Why must there be two chlorine atoms for every one calcium atom

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.