#
APS April Meeting 2013

## Volume 58, Number 4

##
Saturday–Tuesday, April 13–16, 2013;
Denver, Colorado

### Session X7: Invited Session: 100 Years of the Bohr Atom

10:45 AM–12:33 PM,
Tuesday, April 16, 2013

Room: Governor's Square 16

Sponsoring
Unit:
FHP

Chair: Peter Pesic, St. John's College

Abstract ID: BAPS.2013.APR.X7.1

### Abstract: X7.00001 : Niels Bohr and the Third Quantum Revolution

10:45 AM–11:21 AM

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Abstract

####
Author:

Alfred Scharff Goldhaber

(Stony Brook University)

In the history of science few developments can rival the discovery of
quantum mechanics, with its series of abrupt leaps in unexpected directions
stretching over a quarter century. The result was a new world, even more
strange than any previously imagined subterranean (or in this case
submicroscopic) kingdom. Niels Bohr made the third of these leaps (following
Planck and Einstein) when he realized that still-new quantum ideas were
essential to account for atomic structure: Rutherford had deduced, using
entirely classical-physics principles, that the positive charge in an atom
is contained in a very small kernel or nucleus. This made the atom an
analogue to the solar system. Classical physics implied that negatively
charged electrons losing energy to electromagnetic radiation would ``dive in''
to the nucleus in a very short time. The chemistry of such tiny atoms would
be trivial, and the sizes of solids made from these atoms would be much too
small. Bohr initially got out of this dilemma by postulating that the
angular momentum of an electron orbiting about the nucleus is quantized in
integer multiples of the reduced quantum constant $\hbar =$ h/2$\pi $.
Solving for the energy of such an orbit in equilibrium immediately produces
the famous Balmer formula for the frequencies of visible light radiated from
hydrogen as an electron jumps from any particular orbit to another of lower
energy. There remained mysteries requiring explanation or at least
exploration, including two to be discussed here: 1. Rutherford used
classical mechanics to compute the trajectory and hence the scattering angle
of an $\alpha $ particle impinging on a small positively charged target. How
could this be consistent with Bohr's quantization of particle orbits about
the nucleus? 2. Bohr excluded for his integer multiples of $\hbar$ the
value 0. How can one justify this exclusion, necessary to bar tiny atoms of
the type mentioned earlier?

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2013.APR.X7.1