AtkinsQuantumtheory - Chapter 12 1 Peter Atkins • Julio...

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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 λ ≈ 380…700 nm – colors = brain’s interpretation of EM radiation of different λ ’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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AtkinsQuantumtheory - Chapter 12 1 Peter Atkins • Julio...

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