Question

(a) What is the shape of the thionyl chloride molecule, \(\mathrm{SOCl}_{2}\) ? Sulfur is the central atom. (b) How many different OSCl bond angles are there in this molecule? (c) What values are expected for the OSCl and CISCl bond angles?

Short answer

The shape of SOCl2 is trigonal pyramidal. There are two different OSCl bond angles in the molecule. The expected OSCl bond angle is slightly less than 109.5 degrees, while the ClSCl bond angle should be close to 109.5 degrees.

Step-by-step solution

Determining the Shape of SOCl2

To determine the shape of the SOCl2 molecule, we will use the VSEPR (Valence Shell Electron Pair Repulsion) theory. First, we count the valence electrons of each atom (Sulfur has 6, Oxygen has 6, and each Chlorine has 7 electrons) and add them together to get the total. Then, we use these electrons to form bonds and lone pairs around the sulfur atom. Sulfur forms two single bonds with chlorine atoms and one double bond with the oxygen atom. This arrangement leaves sulfur with no lone pairs. The molecular shape with three attachment points and no lone pairs on the central atom is trigonal pyramidal.

Counting the Different OSCl Bond Angles

In a trigonal pyramidal molecule like SOCl2, there are three different OSCl bond angles. Each chlorine atom forms an angle with the oxygen atom and the sulfur atom at the vertex. Since the molecule is not symmetrical due to the presence of the double bond with oxygen, these angles are not all equal.

Determining the Bond Angle Values

For a trigonal pyramidal molecule, the ideal bond angle is approximately 109.5 degrees, which is the bond angle for a perfect tetrahedral shape. However, due to the repulsion of the double-bonded oxygen having greater electron density, it will slightly compress the OSCl bond angles. Consequently, the expected OSCl bond angle will be slightly less than 109.5 degrees, while the ClSCl bond angle will likely be close to 109.5 degrees due to repulsion among the bonding pairs.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Understanding the molecular shape is crucial because it influences many properties, such as polarity, reactivity, and phase of matter under certain conditions. VSEPR theory provides a systematic approach to determine molecular geometry by considering the numbers of bonding pairs and lone pairs of electrons around the central atom.

Using VSEPR theory as a guideline, we can predict the molecular shape through simple steps, like counting valence electrons and identifying electron pairs. For instance, when determining the shape of the thionyl chloride molecule (SOCl₂), we observe sulfur as the central atom with three bonds (two single and one double) and no lone pairs, leading to a trigonal pyramidal shape. This shape is characterized by a pyramid-like structure with a triangular base, formed by the three atoms (one oxygen and two chlorine atoms) bonded to sulfur.

Bond Angles

Bond angles are the angles between adjacent lines representing bonds that emanate from a common central atom. These angles affect the spatial distribution of electrons and can greatly influence the physical and chemical properties of a molecule. VSEPR theory aids in predicting bond angles by considering that electron pairs, both bonding and nonbonding, repel each other and tend to occupy positions that minimize this repulsion, leading to the most stable structure.

In molecules with trigonal pyramidal geometry like SOCl₂, the bond angles are affected by the nature of the attached atoms and the presence of lone pairs or multiple bonds. For example, the differing electronegativity of the atoms and the double bond to oxygen cause discrepancies in the OSCl and ClSCl bond angles, influencing their predicted values away from the ideal tetrahedral angle of 109.5 degrees.

Trigonal Pyramidal

The trigonal pyramidal shape is a molecular geometry that arises when a central atom is covalently bonded to three other atoms, with no lone pairs on the central atom itself. The name 'trigonal pyramidal' reflects the three-sided pyramid-like shape of such molecules.

For SOCl₂, sulfur is the central atom bonded to two chlorines and an oxygen. Although based on a tetrahedral arrangement, the presence of the double bond with oxygen disrupts the symmetry, causing the three OSCl bond angles to differ. Additionally, the more significant repulsion from the double bond causes compression in the bond angles surrounding the oxygen atom, often leading to deviations from the ideal bond angles seen in a perfect tetrahedron.

Valence Electrons

Valence electrons are the electrons in an atom's outermost shell that participate in the formation of chemical bonds. These electrons determine an atom's bonding behavior and its ability to form molecules. By counting the valence electrons in an exercise like the one involving the SOCl₂ molecule, we determine the total number available for bonding and the resulting molecular structure.

Sulfur has six valence electrons, oxygen also has six, and each chlorine atom has seven. After forming the two single bonds with chlorine and the double bond with oxygen, all of sulfur's valence electrons are engaged, leading to no lone pairs remaining on the central atom. This count of valence electrons is an essential first step in predicting molecular geometry via the VSEPR theory and lays the foundation for understanding the shape and bond angles in molecules.