1. Determine the Valence Electrons:

* Calcium (Ca): Group 2, so it has 2 valence electrons.

* Fluorine (F): Group 17, so it has 7 valence electrons.

2. Achieve a Stable Octet:

* Calcium: Tends to lose its 2 valence electrons to achieve a stable electron configuration like the noble gas Argon (with 8 valence electrons).

* Fluorine: Tends to gain 1 electron to achieve a stable electron configuration like the noble gas Neon (with 8 valence electrons).

3. Balance the Transfer:

* Since calcium loses 2 electrons and each fluorine gains 1, we need two fluorine atoms to accept the two electrons from calcium.

4. Determine the Formula:

* Calcium loses 2 electrons, becoming a +2 cation (Ca²⁺).

* Fluorine gains 1 electron each, becoming -1 anions (F⁻).

* To balance the charges, the formula is CaF₂.

Therefore, the formula for the compound between fluorine and calcium, predicted by Lewis theory, is CaF₂.