What is the electron domain geometry (EDG) and molecular geometry (MG) of AsH3? A. trigonal bipyramidal, T-shaped B. tetrahedral, trigonal pyramidal C. trigonal planar, trigonal pyramidal D. octahedral, T-shaped E. tetrahedral, tetrahedral
The Correct Answer and Explanation is:
Correct Answer: B. tetrahedral, trigonal pyramidal
Explanation:
Arsine (AsH₃) is a molecule made up of one arsenic atom and three hydrogen atoms. To determine its electron domain geometry (EDG) and molecular geometry (MG), we use the VSEPR (Valence Shell Electron Pair Repulsion) theory. This theory is based on the idea that electron pairs around a central atom will arrange themselves in a way that minimizes repulsion, leading to a specific three-dimensional shape.
Arsenic (As) is in Group 15 of the periodic table and has five valence electrons. In AsH₃, three of these valence electrons are used to form single bonds with hydrogen atoms. This leaves one lone pair of electrons on the arsenic atom.
Thus, the total number of electron domains around the arsenic atom is four: three bonding pairs and one lone pair. According to VSEPR theory, four electron domains around a central atom create a tetrahedral electron domain geometry. This arrangement ensures that the electron domains are as far apart as possible, reducing repulsive forces.
However, molecular geometry focuses only on the positions of atoms and not lone pairs. In AsH₃, the lone pair occupies one of the tetrahedral positions, but it is not counted when naming the molecular shape. As a result, the molecule has trigonal pyramidal molecular geometry.
This geometry is similar to ammonia (NH₃), which also has a central atom with three bonding pairs and one lone pair. The lone pair in both molecules causes a slight compression of the bond angles, making them slightly less than the ideal tetrahedral angle of 109.5 degrees.
To summarize:
- Electron Domain Geometry (EDG): Tetrahedral
- Molecular Geometry (MG): Trigonal pyramidal
So, the correct option is B. tetrahedral, trigonal pyramidal.
