Which element forms an ionic compound when it reacts with lithium

Fluorine

Explanation:

Flourine forms an ionic bond when combined with lithium. The valence electronic shell of lithium has just one single electron. Fluorine has 7 electrons in its valence shell but requires just a single electron to complete its octet. When lithium and fluorine combines, fluorine achieves its octet and lithium resembles Helium, a noble gas. This transfer of electron between Li and F is what results in an ionic bond between the two elements. It is also worthy to note that for an ionic bond to form, the electronegative difference between the two atoms should be greater than 0.7.

Fluorine has an electronegativity of 4.0 and that of Li is 1.0. The difference is 3.0. This implies that an ionic compound would be formed. The compound is LiF, Lithium Fluoride.