Answer:
2:1
Step-by-step explanation:
When a Group 2A (alkaline earth) metal reacts with a Group 7A (halogen) nonmetal, they form an ionic compound with a 1:2 ratio of cations to anions. The Group 2A metal ion loses two electrons to form a cation with a 2+ charge, while the Group 7A nonmetal gains one electron to form an anion with a 1- charge. In order for the compound to be electrically neutral, two anions are needed for each cation. So the ratio of anions to cations in the compound is 2:1.