If you wanted to find an element that would react similarly to chlorine (CI),
where would you look and why?

 Fluorine (F) and chlorine (Cl) are two good choices.  See reasoning below.

Explanation:  Mendeleev arranged the elements in the periodic table in a manner that each group, or vertical column, contained elements that exhibited similar chemical properities.  He even reserved spaces for undiscovered elements when none of the known elements had properties similar to those above or below the known elements in a group.  An interesting event illustrates this property.  In December 2010, NASA announced an important new discovery that was to be broadcast on live TV, creating much speculation (e.g., UFO's are real and we'll see the aliens sequestered at Roswell!!!  I knew it.).  

The actual announcement:

"Dec. 2, 2010:  NASA-supported researchers have discovered the first known microorganism on Earth able to thrive and reproduce using the toxic chemical arsenic. The microorganism, which lives in California's Mono Lake, substitutes arsenic for phosphorus in the backbone of its DNA and other cellular components."

A look at the periodic table shows arsenic just below phosphorus, in the same group.   This was an unexpected find, but it might have been predicted with an understanding of how the elements are organized.  

Side note:  look how close silicon is to carbon.  Can there be a life form whose being is made up of silicon instead of carbon?  Sure . . . why not?  Maybe not on Earth, but . . . wait . . . did that marble statue just wink?

where is the circuit diagram ?