Answer:
There must be two Chlorine atoms for every one Calcium atom in order to fulfill Chlorine's octet rule and pair Calcium's unpaired electrons.
Explanation:
Calcium has two unpaired electrons in its Lewis dot structure, while Chlorine has one unpaired electron.
So why can't we just make a double bond for one Chlorine?
Chlorine has seven valence electrons, so once it shares electrons with Calcium, the octet rule is accomplished, and no more pairs can be made.