Draw the lewis structure of BH3 and then determine its electrondomain and molecular geometries

Draw the lewis structure of BH3 and then determine its electron
domain and molecular geometries.
a) trigonal / planar
b) trigonal / tetrahedral
c) trigonal planar/ tetrahedral
d) trigonal / bent (120

The Correct Answer and Explanation is :

To determine the Lewis structure, electron domain geometry, and molecular geometry of borane (BH₃), let’s go through each step carefully.

Step 1: Draw the Lewis Structure of BH₃

• Boron (B) is in Group 13 of the periodic table and has 3 valence electrons.
• Hydrogen (H) is in Group 1 and has 1 valence electron. Since there are three hydrogens, the total number of valence electrons for BH₃ is: [
  3 (\text{from B}) + 3 \times 1 (\text{from each H}) = 6 \text{ total valence electrons}
  ]
• In the Lewis structure, the boron atom will be placed at the center with three hydrogen atoms bonded to it, using all 6 electrons to form three B-H single bonds. Each hydrogen atom achieves its stable duet, while boron has only six electrons (an incomplete octet), which is typical for boron.

The Lewis structure looks like this:

  H
  |
H-B-H

Step 2: Determine Electron Domain Geometry

The electron domain geometry is based on the regions of electron density (bonding or lone pairs) around the central atom. In BH₃, boron has 3 bonding pairs of electrons and no lone pairs.

• Electron domain geometry is determined by the number of electron pairs around the central atom. Since there are 3 bonding pairs, the electron domain geometry is trigonal planar.

Step 3: Determine Molecular Geometry

• Molecular geometry is determined by the positions of the atoms, not the electron pairs. Since BH₃ has no lone pairs and the 3 hydrogen atoms are arranged symmetrically around the boron atom, the molecular geometry is also trigonal planar.

Correct Answer

The correct answer is (a) trigonal / planar.

Explanation

• Boron in BH₃ has three bonding pairs and no lone pairs, resulting in 3 regions of electron density. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, the arrangement that minimizes repulsion between these regions is a trigonal planar shape.
• In a trigonal planar geometry, the bond angles are approximately 120°, as the atoms and electron pairs spread out evenly in a flat, triangular shape around the central atom.
• Boron does not obey the octet rule here, as it only has 6 valence electrons, which is common for elements in Group 13.