How do you represent molecules in 3D? Does it matter that you draw hashes or wedges?

Tetrahedra are of course difficult to represent on paper. The simple rule is that the wedge projects out, the dotted line projects into the page.

A tip with regard to representation of stereochemistry. If you have represented a chiral tetrahedron on paper; the interchange of any #pair# of substituents, results in depiction of its optical isomer, its #enantiomer#.

Of course, if any pair of this #enantiomer# is interchanged, we revert back to the original stereoisomer (it doesn't matter if I don't interchange the original pair; interchange about a chiral tetrahedron always results in the enantiomer.) For instance, if I have depicted an #S# isomer of 4 coordinate carbon, interchange of any pair of substituents gives the #R# isomer; interchange any pair again of the #R# isomer, and the original #S# isomer is restored. You should try this operation with molecular models to convince yourself (and such models are always permitted in tests).

In early/General Chemistry classes, so long as you draw solid wedges to represent atoms coming out of the paper, dashed/hashed wedges to represent atoms going into the paper, and a line for atoms in the plane of the paper, that's fine.

You generally learn how to draw like this in the context of VSEPR theory, which, for simplicity, tends to be taught using symmetrical molecules that don't have specific stereochemistry (different properties based on which atoms are oriented in what way).

If an organic chemistry student reads this, however, and they are drawing stereoisomers such as meso, R, S, D-sugar, L-sugar, or other stereoisomers, it does matter that you draw the atomic orientations properly to represent specific molecules. These two molecules are different:

You can see that the hydroxyl (#"OH"#) group is coming out of the paper in the R isomer, but into the paper on the S isomer. If these R and S labels are necessary to distinguish these two molecules, they must be different molecules.

In more complex cases, such as in medicinal chemistry, whether a molecule is R or S may actually mean the difference between it being either medicinally useful or poisonous.