Account for the following:
a) The `C-=C` distance is shorter than `C=C` and `C-C` distance.
b) The stability of alkenes is in decreasing order of substitution by R.

Explanation

a) The carbon-carbon distance depends upon the shielding of carbon nuclei by bonded electrons. In `C-=C`, each carbon nuclei is shielded by six electrons while in `C=C` and `C-C` by four and two electrons respectively. As a result of this, the carbon atoms come closer and undergoing more of overlapping of atomic orbitals and thus form stronger bonds.
It can also be expllained on the basis of hybridisation concept. In `C-=C`, each carbon atom is sp-hybridized while in `C=C` and `C-C`, each carbon atom is sp-hybridzed while in `C=C` and `C-C`, each carbon atom is `sp^(2)`- and `sp^(3)` -hybridized respectively. More the s-character of the orbital, shorter will bond formed by the orbital. In sp, the s-character is maximum, hence `C-=C` distance is minimum.
b) According to Saytzeff's rule, stability of alkenes depends upon the substitution in `C=C` bond. More of substitution by R, the more stable is the alkene. Hence, the order of stability is:
`R_(2)C=CR_(2) gt R_(2)C gt RCH=CHR gt RCH=H_(2)Cgt H_(2)C=CH_(2)`