Which element has the highest electronegativity? what is the numerical value?

Fluorine; 4. This is because it only needs one more electron to have a full octet and has low electron shielding between the nucleus and outermost electrons due to few energy levels (so the nucleus holds on tightly). That is why it has the largest electronegativity; it really does not want to lose an electron.

Cause its being chemically applied?

Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more