Which of the following molecules contain a C3 axis but not a σh plane?
AlCl3
SeO3
SbBr3
PCl3
To determine which of the molecules contain a C3 axis but not a σh plane, we need to analyze the symmetry elements of each molecule.
1. AlCl3:
- AlCl3 has a trigonal planar shape.
- It contains a C3 axis, which is a rotation axis that passes through the central atom and divides the molecule into three equivalent positions.
- It also has a σh plane, which is a horizontal mirror plane that bisects the molecule into two equivalent halves.
Conclusion: AlCl3 contains a C3 axis and a σh plane, so it does not meet the criteria.
2. SeO3:
- SeO3 has a trigonal pyramidal shape.
- It does not have a C3 axis, as the rotation axis would have to pass through the central atom and divide the molecule into three equivalent positions, which is not the case.
- SeO3 does not have a σh plane either.
Conclusion: SeO3 does not have a C3 axis or a σh plane.
3. SbBr3:
- SbBr3 has a trigonal pyramid shape.
- It contains a C3 axis, which is a rotation axis that passes through the central atom and divides the molecule into three equivalent positions.
- SbBr3 does not have a σh plane, as there is no horizontal mirror plane that bisects the molecule into two equivalent halves.
Conclusion: SbBr3 contains a C3 axis but does not have a σh plane.
4. PCl3:
- PCl3 has a trigonal pyramid shape.
- It contains a C3 axis, which is a rotation axis that passes through the central atom and divides the molecule into three equivalent positions.
- PCl3 does not have a σh plane, as there is no horizontal mirror plane that bisects the molecule into two equivalent halves.
Conclusion: PCl3 contains a C3 axis but does not have a σh plane.
In summary, the molecules SbBr3 and PCl3 both contain a C3 axis but not a σh plane.
To determine which of the given molecules contain a C3 axis but not a σh plane, we need to understand the symmetry elements of each molecule.
First, let's define the two symmetry elements in question:
1. C3 axis: A molecule possesses a C3 axis if it has a rotational symmetry of 120 degrees around an axis. This means that if you rotate the molecule by 120 degrees around the axis, it will align with its original orientation.
2. σh plane: A molecule possesses a σh plane if it has a horizontal plane of symmetry. This means that if you fold the molecule along the plane, the two halves will be identical.
Now, let's determine the presence of these symmetry elements in each molecule:
AlCl3:
- AlCl3 does not possess a C3 axis because rotation by 120 degrees does not align the molecule with its original orientation.
- AlCl3 does not possess a σh plane because folding the molecule along any horizontal plane does not result in identical halves.
SeO3:
- SeO3 possesses a C3 axis because rotation by 120 degrees aligns the molecule with its original orientation.
- SeO3 does not possess a σh plane because folding the molecule along any horizontal plane does not result in identical halves.
SbBr3:
- SbBr3 possesses a C3 axis because rotation by 120 degrees aligns the molecule with its original orientation.
- SbBr3 does not possess a σh plane because folding the molecule along any horizontal plane does not result in identical halves.
PCl3:
- PCl3 possesses a C3 axis because rotation by 120 degrees aligns the molecule with its original orientation.
- PCl3 does possess a σh plane because folding the molecule along a horizontal plane results in identical halves.
Therefore, the molecule that contains a C3 axis but not a σh plane is AlCl3.