Determine the molecular geometry about each interior atom and draw each molecule. (Skeletal structure is indicated in parentheses.) a. C2H2 (HCCH) b. C2H4 (H2CCH2) c. C2H6 (H3CCH3)

Understanding the shapes of molecules is integral in the study of chemistry, and the Valence Shell Electron Pair Repulsion (VSEPR) theory offers a practical method to predict molecular geometry. It operates on a simple premise—the electron groups (bonding and nonbonding electron pairs) around the central atom will arrange themselves to minimize repulsion.

For instance, when examining molecules like ethyne (C2H2), ethene (C2H4), and ethane (C2H6), VSEPR theory can be applied to forecast the spatial arrangement around each carbon atom. A linear geometry is observed in C2H2 due to two regions of electron density; in C2H4, three regions create a trigonal planar shape. Finally, C2H6's four regions of electron density result in a tetrahedral geometry. Each scenario illustrates how electron pairs orient themselves to be as far apart as possible, ensuring minimal repulsion, as per VSEPR guidelines.

Closely related to VSEPR theory is the concept of electron geometry, defined by the spatial arrangement of electron groups. These groups include both bond pairs and lone pairs of electrons. Electron geometry is crucial as it lays the foundation for molecular geometry.

For example, in the case of C2H2, the electron geometry is linear because the electron groups extend in opposite directions. In C2H4, the electron geometry is trigonal planar—a flat, triangular shape—owing to three electron groups. In C2H6, the electron geometry is tetrahedral, an angular, pyramid-like shape defined by four electron groups. Interestingly, the presence of lone pairs—unshared electron pairs—can alter the observed molecular shape, but these compounds do not have lone pairs. Understanding electron geometry helps clarify why molecules take on their specific 3D forms.

Chemical bonding is the force of attraction that holds atoms together in molecules. In the context of the VSEPR theory and predicting molecular geometries, the type of chemical bonds—single, double, or triple—plays a significant role. Bonds are regions of high electron density and thus are taken into account when applying VSEPR principles.

In ethyne (C2H2), the carbon atoms are held together by a triple bond, requiring each to form a single bond with hydrogen. This triple bond counts as one region of electron density, meaning each carbon has two electron groups. In comparison, ethene (C2H4) has a double bond between carbons and two single bonds with hydrogens per carbon, leading to three regions of electron density. Conversely, the single bonds throughout ethane (C2H6) result in four regions around each carbon. Appreciating the type of bonds between atoms aids in predicting how electrons are distributed around a central atom, thus determining the molecular shape.