Ch 12 Quantum

# Ch 12 Quantum - Quantum Mechanics and Atomic Theory(Chapter...

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1 Quantum Mechanics and Atomic Theory (Chapter 12) vanKoppen Chem 1B 2005 Quantum Mechanics was developed to account for the behavior of light and atoms. Classical mechanics works very well for macroscopic particles (such as billiard balls, cars, etc.) but fails when applied to atomic particles. Composition of Atoms : The atom (atomic radius 1Å = 1 x 10 –10 m) consists of a small dense nucleus containing protons and neutrons (nuclear radius = 10 –13 to 10 –12 m) surrounded by electrons. One proton has one unit of positive charge (+1) The mass of a proton = 1.67 x 10 –27 kg One electron has one unit of negative charge (–1) The mass of an electron = 9.1 x 10 –31 kg The neutron has no charge. The magnitude of positive charge = the magnitude of negative charge = 1.602 x 10 –19 C In a neutral atom: the number of protons = the number of electrons. The mass of a proton the mass of a neutron 2000 times the mass of an electron. The number of protons in an atom = Z = atomic number. Z determines the element and its position in the periodic table. Generally, as Z increases the atomic mass increases (some exceptions, e.g. Ar —> K, why?) Chemical properties of atoms and molecules are determined by their electronic structure (i.e. the arrangement of the electrons in the atom). Thus, we need to understand electron motion and energy. The Rutherford Atom . Because the mass of protons and neutrons are much greater than the mass of electrons, the nucleus essentially remains stationary relative to the motion of the electrons. Rutherford's "planetary" model of the atom, where the electron orbits the nucleus is not predicted to be stable according to classical electromagnetic theory, which states that an accelerated charged particle radiates energy in the form of electromagnetic waves (light). In Rutherford's atom, the electron orbiting the nucleus is accelerated (because the velocity vector keeps changing) and, therefore, the electron should lose energy continuously and spiral into the nucleus. This implies that this model of the atom is unstable. Quantum mechanics was developed in the early 1900's to explain the stability of the atom as well as chemical bonding. The Nature of light and matter . Classically light consists of oscillating electric and magnetic fields (waves). However, Planck and Einstein showed that light also consists of discrete particles, photons, of energy h ν . E photon = h ν = 1 quantum of energy E = h ν , ν = c/ λ , E = hc/ λ , ν = frequency, λ = wavelength, c = speed of light = 2.99 x 10 8 m/sec, 1 J = 1 kg m 2 /sec 2 Thus, light is dual in nature : both particle and wave . Similarly, matter is dual in nature : both particle and wave . For billiard balls the wavelength associated with the balls does not effect their behavior. However, for atomic particles, such as electrons, the associated wavelengths are very important. By applying wave mechanics (quantum mechanics) we can understand the nature of atoms and chemical bonding.

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