7.1-7.3 The Nature of Light – Atomic Spectroscopy and the Bohr Model

The Nature of Light The quantum-mechanical model (a.k.a. the electron cloud model) – An atomic model that explains the strange behavior of electrons. Electrons determine the chemical and physical properties of elements. Electromagnetic radiation – a type of energy embodied in oscillating electric and magnetic fields. The speed of light (c): 2.998X108 m/s  on your reference table.

The Nature of Light (Continued) Amplitude: the vertical height of the crest (or the depth of the trough. In light a higher amplitude means more intense light. Wavelength (λ): the distance between two adjacent crests. Units are meters (or nano- or…) More energetic waves have higher amplitudes and shorter wavelengths. Frequency (ν): the number of waves (cycles/s), or s-1, or Hz. c = λ ν For Visible light: light that can be seen by the human eye, wavelength determines color. Red has the longest wavelength and violet has the shortest.

Let’s Try a Sample Problem A laser dazzles the audience in a rock concert by emitting green light with a wavelength of 515 nm. Calculate the frequency of the green light. c = λ ν c 2.998X108 m/s ν = --- = --------------------- = 5.82X1014 Hz λ 5.15X10-7 m

The Electromagnetic Spectrum Other than visible light, what other types of electromagnetic radiation are there? Low energy waves: radio waves, microwaves, Infrared (IR) waves. High energy waves: Ultraviolet, x-rays, and gamma rays

The Particle Nature of Light The photoelectric effect: the observation that many metals emit electrons when light shines upon them. Only high frequency light, transfers energy into the electron, excites and dislodges the electrons within the metal. The energy of a photon has to at least be equal to the binding energy of the electron to eject the electron. Albert Einstein explained that light energy comes in packets (a photon or quantum) of light. E = hν E = amount of energy in a light packet h = Planks constant = 6.626X10-34 J s  in your reference table hc E = ----- λ

Let’s Try a Practice Problem The energy required to dislodge electrons from sodium metal via the photoelectric effect is 275 kJ/mol. What wavelength in nm of light has sufficient energy per photon to dislodge an electron from the surface of sodium? hc E = ----- λ hc (6.626X10-34 J s)(2.998X108 m/s) λ = ------ = ------------------------------------------ ----------------------- = E (2.75X105 J/mol) (1 mol/6.02X1023 electrons) λ = 4.35X10-7 m = 435 nm

Atomic Spectroscopy and the Bohr Model Atomic spectroscopy: the study of electromagnetic radiation absorbed and emitted by atoms. When atoms absorb a specific amount of energy, electrons become excited to a higher energy level, and then relax and emit the energy in the form of light energy (photons). If we slow down this light using a prism or spectrometer, we can see the constituent colors that make up the color light that we are seeing. This series of lines is called the emission spectrum. This bright line spectrum is used to identify elements. Example on the next page.

Bohr’s Model of The Atom In Bohr’s model (this is not the modern model of the atom), electrons traveled around the nucleus in circular orbits, and each orbit existed at a fixed distance from the nucleus. Each orbit represented a different energy level, and the electron only released the amount of energy analogous to the difference between the two stationary orbits.

7.1-7.3 pg. 330 #’s 38, 40 and 42. Read 7.4-7.5 pgs. 309-320