Draw the Lewis structure for SeH2 in the window below and then answer the questions that follow.
- Do not include overall ion charges or formal charges in your drawing.
D What is the electron-pair geometry for Se in SeH₂?
What is the the shape (molecular geometry) of SeH2?
The Correct Answer and Explanation is :
Electron-pair Geometry and Molecular Geometry of SeH₂:
Electron-pair Geometry:
In SeH₂, selenium (Se) is the central atom, and it is bonded to two hydrogen atoms. Selenium has six valence electrons, and each hydrogen atom contributes one electron, giving a total of eight valence electrons. The two Se-H bonds are single bonds, and there are two lone pairs of electrons on the selenium atom.
To determine the electron-pair geometry, we use the VSEPR (Valence Shell Electron Pair Repulsion) theory. In SeH₂, the central selenium atom has four regions of electron density (two bonding pairs and two lone pairs). According to VSEPR theory, the electron pairs will arrange themselves in a tetrahedral geometry to minimize repulsion. Therefore, the electron-pair geometry of Se in SeH₂ is tetrahedral.
Molecular Geometry:
Molecular geometry refers to the spatial arrangement of atoms in a molecule, and it considers only the positions of the atoms, not the lone pairs. In SeH₂, since the lone pairs are positioned to minimize repulsion, they push the bonding pairs of electrons closer together. This results in a bent or V-shaped molecular geometry.
Thus, the shape (molecular geometry) of SeH₂ is bent, with an angle of approximately 92° between the H-Se-H bonds. This is similar to the shape of water (H₂O), where lone pairs on oxygen cause the molecule to adopt a bent shape rather than a linear configuration.
Summary:
- Electron-pair geometry: Tetrahedral
- Molecular geometry (shape): Bent
This structure is consistent with the predictions of the VSEPR model, where lone pairs exert more repulsion than bonding pairs, leading to a bent shape for the SeH₂ molecule.