If atoms are made up of smaller particles, such as protons, neutrons, and electrons, then why are atoms considered the smallest particles of an element?

Light traveling in water, nwater = 1.33, strikes a plastic block at an angle of incidence of 51.4°; part of the beam is reflected and part is refracted. if the index of refraction of the plastic is 2.0, what is the angle made by the reflected and refracted beams?