difference between the bond angles and the bond lengths of alkane and alkene groups.

Dear student,

The bond angle is approximately 120 degrees (there are three approximately equivalent hybrid orbitals in the xy plane. The full circle of 360 degrees is thus divided into three approximately equivalent angles. We use the term "approximately" here, because the angles are not exactly equivalent and are not exactly 120 degrees. This is because the atoms to which carbon is bonded are not the same (2H's and 1 C), therrefore the three sigma bonds are not exactly equivalent. The HCH angle is less than 120 and the HCC angle is greater than 120.

You should know that the C=C is comprised of one sigma and one pi bond, and that optimum pi overlap requires the two 2p_z orbitals to be "parallel", i.e., to have a 0 degree dihedral angle for the most efficient overlap. Recall that pi overlap is lateral and thus is less efficient than sigma overlap. Hence the pi bond is weaker than the sigma bond, easier to break, making alkenes much more reactive than alkanes. You should know that the pi bond strength is ca. 63 kcal/mol, and that the pi bond is completely broken by a relative rotation of 90 degrees.