may rearrange from one geometry (such as a tetrahedral arrangement from sp3 hybridization) to another (for example, a planar geometry from sp2 hybridization) in order to lower the overall energy by reducing electron repulsions.

VSEPR Theory

This theory explains that the spatial arrangement of electron domains (bonding pairs and lone pairs) around a central atom is determined by the repulsive interactions among them. The key idea is that each electron pair will orient itself as far as possible from the others to minimize repulsion, thereby dictating the molecule’s overall geometry rather than only the positions of the nuclei.

Electron Pair Repulsion

Electron pairs, whether involved in bonding or existing as lone pairs, repel each other due to their negative charges. The intensity of these repulsions depends on the electron density associated with each pair. When an imbalance occurs—for instance, when not all expected electrons are present—the distribution of repulsive forces changes, prompting the molecule to adjust its geometry to achieve a more stable configuration with minimized repulsion.

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