Question

name the element having
1.) lowest work function
2.) Highest work function

Answer 1

highest work function is of platinum 6.35 ev
lowest work function is of  cesium 2.14 ev

Answer 2

Work function of an element is the minimum energy need to remove an electron from the surface of the solid to vacuum.
Reactive metals have very low work function whereas inert or relatively less reactive metals have high work function.

Element with lowest work function : Cesium (Cs)
Element with highest work function : Platinum (Pt)