General question on Molecular Shape

[Ashley Odibo Dis3E]

Generally speaking, when determining the shape of a molecule how can you tell which atoms are going to fall in the same plane?

[Fanny Lee 2K]

I would first draw out the lewis dot structure and use the steric numbers to determine the VSEPR shape of the molecule. If it is linear, trigonal planar, square planar, T shaped, and bent, then it is on the same plane. If the VSEPR shape is drawn using wedges, then it does not lie on the same plane.

[Matthew Mar 1J]

I think another way to think of it is to count the regions of electron density there are around the central atom and see which regions correspond to atoms that are bonded to the center and which regions are lone pairs.
If you use the notation professor Lavelle used in class (AXE, A=central atom, X=bonded atoms, E=lone pairs), then generally any molecule with X=3 or less will be planar (AX2=linear, AXE3=linear, AX3=trigonal planar, AX2E=bent, AX2E2=bent, AX3E2=T-shape). So molecules with 3 or less regions of electron density from bonded atoms will be planar. The only exceptions are AX3E which is trigonal pyramidal (i.e. not planar), and AX4E2 which is square planar (X>3, but still planar).