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Unformatted text preview: Chapter 12 1/12/2011 1 Peter Atkins Julio de Paula Elements of Physical Chemistry Chapter 12: Quantum theory 2 Structural chemistry study of individual atoms and molecules study of motions of electrons and nuclei motion of particles in response to forces acting on them Quantum mechanics the most fundamental description of matter that we currently possess the only correct way to account for the structures of atoms forms basis for virtually all modes of spectroscopy and microscopy Atomic structure is essential part of Chemistry arrangement of electrons in atoms basis for description of molecular structure and molecular interactions 3 Classical mechanics introduced in 17 th century by Sir Isaac Newton very successful at explaining motion of everyday objects and planets Classical mechanics Classical mechanics is based on three principles a particle travels in a trajectory, a path with precise position and momentum at each instant any type of motion (translational, rotational and vibrational) can be excited to any energy simply by controlling forces, torques and impulses applied waves and particles are distinct concepts These assumptions agree with everyday experience everyday experience does not extend to individual atoms 4 Classical theory of light In classical physics, light is described as electromagnetic (EM) radiation (field) oscillating electric and magnetic disturbance that spreads as a harmonic wave through empty space, vacuum EM field has two components electric field acts on charged particles (stationary or moving) magnetic field acts only on moving charged particles 5 for example, oscillating motion of electrons in the antenna of a radio transmitter EM waves are generated by acceleration of electric charge In vacuum, EM waves travel at a constant speed c ~3 10 8 m/s, called speed of light 6 EM spectrum description and classification of EM field according to its frequency and wavelength White light mixture of EM radiation with 380700 nm colors = brains interpretation of EM radiation of different s Chapter 12 1/12/2011 2 7 The failures of classical mechanics Toward the end of 19 th century, experimental evidence accumulated showing that classical mechanics failed to explain motion of very small particles Classical mechanics is only an approximate description of the motion of particles classical mechanics fails when dealing with transfers of very small quantities of energy approximation fails when small masses and small transfers of energy are involved approximation is invalid when it is applied to molecules, atoms and electrons 8 Crucial experiments Atomic and molecular spectra show that energy can be transferred between systems only in discrete amounts The photoelectric effect shows that EM radiation (light), which was considered to be a wave, behaves like a stream of particles...
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