Problem 38 - Chemistry The Science in Context

Question

The first compound containing a xenon-sulfur bond was isolated in \(1998 .\) Draw a Lewis structure for HXeSH and determine its molecular geometry at Xe.

Step-by-Step Solution

Answer

Based on the Lewis structure for HXeSH found through the systematic approach, the molecule has a central Xenon (Xe) atom bonded to two Hydrogen (H) atoms and one Sulfur (S) atom, with two lone electron pairs on the Xe atom. The molecular geometry around the Xe atom, according to VSEPR theory, is T-shaped.

Step 1: Determine total valence electrons

First, we need to determine the total number of valence electrons for the HXeSH molecule. Hydrogen (H) has 1 valence electron, Xenon (Xe) has 8 valence electrons (Group 18), and Sulfur (S) has 6 valence electrons (Group 16). So, the total number of valence electrons in the molecule is: Total valence electrons = 1 (for H) + 8 (for Xe) + 6 (for S) + 1 (for H) = 16 electrons

Step 2: Form single bonds

Now, we distribute the electrons to form single bonds between the central atom (Xe) and the surrounding atoms (H, S, and H). Xe has 4 unpaired electrons in its outer shell and can form single bonds with the two H atoms and the S atom. So, we use 6 electrons to form the single bonds: HXeSH After forming the single bonds, we have 10 electrons left (16 - 6 = 10).

Step 3: Distribute remaining electrons

Now we distribute the remaining 10 electrons to complete the octets for the non-hydrogen atoms. Hydrogen only requires 2 electrons to be stable, so their octets are already full. We distribute 6 electrons (3 lone pairs) around Sulfur and 4 electrons (2 lone pairs) around Xenon. HXeS H : : #: | :#

Step 4: Determine molecular geometry

Now, let's determine the molecular geometry around the central atom Xe. The VSEPR (Valence Shell Electron Pair Repulsion) theory states that the electron pairs around an atom will arrange themselves to minimize repulsion. The electron pairs are arranged based on the number of bond pairs and lone pairs. Around Xe, we have 3 bond pairs (Xe-H bond, Xe-S bond, and another Xe-H bond) and 2 lone pairs. Using the VSEPR notation, AX_3E_2 represents this configuration (A = central atom, X = surrounding atoms, E = lone pairs). For AX_3E_2, the molecular geometry is T-shaped. Therefore, the molecular geometry around Xe in HXeSH is T-shaped.

Key Concepts

Molecular Geometry

The molecular geometry of a compound provides a visual model of how atoms are arranged in a molecule. It is a key aspect in understanding a molecule's properties and reactivity. In the given exercise, we are asked to determine the molecular geometry of HXeSH, focusing on the xenon (Xe) atom.

To deduce the geometry, we consider both the bonding atoms and lone pairs of electrons around the central atom. In the case of HXeSH:

• Xenon (Xe) is the central atom.
• The molecular geometry depends on the arrangement of 3 bond pairs and 2 lone pairs around Xe.

Applying the Molecular geometry theory, a molecule with this configuration corresponds to a T-shaped geometry. This shape arises because the bonds and lone pairs arrange themselves to minimize electron repulsion, according to the VSEPR theory. Visualizing a T-shape can help predict other molecular attributes, such as polarity and intermolecular forces.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a significant role in chemical bonding. For any molecule, determining the total number of valence electrons is the first crucial step in drawing its Lewis structure and understanding its bonding behavior.

For HXeSH, we calculate the total valence electrons as follows:

• Hydrogen (H) contributes 1 valence electron, but there are two hydrogens, so a total of 2.
• Xenon (Xe), a noble gas, contributes 8 valence electrons.
• Sulfur (S), being in Group 16, contributes 6 valence electrons.

In total, HXeSH has 16 valence electrons. These electrons are used to form bonds and lone pairs in the molecule, guiding us to the correct Lewis structure and allowing us to predict its molecular geometry.

VSEPR Theory

VSEPR stands for Valence Shell Electron Pair Repulsion theory, a fundamental tool used to predict the shape of a molecule. It explains that electron pairs, both bonding and non-bonding, repel each other and arrange themselves as far apart as possible around the central atom to minimize repulsion.

In the case of HXeSH, VSEPR theory predicts the arrangement around Xe by considering:

• The number of bonding pairs (3) around Xe, forming bonds with two H atoms and one S atom.
• The lone pairs of electrons (2) present on Xe.

According to VSEPR theory, this arrangement (\( AX_3E_2 \), where A is the central atom, X is the number of bond pairs, and E is the number of lone pairs) results in a "T-shaped" molecular geometry. This structural prediction is crucial for understanding the physical and chemical properties of the molecule, such as angles between bonds and potential reactivity.