Question

Predict the geometries of the following species using the VSEPR method: (a) \(\mathrm{PCl}_{3},\) (b) \(\mathrm{CHCl}_{3}\), (c) \(\mathrm{SiH}_{4}\), (d) \(\mathrm{TeCl}_{4}\)

Short answer

(a) Trigonal pyramidal, (b) Tetrahedral, (c) Tetrahedral, (d) Seesaw.

Step-by-step solution

- Determine the central atom and valence electrons for PCl3

The central atom in \( \mathrm{PCl}_{3} \) is phosphorus (P). Phosphorus has 5 valence electrons and each chlorine (Cl) atom contributes 1 electron. Since there are 3 Cl atoms, the total electron count is \( 5 + 3 = 8 \) electrons.

- Apply VSEPR theory to PCl3

Phosphorus in \( \mathrm{PCl}_{3} \) has 3 bond pairs and 1 lone pair. VSEPR theory predicts that \( \mathrm{PCl}_{3} \) will have a trigonal pyramidal geometry due to the lone pair-bond pair repulsion.

- Determine the central atom and valence electrons for CHCl3

The central atom in \( \mathrm{CHCl}_{3} \) is carbon (C). Carbon has 4 valence electrons. Each hydrogen (H) contributes 1 electron, and each chlorine (Cl) also contributes 1 electron. The total electron count is \( 4 + 1 + 3 = 8 \) electrons.

- Apply VSEPR theory to CHCl3

Carbon forms 4 bonds in \( \mathrm{CHCl}_{3} \), with no lone electron pairs. This leads to a tetrahedral geometry according to VSEPR theory.

- Determine the central atom and valence electrons for SiH4

The central atom in \( \mathrm{SiH}_{4} \) is silicon (Si), which has 4 valence electrons. Each hydrogen atom contributes 1 electron. Total electrons are \( 4 + 4 = 8 \).

- Apply VSEPR theory to SiH4

Silicon forms 4 bonds, with no lone pairs, in \( \mathrm{SiH}_{4} \). According to VSEPR theory, \( \mathrm{SiH}_{4} \) adopts a tetrahedral geometry.

- Determine the central atom and valence electrons for TeCl4

The central atom in \( \mathrm{TeCl}_{4} \) is tellurium (Te). Tellurium has 6 valence electrons and each chlorine contributes 1 electron, leading to \( 6 + 4 = 10 \) total electrons.

- Apply VSEPR theory to TeCl4

Tellurium in \( \mathrm{TeCl}_{4} \) forms 4 bonds and has 1 lone pair. The VSEPR model predicts a seesaw geometry.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. The shape of a molecule is determined by several factors, primarily the number of bonds and lone pairs of electrons around the central atom.
Understanding molecular geometry is crucial because it influences several physical and chemical properties of the molecule, such as polarity, reactivity, and color. One of the most effective ways to predict the shape of a molecule is through the VSEPR theory, which stands for Valence Shell Electron Pair Repulsion.

• The theory is based on the idea that electron pairs around a central atom will position themselves as far apart as possible to minimize repulsion.
• This prevents overlap and allows us to predict the shape of molecules.

For instance, in the molecule of \(\mathrm{PCl}_{3}\), VSEPR theory helps predict a trigonal pyramidal geometry due to the presence of a lone pair on phosphorus, which changes the typical tetrahedral distribution.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining an atom's chemical behavior. These electrons take part in chemical bonding, and their arrangement can be used to predict molecular shapes according to VSEPR theory.
For example, \(\mathrm{CHCl}_{3}\) has carbon as its central atom, which has four valence electrons and forms bonds with hydrogen and chlorine atoms. The total electron count around carbon remains 8.

• The total number of valence electrons influences how atoms pair up to form bonds.
• Free valence electrons can form lone pairs that influence molecular geometry, as seen in molecules like \(\mathrm{TeCl}_{4}\).

Identifying the number of valence electrons is the first step in applying the VSEPR model to predict 3D molecular structures.

Trigonal Pyramidal

Trigonal pyramidal geometry occurs when a molecule has three bonded atoms and one lone pair around the central atom. This geometry is different from a flat configuration like a trigonal planar because the lone pair occupies more space and repels the bonding pairs closer together, forming a pyramid shape.
An example of a molecule with trigonal pyramidal geometry is \(\mathrm{PCl}_{3}\), where phosphorus is the central atom surrounded by three chlorine atoms.

• The lone pair on phosphorus pushes the \(\mathrm{Cl}\) atoms down, creating a three-sided pyramid shape.
• The bond angles are slightly less than 109.5°, the typical angle for a perfect tetrahedral configuration, due to this repulsion.

Understanding these details helps predict how molecules might interact in chemical reactions.

Tetrahedral

A tetrahedral geometry is characterized by four bonds surrounding a central atom, with no lone pairs, resulting in bond angles of about 109.5°. This configuration is common in organic compounds and can significantly affect the molecule’s properties and behavior.
Both \(\mathrm{CHCl}_{3}\) and \(\mathrm{SiH}_{4}\) exhibit tetrahedral geometry. For example, silicon in \(\mathrm{SiH}_{4}\) has four equivalent \(\mathrm{Si-H}\) bonds, forming a symmetrical and stable molecular structure.

• In such geometries, the central atom can freely rotate, providing flexibility to the molecular framework.
• The geometric symmetry impacts molecular polarity and is crucial in determining how molecules react with one another.

Understanding tetrahedral geometry assists in predicting the chemical and physical attributes of a wide range of substances.

Seesaw Geometry

Seesaw geometry is a fascinating structure derived from the trigonal bipyramidal shape but modified due to lone pairs. In the case of \(\mathrm{TeCl}_{4}\), tellurium, the central atom, forms four bonds with chlorine atoms while having one lone pair.
The lone pair occupies one of the equatorial positions in a trigonal bipyramidal configuration, pushing the bonded chlorine atoms into a seesaw shape.

• This configuration leads to bond angles that are irregular, with those between axial and equatorial bonds differing from regular values.
• The presence of a lone pair results in increased repulsion, altering the molecule's geometry from a symmetrical form.

The unique seesaw shape affects how \(\mathrm{TeCl}_{4}\) might engage in chemical reactions and interact with other molecules, emphasizing the role of lone pairs in defining molecular structure.