Methane is an alkane. It has no double bonds which alkenes do have.
The same place as where you get oil... underground. Methane is an organic compound made of carbon and hydrogen just like any other oil molecule.
Methane has the formula CH4. So its molecular weight is 12 + 4 = 16. Is that what you mean?
Methane is just a general word for the substance, while methane ice is frozen methane, in it's solid form. It has to be quite cold for this to happen, it's a gas at normal earth temperatures and...
they contain unreactive atoms