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hi there! i was just wondering how to tell the difference between a tetrahedral molecule and a triangular pyramidal molecule, or if they can sometimes be interchangeable. I remember from the textbook that lone pairs are considered significant regions of high electron configurations just like bonds. So, wouldn't a molecule with three bonds and one lone pair around the central atom be considered a tetrahedral shape?
A tetrahedral shape refers to four electron densities around a central atom. Trigonal pyramidal refers to three atoms around a central atom and then one lone pair. Trigonal pyramidal is technically tetrahedral shape since there are four electron densities around the central atom. But it is more specific to say the shape is trigonal pyramidal if there is a lone pair.
This is where lone pairs come into play. A molecule with four regions of electron density (lone pairs and bonded atoms) is considered tetrahedral, but if there are 3 bonded regions and one lone pair, the shape is trigonal pyramidal. Basically, when you count the regions of electron density, that gives you the basic shape of the molecule (linear, trigonal planar, etc.), but the actual specific shape of the molecule is given when you take into account what makes up those regions of electron density (bonded pairs v. lone pairs). So to answer your last question, a molecule with 3 bonds and 1 lone pair has an electron group shape of tetrahedral, but its actual molecular shape is trigonal pyramidal.
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