What is the molecular geometry at the central atom in CH₂Br₂?
A. trigonal planar
B. trigonal pyramidal
C. square planar
D. tetrahedral
E. None of these
The correct answer and explanation is :
The molecular geometry at the central atom in CH₂Br₂ is tetrahedral.
To understand why, let’s break it down step by step:
Step 1: Draw the structure of CH₂Br₂
In CH₂Br₂ (dibromomethane), the central atom is carbon (C), which is bonded to two hydrogen (H) atoms and two bromine (Br) atoms. Therefore, the structure can be written as:
- C is the central atom.
- It has four single bonds: two to hydrogen atoms and two to bromine atoms.
Step 2: Count the number of electron pairs around the central atom
Carbon (C) in CH₂Br₂ is in Group 14 of the periodic table, so it has 4 valence electrons. In this molecule, each single bond involves one of these valence electrons. Since C is bonded to 4 other atoms (2 hydrogens and 2 bromines), it has 4 bonding pairs of electrons.
There are no lone pairs on the carbon atom. Therefore, the central carbon has 4 bonding pairs of electrons, and no lone pairs.
Step 3: Determine the electron pair geometry
The electron pair geometry depends on the number of bonding pairs around the central atom. According to the VSEPR (Valence Shell Electron Pair Repulsion) theory:
- 4 bonding pairs of electrons around a central atom will arrange themselves in a tetrahedral geometry to minimize electron-electron repulsion.
Step 4: Conclusion – Molecular Geometry
Since there are no lone pairs of electrons on the carbon atom, the molecular geometry is the same as the electron pair geometry, which is tetrahedral.
Thus, the correct answer is:
D. tetrahedral
Why not the other options?
- A. Trigonal planar: This geometry occurs with 3 bonding pairs of electrons and no lone pairs, which is not the case for CH₂Br₂.
- B. Trigonal pyramidal: This geometry occurs when there are 3 bonding pairs and 1 lone pair, which again, is not the case here.
- C. Square planar: This geometry occurs in molecules with 4 bonding pairs and 2 lone pairs (e.g., XeF₄), not in CH₂Br₂.
- E. None of these: As we’ve seen, the correct answer is tetrahedral, so this option is incorrect.
Therefore, tetrahedral is the correct molecular geometry for CH₂Br₂.