The Electronic Structure of Atoms

Electromagnetic Spectrum

Electromagnetic Radiation Described using terminology of waves Wavelength: (λ) Distance from crest to crest or valley to valley Frequency: (ν) Number of wave crests passing a given point per unit time. Expressed as cycles per second or s-1 λv=speed of wave propagation or λv = c where c = speed of light in a vacuum c = 3.0 X 108 m s-1

Electromagnetic Radiation All forms of radiation, including visible light, consist of electric and magnetic fields that vary repetitively at right angles to each other.

Light: Wave-Particle Duality Light has been shown to behave as a wave. Light also has been shown to behave as a particle: Photon. Max Planck demonstrated that each photon contains a particular amount (quanta) of energy. The amount of energy that a photon possesses depends on the frequency of light Energy of a photon given by Planck’s equation: E = hv Or E = hc λ h = Planck’s constant h = 6.63 X 10-34 J s

Question: For what did Einstein receive his Nobel Prize in 1921? The photo electric effect When electromagnetic radiation of sufficient minimum energy strikes the surface of a metal (- electrode or cathode) inside an evacuated tube, electrons are knocked off the metal and travel to the + electrode (anode) to produce an electric current. Einstein’s observations: Electrons are ejected only if the light is of sufficiently short wavelengths (high energy), no matter how long or how bright the light shines The number of electrons ejected per second (current) increases as the brightness of the light (intensity) increases.

The Photoelectric Effect

The Photoelectric Effect This phenomenon cannot be explained by the idea that light is behaving as a wave. The explanation comes with the idea that light is behaving as a particle (photon) each with its associated quanta of energy. The photon’s energy gets transferred to a single electron when it collides with it. The intensity (brightness) of light, in wave terms is related to amplitude of light waves. In particle terms it is the number of photons hitting the target. Classical “wave” theory suggests that even low energy light should cause current to flow if the metal is exposed long enough. Electrons should accumulate enough energy and be released when this energy is sufficient for them to escape.

Homework Due Monday 9/29 Read 167 to 184 in text Do Questions: 2,4,7,8,12,13,15,20,24,26,27,28,35,39,40,42,44,47,48