In both propane and cyclohexane all Carbon atoms are sp3 hybridized. But in cyclohexane not all carbon atoms lie in the sme plane as in propane?
Correct.
3 points of a triangle will always be on the same plane.
i want to know why. both are sp3 hybridized. butane will have coplanar carbon also?
No. Only 2 points (ie. a line) and 3 points (ie. a triangle) will have all points on the same plane.
okay. nice summary.
It means cyclopropane will be coplanar.
but not cyclobutane and not methylpropane.
Considering C atoms only, yes. If include H atoms, none of the molecules are co-planar, due to the C atoms being sp3.
Thank you for the additional point.