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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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tommyo0729 tommyo0729
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11 years ago
On my chemistry test, my professor asked for the geometry.  I put tetrahedral, and I got it wrong.  My professor said the right answer was trigonal pyramidal.

But in the book, there are cases when they ask me a similar question, and the answer is tetrahedral, not trigonal pyramidal.

Is one referring to shape, and one referring to VSEPR?
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#1
11 years ago
It cannot be tetrahedral, otherwise it was referring to NH4+ because the structure requires 4 attachments to a center body.

In NH3, there are only 3 attachments to a centered body to make it trigonal pyramidal.

The free electrons on top of the Nitrogen bends it into that shape.
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#2
11 years ago
The molecular geometry simply refers to the shape of the molecule without taking into account the lone pairs of electrons, which is what your professor was asking.  Tetrahedral is the geometry when the lone pairs are considered, called the electron-pair geometry.
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#3
11 years ago
The N-bonding orbitals are sp3, but there is a pair of unshared electrons in one of the orbitals.  The other 3 each overlap the s-orbital of the hydrogen atom.   I suppose they are referring to the orbitals that are actually bonding to another entity.
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#4
11 years ago
Trigonal pyramidal = AB2E where AB2 are the two compounds involved and E the lone pair of electorns it has. A+B2  = 3. the lone pair doesn't matter.
 Tetrahedral is AB3E.
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