Many scientists found Rutherford’s Model to be incomplete He did not explain how the electrons are arranged He did not explain how the electrons were not pulled in by the positive nucleus He did not explain the differences in chemical behavior In the early 1900’s, scientists began to analyze the visible light that elements emitted when heated An element’s chemical behavior is related to the arrangement of its electrons To better understand chemical behavior, we must first better understand light Light By 1900, evidence supported that light consists of waves. Wave Properties Amplitude- wave’s height from zero to crest. Wavelength- (λ) length from one point of one wave to same point on next wave. Frequency- (f) number of wave cycles to pass a given point per unit of time. Hertz- units of frequency = cycles/sec Wavelength and frequency are inversely proportional to one another. Speed = wavelength x frequency (V = λf) Wave Nature of Light Consists of radiation (energy) over several wavelengths. Visible light portion is very small Light consists of electromagnetic waves. Electromagnetic Radiation- includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays. All waves travel at the speed of light (c), which is 3.0 x 108 m/s. Practice If the wavelength of yellow light is 580 nm, determine the frequency. The wave model is great, but it does NOT explain some important aspects of light’s interactions with matter Particle Nature of Light Max Planck studied the light emitted from heated objects He concluded that matter can gain or lose energy in small specific amounts (quanta) Quantum – minimum amount of energy that can be gained or lost by an atom Photoelectric Effect Electrons are emitted from a metal’s surface when certain frequency light shines on it The frequency of light that required to do this varies by metal Albert Einstein- proposed that light could be described as quanta of energy. Light quanta are called photons. Wave-Particle behavior of light. Energy of photon = hv (E=hf ) h- Planck’s constant = 6.626 x 10-34 Js v/f – frequency Also, remember c = v λ So… E = hc / λ Practice How much energy is emitted when light with a wavelength of 350 nm strikes an atom? Atomic Spectra Electrons are normally at ground state By absorbing energy, the energy level rises to 2, 3, 4,or more (excited state) These electrons lose energy by emitting light when they return to lower energy levels. Atomic emission spectrum- frequencies of light emitted by an element separate into specific lines. Each line in an emission spectrum matches to one exact frequency of light emitted by the atom. No two elements have the same emission spectra. They are like the fingerprints of elements!