[dipesh747]

So I'm sure people are aware of elements such as S and P can form 6 and 5 bonds respectively.
But why does Chlorine not use SPD hybrid orbitals to form theoretically 7 bonds.

[Schrödinger]

Well, it does form 7 bonds in the ion ClO₄^- doesnt it?

[dipesh747]

Don't think I've ever come across that one before!! 

[Schrödinger]

HClO₄ - it's called perchloric acid. As a matter of fact, Cl, Br and I all form such acids with +7 oxidation state... perchloric/perbromic/periodic acid

[Borek]

For periodic acid it is more complicated, as it exists in more than one form (google orthoperiodic acid). Perbromic is rather unstable.

[cheese (MSW)]

Although extremely reactive, ClF5 is a well-characterized cmpd.
Cl (and Br) however do not form an analogue of IF7 (pentagonal
bipyramid).
The reason for this is either steric or electronic.  Consider each in turn
and come up with both a steric an electronic reason for the instability of
ClF7.  In particular, how do the np-nd energy gaps behave as one goes from
Cl right arrow Br right arrow I?
(Hybridization is now thought to be an artifact and the term should be avoided.)

"I believe the chemical bond is not so simple as some people seem to think." 
Robert S. Mulliken