electron geomtry vs. molecular geometry

[badasshairday]

Sorry if this is a dumb question, but what the hell is the difference?

What is the difference between trigonal pyramidal and tetrahedral?

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[SpartanBlueJay2]

All you chem geniuses out there, correct me if I'm wrong, but here's my 2 second explanation:

Both trigonal pyramidal and tetrahedral are sp3 hybridized, but differ in the number of atoms attached to the central atom. For example, methane and ammonia are both sp3, but methane has 4 H atoms attached to the C, so it is tetrahedral, while ammonia has 3 H atoms attached to the N, so it is trigonal pyramidal.

 

[canjosh]

Sorry if this is a dumb question, but what the hell is the difference?

What is the difference between trigonal pyramidal and tetrahedral?

NH3 is trigonal pyramidal because of the lone pair. H2O has 2 lone pairs and 2 atoms bonded to the central H atom. H2O's molecular geometry is angular (104.5 degrees). Just remember molecular geometry theory is concerned with only the positions of bonded atoms, not the positions of lone pairs. You still take into account the existence of lone pairs (if that makes sense).

Electron pair geometry IS concerned with positions of bonded atoms AND positions of lone pairs.