What is the molecular geometry of the compound AsH3?
The Correct Answer and Explanation is:
The molecular geometry of arsine (AsH₃) is trigonal pyramidal.
Explanation:
Arsine (AsH₃) consists of one arsenic (As) atom bonded to three hydrogen (H) atoms. To determine the molecular geometry, we use Valence Shell Electron Pair Repulsion (VSEPR) theory.
- Valence electrons:
- Arsenic is in group 15, so it has 5 valence electrons.
- Each hydrogen contributes 1 electron, and there are 3 hydrogen atoms.
- Total valence electrons = 5 (As) + 3(1) = 8 electrons.
- Electron regions around the central atom (As):
- Three bonding pairs (each with one hydrogen atom)
- One lone pair of electrons on the arsenic atom
This makes a total of four electron regions around arsenic. According to VSEPR theory, four regions of electron density arrange themselves in a tetrahedral electron geometry to minimize repulsion. However, because one of these regions is a lone pair rather than a bonding pair, the shape of the molecule is no longer perfectly tetrahedral.
- Molecular geometry:
- When one of the four regions is a lone pair, the molecular shape becomes trigonal pyramidal.
- The three hydrogen atoms form the base of a pyramid, and the lone pair occupies the top position, pushing the hydrogen atoms slightly closer together.
- Bond angle and influence of lone pair:
- The ideal bond angle for a tetrahedral geometry is about 109.5 degrees.
- However, lone pairs exert more repulsion than bonding pairs. This causes the H-As-H bond angle in AsH₃ to shrink slightly, typically to around 91 to 93 degrees.
In conclusion, although AsH₃ has a tetrahedral electron geometry due to four regions of electron density, its molecular geometry is trigonal pyramidal due to the presence of a lone pair on the central arsenic atom.
