Use VSEPR theory to predict the molecular geometry of bromate ion, BrO3-.
1) trigonal-pyramidal
2) trigonal-planar
3) tetrahedral
4) T-shaped
5) trilateral
The Correct Answer and Explanation is:
To determine the molecular geometry of the bromate ion ((\text{BrO}_3^-)) using VSEPR (Valence Shell Electron Pair Repulsion) theory, we need to analyze the structure of the ion.
Step 1: Determine the number of valence electrons
The bromate ion has the chemical formula (\text{BrO}_3^-). Bromine (Br) is in group 17 of the periodic table, so it has 7 valence electrons. Oxygen (O) is in group 16, so each oxygen atom contributes 6 valence electrons. Since the ion has a negative charge, we add one extra electron to account for the negative charge.
- Bromine: 7 valence electrons
- Oxygen: (3 \times 6 = 18) valence electrons
- Extra electron (due to the negative charge): +1 electron
Total valence electrons = 7 + 18 + 1 = 26 electrons.
Step 2: Draw the Lewis structure
In the bromate ion, the bromine atom is the central atom, surrounded by three oxygen atoms. Bromine forms single bonds with each oxygen. To satisfy the octet rule, oxygen atoms will have lone pairs of electrons. The remaining electrons are placed as lone pairs on the oxygen atoms, ensuring that each oxygen has 8 electrons in its valence shell.
Step 3: Apply VSEPR theory
The bromine atom is surrounded by three bonding pairs of electrons (from the three Br-O bonds) and one lone pair of electrons. This gives a total of 4 electron pairs around the central bromine atom.
- Electron pair geometry: According to VSEPR theory, 4 electron pairs arrange themselves in a tetrahedral geometry, as this minimizes electron pair repulsion.
- Molecular geometry: Since one of the electron pairs is a lone pair, the molecular geometry is not tetrahedral but trigonal-pyramidal. Lone pairs occupy positions that cause the bonded atoms to form a pyramid shape rather than a flat plane.
Step 4: Conclusion
Based on the VSEPR theory and the arrangement of electron pairs, the molecular geometry of the bromate ion ((\text{BrO}_3^-)) is trigonal-pyramidal.
Answer: 1) Trigonal-pyramidal