Why doesn't the density of the object change when you adjust the mass and volume of the objects?

Because as you change the mass the volume will change and as you change the volume the mass will change too, it's a constant ratio for each element. Mass and volume are not factors that affect the density. factors that affect the density are temperature and type of the material/element

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.