Lone Pairs

[salvadorramos3k]

How is it that sometimes a molecule has 4 bonding regions (one of them being a lone pair) but the shape is not tetrahedral? In class my TA was showing up examples of identifying shapes and he would erase the lone pair, but I'm not sure why. Do lone pairs not really matter, just the actual bonds?

[Anish Patel 4B]

lone pairs count towards molecular geometry not electron domain geometry. so in the case of 4 electron domains (3 bonding and one lone pair), the electron domain geometry would be tetrahedral while the molecular geometry would be trigonal pyramidal.

[charleejohnson1L]

The geometry is based purely on electron dense areas, but the VSEPR model only uses the bonded parts, as the repulsion will cause all electron-dense areas to move as far away from each other. :) I hope this helps.

[Kayla Maldonado 1C]

lone pairs count towards molecular geometry not electron domain geometry. so in the case of 4 electron domains (3 bonding and one lone pair), the electron domain geometry would be tetrahedral while the molecular geometry would be trigonal pyramidal.

So if asked in a question what shape a certain molecule would have, would we give the electron domain geometry or molecular geometry, in your example tetrahedral or trigonal pyramidal?

[Emily_4B]

lone pairs count towards molecular geometry not electron domain geometry. so in the case of 4 electron domains (3 bonding and one lone pair), the electron domain geometry would be tetrahedral while the molecular geometry would be trigonal pyramidal.

So if asked in a question what shape a certain molecule would have, would we give the electron domain geometry or molecular geometry, in your example tetrahedral or trigonal pyramidal?

I believe you would say the shape is a trigonal pyrimidal

[Ashley R 1A]

This concept also applies to 5 electron densities.

For 5 electron densities where all 5 are bonds, we have a trigonal bi-pyramidal molecular geometry.
However when we have 4 bonds and 1 unshared electron pair we have a seesaw geometry.

[Merin Padayatty 3G]

The actual shape of the molecule is dependent on the bonding electron pairs.