Question

How can you tell if two atoms are in the same plane?

I am trying to draw 3-D molecular bond structures with wedges and lines, and I cannot decipher how I can place a "plane" so as to see if two atoms are in the same plane.

Answer 1

Simply choose three or more atoms to be on the same plane (at least three atoms are required to make a 2D angle), and make the remaining ones not in that plane.

The ones not in the plane are drawn with wedges (front) and dashes (rear).

The simplest 3D example is methane, tetrahedral:

In this case, the lefthand `"H"-"C"-"H"` is in the same plane, while the bottom-right `"H"` is in front, and the middle-right `"H"` is in the rear.

This first way is most common, but if this helps, methane can also be drawn as:

...if you rotate the first image by `120^@` counterclockwise. I drew it both ways since tetrahedral in particular can be difficult to visualize and rotate in your head.

A more complicated example:

This is `"PCl"_5`, phosphorus pentachloride, trigonal bipyramidal.

Here, you can see that the right-hand `"Cl"` (one equatorial and two axial) are in the same plane, since they are all lines. The bottom-left `"Cl"` is in front, and the upper-left `"Cl"` is in the rear.

Basically, if you are unsure, consider buying an actual model kit or borrowing one, and making the structures yourself so you can see it.