What atomic or hybrid orbitals make up the sigma bond between c2 and cl in dichloroethylene, ch2ccl2 ? (c2 is the second carbon in the structure as written.) orbital on c2? orbital on cl ? what is the approximate c1-c2-cl bond angle ?

Answer is:

1) orbital on C2 is sp2 hybrid orbitals and orbital on Cl is p atomic orbital.

Carbon atom is sp2 hybridized, it has three sp2 orbitals and one p orbital, they form four bonds. Three sp2 orbitals form three sigma bonds. Pi bonds are formed between two p orbitals on C1 and C2 atoms.

In sp2 hybridization the 2s orbital mixes with only two of the three available 2p orbitals.

2)  the approximate C1-C2-Cl bond angle is 120°.

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.