Question

Predict the parent structures of the following molecules. (a) \(\mathrm{BeCl}_{2}\) (c) \(\mathrm{SCl}_{2}\) (e) \(\mathrm{H}_{2} \mathrm{Te}\) (g) \(\mathrm{BBr}_{3}\) (b) \(\mathrm{PH}_{3}\) (d) \(\mathrm{SO}_{2}\) (f) \(\mathrm{SiH}_{4}\) (h) \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

The parent structures are: linear for \(\mathrm{BeCl}_{2}\) and \(\mathrm{H}_{2}\mathrm{Te}\), bent or V-shaped for \(\mathrm{SCl}_{2}\) and \(\mathrm{SO}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\), tetrahedral for \(\mathrm{SiH}_{4}\), pyramidal for \(\mathrm{PH}_{3}\), and trigonal planar for \(\mathrm{BBr}_{3}\).

Step-by-step solution

Find the central atoms

Each molecule is composed of a central atom bonded with surrounding atoms or groups. The central atoms will be: Be in \(\mathrm{BeCl}_{2}\), S in \(\mathrm{SCl}_{2}\), Te in \(\mathrm{H}_{2}\mathrm{Te}\), B in \(\mathrm{BBr}_{3}\), P in \(\mathrm{PH}_{3}\), S in \(\mathrm{SO}_{2}\), Si in \(\mathrm{SiH}_{4}\), and O in \(\mathrm{H}_{2}\mathrm{O}\).

Determine the electron pair distribution around the central atom

The number of bonding and lone electron pairs in central atom's shell determines the molecular geometry. For example, in \(\mathrm{BeCl}_{2}\), Be has two pairs of bonding electrons and no lone pair, indicating linear structure.

Predict molecular structures

Applying the VSEPR model, we predict: linear structure for \(\mathrm{BeCl}_{2}\), bent or V-shaped structure for \(\mathrm{SCl}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\), linear structure for \(\mathrm{H}_{2}\mathrm{Te}\), trigonal planar structure for \(\mathrm{BBr}_{3}\), pyramidal structure for \(\mathrm{PH}_{3}\), bent or V-shaped structure for \(\mathrm{SO}_{2}\), and tetrahedral structure for \(\mathrm{SiH}_{4}\).

Key concepts

VSEPR Model

The Valence Shell Electron Pair Repulsion (VSEPR) model is a method used to predict the shape of individual molecules based on the repulsion between electron pairs on the valence shell of the central atom.
According to the VSEPR theory, electron pairs located in the outermost shell of an atom will arrange themselves in a way that minimizes repulsion, thus determining the molecule's geometric structure. This includes both bonding electron pairs (which form bonds with other atoms) and lone pairs (which do not participate in bonding but still repel other electron pairs).

When applying the VSEPR model to predict molecular geometries, one must first count the total number of electron pairs around the central atom, then distinguish between bonding pairs and lone pairs. This count directly influences the molecular shape. For instance, a molecule with two bonding pairs and no lone pairs, like \(\mathrm{BeCl}_{2}\), will adopt a linear shape to keep the electron pairs as far apart as possible.

Central Atom

The central atom in a molecule is typically the one with the highest capacity to form bonds due to having a larger number of available valence electrons.
It acts as the cornerstone of the molecule, with other atoms arranged around it. The central atom's identity is essential in determining the overall structure of the molecule because it directly affects how electron pairs are distributed.

In the example of \(\mathrm{BeCl}_{2}\), beryllium serves as the central atom. Despite having a limited number of valence electrons, it forms bonds with two chlorine atoms, making it the central link in the molecule. This central position is not just a matter of spatial arrangement but also of functional significance since the central atom often defines the molecule's chemical reactivity and interactions.

Electron Pair Distribution

Electron pair distribution refers to the spatial arrangement of electron pairs (both bonding and lone pairs) around the central atom. This distribution is crucial as it determines the molecule's geometry and polarity.
Bonding electron pairs are shared with surrounding atoms, while lone pairs are localized on the central atom. These different types of pairs exhibit varying levels of repulsion: lone pairs repel more strongly than bonding pairs.

For example, in \(\mathrm{H}_{2}\mathrm{O}\), the oxygen atom has two lone pairs and two bonding pairs. The repulsion between these pairs results in a bent geometry. Recognizing the differences in electron pair distributions helps explain why molecules with the same number of electron domains might have different shapes, like \(\mathrm{SO}_{2}\) and \(\mathrm{SiH}_{4}\).

Molecular Structures

Molecular structure, determined largely by the VSEPR model and electron pair distribution, describes the three-dimensional arrangement of atoms within a molecule. Understanding a molecule's geometry can give insights into reactivity, polarity, color, magnetism, and biological activity.
Molecular structures range from simple (linear, bent) to more complex (tetrahedral, trigonal bipyramidal, octahedral).

Returning to our examples, \(\mathrm{BBr}_{3}\) has a trigonal planar structure, and it is easy to visualize this flat arrangement of three equivalent bromine atoms around a central boron. Meanwhile, \(\mathrm{SiH}_{4}\) features a tetrahedral structure with silicon at the center, bonded to four hydrogen atoms equally spaced out in three dimensions, resembling a pyramid with a triangular base. Familiarity with these structural types aids in predicting and explaining the physical and chemical properties of molecules.