Hope my question is understood.
No it isn't. Please note that if you're too lazy to explain the context of your question, then it is foolish of you to expect people to spend time and effort to give you thoughtful responses.
your identifying a special case. You would need to delve into quantum mechanics. In addition, to which molecule are your referring?
To the original poster: try a Google search on "tetrahedral angle" (including the quotes, to keep the words together). Some of the first few hits look like they might be what you're looking for.
As posed, the question in the OP has nothing to do with physics (or chemistry) - it is a standard high school level exercise in either co-ordinate geometry or vectors.
Sorry for forgetting my question for a long time, I am really sorry.
I wanted to ask that in methane the HCH angle is 109 deg 28 min. How in theory is this calculated ? If this has to something with co-ordinate geometry, I would like a bit clarification.I hope my question is clear this time and I'm in the right forum.
See these links:http://jchemed.chem.wisc.edu/journal/issues/1997/Sep/abs1086.html [Broken]
The tetrahedral angle can be formed by taking the midpoint of a cube and drawing four lines from this midpoint out to every other corner. It's then pretty easy to prove the angle.
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