Final answer:
The correct formula for the ionic crystal formed between calcium and fluoride is CaF₂, as two fluoride ions are needed to balance the charge of one calcium ion (option B).
Step-by-step explanation:
The question asks for the correct formula for an ionic crystal formed between calcium (Ca²⁺) and fluoride (F⁻). In ionic bonding, the total positive charge must balance the total negative charge, resulting in a neutral compound. Calcium has a charge of 2+ (Ca²⁺), and fluoride has a charge of 1- (F-). To balance the charges, two fluoride ions are needed for every calcium ion, so the correct formula is CaF₂.
Hence, the answer is option B.