Hydrogen selenide, represented by the chemical formula \(\text{SeH}_2\), is a simple hydride compound that serves as an excellent example for understanding molecular architecture. The molecular geometry of \(\text{SeH}_2\) is Bent or Angular. \(\text{SeH}_2\) is a colorless, flammable gas under standard conditions and is known to be the most toxic selenium compound. This structure is responsible for many of the compound’s chemical behaviors.
Determining the Lewis Structure of \(\text{SeH}_2\)
The initial step in determining molecular geometry involves mapping the arrangement of valence electrons using the Lewis structure. Selenium (\(\text{Se}\)) is the central atom because it is less electronegative than Hydrogen (\(\text{H}\)). As an element in Group 16, selenium contributes six valence electrons, and the two hydrogen atoms contribute one each, totaling eight valence electrons for the \(\text{SeH}_2\) molecule.
These eight electrons are distributed to satisfy the bonding requirements. Two single covalent bonds form between the central selenium atom and the two hydrogen atoms, using four valence electrons. The remaining four electrons are placed on the central selenium atom as two non-bonding pairs, commonly referred to as lone pairs. This arrangement of two bonding pairs and two lone pairs is the fundamental information needed to predict the molecule’s three-dimensional shape.
Principles of Valence Shell Electron Pair Repulsion Theory
The three-dimensional shape of a molecule is predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that all electron domains surrounding a central atom will repel one another. An electron domain can be a single bond, a double bond, a triple bond, or a lone pair of electrons.
To minimize this repulsion, these domains arrange themselves in three-dimensional space to achieve maximum separation. The resulting arrangement of all electron domains defines the electron geometry of the molecule. The molecular geometry is then determined by looking only at the positions of the atoms within that arrangement, ignoring the lone pairs.
Applying VSEPR to Determine Electron and Molecular Geometry
Applying the VSEPR principles to \(\text{SeH}_2\) begins with counting the total number of electron domains around the central selenium atom. The Lewis structure revealed four electron domains: two bonding pairs and two non-bonding lone pairs. This number of four electron domains dictates the electron geometry.
The arrangement that provides the greatest separation for four electron domains is the Tetrahedral Electron Geometry, which has an ideal bond angle of \(109.5^\circ\). However, the presence of lone pairs modifies the final molecular shape. The two lone pairs on the selenium atom exert a stronger repulsive force than the bonding pairs, pushing the two \(\text{Se-H}\) bonds closer together.
This stronger repulsion compresses the \(\text{H-Se-H}\) bond angle significantly below the ideal \(109.5^\circ\). The measured bond angle for \(\text{SeH}_2\) is approximately \(91^\circ\), confirming this compression effect. The resulting shape, where the three atoms form a V-shape, is correctly classified as Bent or Angular molecular geometry.
Why the Specific Geometry Matters: Physical Properties of \(\text{SeH}_2\)
The bent molecular geometry of \(\text{SeH}_2\) influences its physical and chemical properties by giving the molecule a net dipole moment. Polarity is determined by both the polarity of its individual bonds and the overall molecular shape. The \(\text{Se-H}\) bond is slightly polar because selenium has a higher electronegativity than hydrogen.
Because the molecule is bent, the individual bond dipoles do not cancel each other out. This means that hydrogen selenide is a polar molecule, though its polarity is quite low. This polarity influences physical characteristics, such as its relatively high boiling point compared to non-polar molecules of similar size.
Hydrogen selenide is a colorless gas highly soluble in water, forming a weak acid known as hydroselenic acid. The polar \(\text{SeH}_2\) molecule forms dipole-dipole interactions with water, contributing to this high solubility. The compound is also notable for its extreme toxicity and its foul, characteristic odor, often described as similar to rotten eggs or decaying horseradish.