General Chemistry I, Lecture 1: Atomic Models

General Chemistry I, Lecture No. 1: Models of Atom

•  The lecture covers atomic models, beginning with Thomson's model.
•  By 1911, atomic components (protons and electrons) were discovered.
•  Thomson's model proposed electrons and protons uniformly mixed within the atom.
•  Rutherford's model challenged Thomson's via the gold foil experiment.
•  Rutherford bombarded a gold foil with alpha particles (Helium nuclei).  - Most alpha particles passed straight through, some were deflected slightly, and a few were deflected greatly or bounced back.  - This suggested most of the atom was empty space with a dense, positively charged nucleus. 
•  The nucleus contains the positive charge and most of the atom's mass.
•  The nucleus is significantly smaller than the atom (approximately 100,000 times smaller).
•  Rutherford's model, however, couldn't explain atomic stability or atomic spectra using classical physics.
•  Bohr's model addressed this instability by proposing quantized stationary states of electron motion within the atom.
•  Electrons exist in specific energy levels and don't radiate energy while in these levels or when unchanging.
•  Transition between energy levels involves emitting or absorbing a quantum of energy.
•  Electron motion in quantized states involves circular orbits around the nucleus.
•  Bohr's first two postulates are correct and fundamental to modern quantum theory.
•  His fourth postulate (about angular momentum) is partially correct.
•  Third postulate is incorrect and not used in current quantum theory.
•  Bohr derived the energy expression of electron states in an atom, balancing Coulombic and centrifugal forces.