Three Electron Domains

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Three electron domains around a central atom is known generally as trigonal planar (sometimes triangular planar) and has two major variations you should know:

AX₃ - trigonal planar
AX₂E - angular

AX₃ - Trigonal planar

Molecule	Lewis Structure	3-D Structure	Comments
BF3			Note that BF3 is electron-deficient, with only six electrons in boron's valence shell. This will make it a good Lewis acid. BH3 does not exist as an independent species, but B2H6 (named diborane) does.
CO32¯			Formal charge places a -1 on each of the oxygens with three electron pairs. This ion shows resonance structures. All three bond lengths are equal and intermediate between a C-C bond (134 pm) and a C=C bond (154 pm) at about 147 pm.
NO3¯			Formal charge places a -1 on the oxygens with three electron pairs and a +1 on the N. This ion shows resonance structures. All three bond lengths are equal and intermediate between a N-O bond (136 pm) and a N=O bond (116 pm) at about 130 pm.

AX₂E - Angular

Molecule	Lewis Structure	3-D Structure	Comments
BrNO			ClNO has a bond angle of 113.3° and FNO has a bond angle of 110.1° Can you explain why? Hint: think about what the increasing electronegativity of Br to Cl to F does to the electron density (hence repulsive power) of that bonding domain.