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Title says the question, pretty much.

For example, in the Newman projection of ethane, the carbon hydrogen bond angles are $120$ degrees, not $109.5$ degrees, as they actually are in an $\text{sp}^3$ hybridized center. Why is this so, and does it have any effect on the arguments traditionally derived from Newman projections?

enter image description here

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  • $\begingroup$ why the downvote? this is a legitimate question $\endgroup$ – DrPepper Dec 29 '19 at 20:29
  • $\begingroup$ I also thought to downvote as for is at least bizarre $\endgroup$ – Alchimista Dec 30 '19 at 8:23
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It is a projection. Depending on the orientation of a water molecule with respect to the projection plane, the projected angle can be anywhere from zero to 180 degrees. Similarly, the angles in ethane projected to a 2D plane will be different from 109.5 degrees unless the three atoms making up the angle are in the plane:

enter image description here

Other projections (Fisher projection) make the angles appear as right angles. In either case, the 3D angles remain at 109.5 degrees.

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