# What is the formula for calcium fluoride? How many fluorine atoms does it contain?

$C a {F}_{2}$. Why should this be so? Look at the Periodic Table. Where is calcium? Where is (the element) fluorine.
Calcium is a Group 2 metal. Fluorine is a Group 17 gas. Towards the right of the Periodic table, ELEMENTS tend to get smaller as the increased nuclear charge exerts a greater attractive force on the valence electrons. The small fluorine atom tends to attract electrons, and is energetically most stable when it forms the ${F}^{-}$ ion.
On the other hand, calcium is a large Group 2 metal. It has 2 valence electrons, which are in the valence (the outermost) shell, and nuclear charge is somewhat diminished for these electrons. Calcium tends to be oxidized (to lose electrons) to form $C {a}^{2 +}$ ions.
So if calcium and fluorine want to make music together (or at least form a chemical bond), then a formula of $C a {F}_{2}$ is entirely reasonable (why? because this combination is electrically neutral!). And in fact in nature the mineral fluorite ($C a {F}_{2}$) is very widespread.