All atoms (except for the noble gases) are unstable because their outer electron shell in not full.  Atoms react with other atoms to gain or lose electons to fill their outer shell.

Flourine has seven electrons in its outer shell and requires only one electron to fill its outer shell.  It has a strong desire to gain one electron to fill its outer electron shell (electronegative).

Lithium has only one electron in its outer shell.  It is much easier to give up one electron than to gain 7 electrons to get a full outer shell.  It has a strong desire to lose one electron to get a full outer shell. (electropositive)