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1/ With one sentence explain why two compounds with the same generic formula, NH3 and BH3 have different molecular geometries.
My answer :The shape of the molecule depends on how many electrons and bonds you have in the outer shell of the central atom.
For NH3, it has 3 covalent bonds and a lone pair, so the molecule will look as a trigonal pyramid For BH3, it is the central atom and has no lone pair apart from the three bonds, so the molecule will look as a trigonal planar.
2/ With one sentence explain why the structures of H2S and H2O are so similar? Why are structure of CH4 and SiH4 so similar?
My answer:
- The structures of H2S and H2O are so similar because they have 2 Ione pair of electron and 2 bonds.
- The structure of CH4 and SiH4 are so similar because they have no Ione pair of electron and 4 bonds.
3/ Although PH3 and BH3 both contain bonds of similar polarity; one molecule is polar while the other is nonpolar. Explain
P/s: I still don't know how to explain question 3. Please help me check my answer (#1 and #2) is true and answer question 3?? Thanks a lot
The answers to the first two questions look very good.
To answer the third, consider that phosphine is a trigonal pyramid while borane is a trigonal planar molecule. While the bonds have similar polarity, they are arranged differently.
In a trigonal planar arangement, the vector sum of the bond polarity is zero; they balance out. In a trigonal pyramid arangement, there is an added third dimension to the structure, along which the bond polarity isn't balanced out.
