Why do beryllium and fluorine combine in a 1: 2 ratio?

Since Beryllium is in the second group on the periodic table, we know that it has 2 valence electrons that it wants to 'give up', while fluorine only wants to 'take' 1 electron.

Be gives 2+2

F takes 1 -1

So in order for it to be balanced, there must be 2 fluorine atoms for every beryllium atom.

2f:b
2:1