Later Contributors to Atomic Theory Pg nd Note Taking Sheet ©2011 University of Illinois Board of Trustees

Interaction of Light and Matter In order to understand the contributions of the next scientist, it is important to understand the characteristics of light and how it can interact with matter. Other names for light are radiant energy or electromagnetic radiation (emr for short). In the early 1900s there were observable phenomena involving light and its interaction with matter that could not be explained. ©2011 University of Illinois Board of Trustees

Electromagnetic Radiation Light consists of an oscillating electric field at right angles to an oscillating magnetic field, thus its name (emr). ©2011 University of Illinois Board of Trustees

Characteristics of Light (p.92-93) In order to understand the contributions of the next scientist, it is important to understand the characteristics of light and how it can interact with matter. At this time in history scientists thought of light as waves that propagated (moved) outward perpendicular from the source. In a vacuum scientists knew that light in a vacuum traveled at x 10 8 m/s. This maximum limit on the speed of light is a universal constant represented by the letter “c”. All types of light travel at this speed in a vacuum. If light travels through a denser material it will slow down and different energies of light will be bent differently. ©2011 University of Illinois Board of Trustees

Characteristics of Light Continued The electromagnetic radiation spectrum is all the possible energies of light. –Note that humans can see only a very small portion of emr called the visible range. ©2011 University of Illinois Board of Trustees

Characteristics of Light Continued Wavelength Light of a certain energy has a characteristic frequency and wavelength. A wavelength is the distance from peak to peak or trough to trough. It is a length measurement. ©2011 University of Illinois Board of Trustees

Characteristics of Light Continued Frequency The frequency of light is the number of wavelengths that can pass through a point in a second. Frequency has units of 1/s or s -1 or Hertz (Hz) The higher the frequency the higher the energy of the light ©2011 University of Illinois Board of Trustees

Characteristics of Light Continued Speed In a vacuum all light travels at x 10 8 m/s The speed of light, its frequency and wavelength are all related by the equation: C = λ ∙ ν Where λ (“lambda”) is wavelength and ν (“nu”) is the frequency Note that if frequency becomes greater the wavelength becomes smaller. ©2011 University of Illinois Board of Trustees

Electromagnetic Radiation Spectrum ©2011 University of Illinois Board of Trustees

Characteristics of Light Continued Energy In 1901 Max Planck found that atoms can only adsorb and emit energy in distinct quantities; this showed that energy is quantized. He also determined that the energy of the light is given by the equation E = h ∙ ν Where E is energy (J) h is Planck's constant = x J∙s V is the frequency in Hz or 1/s or s -1 or cycles/sec ©2011 University of Illinois Board of Trustees

Inexplicable New EvidenceNew Evidence Photoelectric effect occurs when light hits a piece of metal and the metal ejects an electron. The energy of the light had to be at least a certain minimum value that was different for different metals. Black body radiation. ©2011 University of Illinois Board of Trustees

Albert Einstein (1905) To explain the photoelectric effect, in 1905 Einstein suggested that light can behave like particles as well as waves. The way in which you consider light depends on the phenomenon you are observing. This is known as the dual nature of light. Light can be considered to be little discrete packets of energy called photons. ©2011 University of Illinois Board of Trustees

Hydrogen Line Emission Spectrum Scientist were very surprised that they didn’t get a continuous spectrum for the light emitted by hydrogen. ©2011 University of Illinois Board of Trustees

There is a fingerprint line emission spectrum for all the elements. Use the spectroscope to see the emission spectrum of other elements. ©2011 University of Illinois Board of Trustees

Neils Bohr (1913) Explained the unexpected result of a hydrogen line emission spectrum. Proposed a model of the atom in which the electrons have quantized energy. quantized Electrons of an atom could only be certain allowed distances from the nucleus which corresponded to specific energy values. ©2011 University of Illinois Board of Trustees

Bohr Continued When electrons are in their lowest energy state, Bohr called this their ground state. He said the when electrons are in a higher energy orbit, they are in the excited state. Energy is absorbed to excite an electron and released when an electron goes back to its ground state. Energy is released in the form of light. ©2011 University of Illinois Board of Trustees

Hydrogen Line Emission Spectrum ©2011 University of Illinois Board of Trustees

Louis DeBroglie (1923) In writing his doctoral thesis Louis DeBroglie suggested that electrons could behave like waves as well as particles. In fact he stated that all matter has a wave nature given by the formula λ = h/mv Where m is the mass and v is the speed. ©2011 University of Illinois Board of Trustees

Louis DeBroglie Continued This meant that an electron “orbiting” the nucleus can be thought of as a wave. Each electron moves with a characteristic energy. The energy of the electron depends on the wavelength. All matter has a characteristic wave called a matter wave – even you! ©2011 University of Illinois Board of Trustees

Erwin Schrodinger (1926) Schrodinger developed a wave theory about how electrons can exist in atoms. His wave theory explains the different energy states of an electron and is based on electrons behaving as waves. ©2011 University of Illinois Board of Trustees

Warner Heisenberg (1927) Realized that if electrons behave as waves their existence is more spread out and it is impossible to know exactly where the electron is or how fast it is moving. This is a statement of the uncertainty principle. ©2011 University of Illinois Board of Trustees