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
