Calcium forms an ion with a positive 2 charge and chlorine forms an ion with a negative one charg, so the formula is ##CaCl_2##
Group 1 metals and group 2 metals form positive ions by losing 1 and 2 electrons respectively. Non-metals in group 17 gain 1, group 16 gain 2 and group 15 gain 3. which lose electrons form positive ions while elements that gain electrons form negative ions.
To write a formula, you must balance charges so the overall charge is zero. A simple way to do this is to swap the of the ion’s charge and make it the subscript of the other ion. However, leave off the number 1 and reduce to lowest whole number ratio.
Here is a video which discusses how to write formulas for ionic compounds. Video from: Noel Pauller
Very detailed explanation, with nice figure on this website: http://study.com/academy/lesson/calcium-chloride-uses-structure-formula.html
