steric number of BH3, AlH3, H2Se?
The correct answer and explanation is:
Correct Answers:
- BH₃ (Borane): Steric number = 3
- AlH₃ (Aluminum hydride): Steric number = 3
- H₂Se (Hydrogen selenide): Steric number = 4
Explanation (300 Words):
The steric number is a concept used in molecular geometry to predict the shape of molecules. It is calculated using the formula:
Steric number = Number of bonded atoms + Number of lone electron pairs on the central atom
Let’s analyze each compound:
1. BH₃ (Borane):
- Central atom: Boron (B)
- Bonded atoms: 3 hydrogen atoms
- Lone pairs on B: 0 (boron only has 3 valence electrons and forms 3 bonds; it does not complete an octet)
- Steric number = 3 + 0 = 3
Geometry: Trigonal planar
Bond angle: Approximately 120°
Note: BH₃ is electron-deficient and can act as a Lewis acid.
2. AlH₃ (Aluminum hydride):
- Central atom: Aluminum (Al)
- Bonded atoms: 3 hydrogen atoms
- Lone pairs on Al: 0 (similar to boron, aluminum is in group 13 and forms 3 bonds without lone pairs)
- Steric number = 3 + 0 = 3
Geometry: Trigonal planar
Note: AlH₃ also does not satisfy the octet rule fully, and it is unstable as a monomer. It often exists as a polymer in solid state.
3. H₂Se (Hydrogen selenide):
- Central atom: Selenium (Se)
- Bonded atoms: 2 hydrogen atoms
- Lone pairs on Se: 2 (Se is in group 16, has 6 valence electrons, uses 2 for bonding, leaving 4 electrons or 2 lone pairs)
- Steric number = 2 + 2 = 4
Geometry: Bent (angular)
Bond angle: Slightly less than 109.5° due to lone pair repulsion
In summary, the steric number provides insight into a molecule’s 3D shape by counting bonded atoms and lone pairs. It plays a crucial role in predicting molecular geometry using VSEPR theory.