Four Electron Domains

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Four electron domains around a central atom is known generally as tetrahedral and has three major variations you should know:

AX4 - tetrahedral
AX3E - trigonal pyramidal
AX2E2 - angular

AX4 - Tetrahedral

Molecule Lewis Structure 3-D Structure Comments
CH4 The H-C-H angle is the classic tetrahedral angle of 109.5°
A very interesting result of molecular orbital theory is that CH4 has one bond different than the other three.
CCl4 tetrahedral
NH4+ tetrahedral

AX3E - Trigonal pyramidal

Molecule Lewis Structure 3-D Structure Comments
NH3 Since the nonbonding pair is larger (and more repulsive) than a bonding pair, the hydrogens are pushed together and the H-N-H bond angle is 107.2° The bond angle in NF3 is 102.3° due to the greater electronegativity of F.
H3O+ The H-O-H bond angle varies depending on the presence of other ions in the solid.

AX2E2 - Angular

Molecule Lewis Structure 3-D Structure Comments
H2O The H-O-H bond angle is 104.5°

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