# How can it be certain that some bonds are 180 degrees?

Take acetylene, for example.

It is defined that the two outer bonds are at 180 degrees.

How is it possible to theoretically define all $\ce{C-H}$ bonds in acetylene as being 180 degrees?

• With a protractor? — Kidding aside, we have no idea what's your background, what sort of explanation you want (theoretical? experimental?), and at what level. Thus this question can hardly be answered in its present form.
– F'x
Dec 9, 2013 at 15:53
• Theoretical, I mean, why can't those outer bonds be, say, 60 degrees? Dec 9, 2013 at 15:54
• Again: what's your level of understanding of molecules and their shapes?
– F'x
Dec 9, 2013 at 15:56
• Not a whole lot. I have a working knowledge of the octet rule and the purpose of bonds, but the measure of those bonds is what I do not understand Dec 9, 2013 at 15:58
• I think this question is clear, and shouldn't be on hold. There is a simple and definite answer: acetylene is perfectly linear when you 'freeze' its vibrations, because there is nothing pushing H's to bend. Look at its charge density at uam.es/departamentos/ciencias/qfa/DAM/acetylene.html Dec 10, 2013 at 2:44