Amplitude modulated Kelvin probe force microscopy was performed on molybdenum (Mo)-thin films deposited on Si(001) substrates by RF magnetron sputtering. Evolution of film microstructure from amorphous to crystalline was observed with increasing RF power from 25 W to 200 W. Spatial mapping of work function across the film surface revealed that the Mo-thin film deposited at 200 W possesses an average work function similar to 4.94 +/- 0.06 eV while higher values were observed at lower RF powers. The genesis of distinct periodic changes in work function is attributed to the formation of the surface dipole layer associated with the adsorbates of different polarities (O-2(-) /OH- or H+). A phenomenological model is also presented to elucidate their effect.