What is the difference atomic structure between the two isotopes of chlorine? How would you account for the gr?

The number of Neutrons in the nucleus.  The Number of Protons and Electrons is constant.

The outer shell electrons does not change ===>  Chemical similarity

http://en.wikipedia.org/wiki/Isotopes_of_chlorine

You mean the two non-radioactive isotopes, ³?Cl and ³?Cl?
Because there are many others that are radioactive; 22 appear on the Brookhaven chart of nuclides.

There is no difference in the atomic structure between two isotopes of any element; their only differences are in number of neutrons in their nuclei. This leads also to a mechanical difference, in that more nucleons (neutrons + protons) means more mass (one nucleon has the mass of 1800 to 1900 electrons). Chemically, this affects only the rates of reaction.

The great chemical similarity is due to their being the same element, which means they have the same electron structure, and that is what determines their chemical behavior.