Use VSEPR theory to predict the electron-pair arrangement and the molecular geometry of tetrahydroborate ion, BH4″.
a. The electron-pair geometry is trigonal-pyramidal, the molecular geometry is trigonal-pyramidal.
b. The electron-pair geometry is trigonal-planar, the molecular geometry is trigonal-planar.
C. The electron-pair arrangement is trigonal-planar, the molecular geometry is trigonal pyramidal.
d. The electron-pair arrangement is tetrahedral, the molecular geometry is trigonal-pyramidal.
e. The electron-pair arrangement is trigonal-pyramidal, the molecular geometry is t-shaped. Of. The electron-pair arrangement is tetrahedral, the molecular geometry is tetrahedral.
The correct answer and explanation is :
Let’s break down the electron-pair arrangements and molecular geometries described in the options:
- Option D: The electron-pair arrangement is tetrahedral, the molecular geometry is trigonal-pyramidal. This is a correct answer and represents a common molecular geometry scenario. The electron-pair arrangement being tetrahedral refers to a central atom surrounded by four electron pairs. In this case, the central atom has one lone pair of electrons and three bonded atoms. The lone pair occupies one of the four positions, but the geometry of the molecule is determined by the positions of the atoms. As a result, the shape is trigonal-pyramidal, where the three bonded atoms form a pyramid with a triangular base, and the lone pair pushes the atoms down, resulting in an asymmetrical shape. An example of this arrangement is ammonia (NH₃), where nitrogen has three hydrogen atoms bonded to it and one lone pair.
- Option E: The electron-pair arrangement is trigonal-pyramidal, the molecular geometry is t-shaped. This is not accurate. If the electron-pair arrangement is trigonal-pyramidal, the molecular geometry cannot be t-shaped. A trigonal-pyramidal electron-pair arrangement means there are four regions of electron density (three bonded atoms and one lone pair), and this arrangement leads to a pyramidal shape with three atoms forming the base of the pyramid. A t-shaped molecular geometry typically occurs when there are five regions of electron density, and three of them are bonded to atoms while two are lone pairs (such as in chlorine trifluoride, ClF₃). So, this option is incorrect.
- Option F: The electron-pair arrangement is tetrahedral, the molecular geometry is tetrahedral. This is another correct answer. When the electron-pair arrangement is tetrahedral, and there are no lone pairs on the central atom, the molecular geometry is also tetrahedral. This means that the central atom is bonded to four atoms in a symmetrical arrangement, with bond angles of approximately 109.5°. An example of this is methane (CH₄), where carbon is bonded to four hydrogen atoms, resulting in a perfect tetrahedral geometry.
Conclusion:
- Option D correctly describes a tetrahedral electron-pair arrangement leading to a trigonal-pyramidal molecular geometry (e.g., NH₃).
- Option E is incorrect because a trigonal-pyramidal electron-pair arrangement does not lead to a t-shaped geometry.
- Option F is correct when describing a tetrahedral electron-pair arrangement with a tetrahedral molecular geometry (e.g., CH₄).
Thus, options D and F are correct, while option E is incorrect.