The atomic number of Lithium is 3. Therefore its electronic configuration is 1s² 2s¹.
It is easier for the atoms with fewer valence electrons to lose electrons to get a full outer shell than gain electrons. Thus Lithium with only one electron will lose the electron to get a full outer shell.
As it can be seen from the electronic configuration of the Lithium, there is only one electron in the valence shell of the Lithium. Thus Lithium needs to lose one electron to get a full outer shell.
