18 which usually just referred to as the noble gasses. The whole purpose of chemical reaction is for atoms to either share electrons or exchange electrons in order to have full valence electron orbitals which is also called having a noble gas configuration. In this example Li was one above noble gas configuration and F was one below noble gas configuration so Li gave an electron to F so that they could both have a noble gas configuration.
I hope this helps. Let me know if anything is unclear.
