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Reading #6: Bohr and Beyond
The Bohr Model and the Bohr-Sommerfeld Model
Niels Bohr's model showed that the negatively charged particles moved in particular orbits
based on energy levels. Bohr continued to work on refining the model of the atom. In collaboration
with Arnold Sommerfeld, the Bohr model was revised and expanded to include identifying features of
different orbits. Orbits or shells could be described by the level, shape, tilt, and spin of the negatively
charged particles.
Information in the Bohr-Sommerfeld Model of the Atom
Image from: http://nobeliefs.com/atom.htm
Heisenberg and Electron Uncertainty
Werner Heisenberg stated and supported with experiments the idea that the exact location or
position of a single negatively charged particle in an atom and the particle's momentum cannot both be
known at the same time. Scientists call this the "Heisenberg Uncertainty Principle." A common
representation of this idea is to place the negatively charged particles in a cloud surrounding the
nucleus.
Electrons, Protons, and Neutrons
Negatively charged particles in the atom had been a focus since early investigations of the atom.
Negatively charged particles are called electrons and move around a positively charged center of the
atom. The positively charged center was introduced by Rutherford. In 1932, English physicist James
Chadwick discovered the neutron, which was heavier than protons and electrons. The neutrons are part
of the nucleus with the protons. Protons are postively charged particles and neutrons are "neutral" or
have no charge. Chadwick originally termed the proton-neutron combination the "nucleon."
Useful Models of Atoms
Scientists continue to investigate matter and even smaller components of atoms. Work focusing
on quarks, dark matter, anti-matter, and other ideas continues to expand what we know about matter
and atoms. While the understanding of atoms has continued to improve, the actual modeling of atoms
has become messy. It is easier to draw orbits of electrons rather than clouds of electrons! Because the
Bohr model provides an easy to draw and useful model of the atom, many scientists rely on the Bohr
model to represent elements and their arrangement of electrons around the nucleus as shown in the
diagram below.
Bohr Models of the First Eleven Elements
Image from: http://nobeliefs.com/atom.htm
Atomic Models
Review the ideas about atomic models and the usefulness of these models, then answer the
questions below.
Questions:
1. Based on the work of scientists throughout history and present day scientists, we have improved our
understanding of the atom. How can understanding the atom impact our lives today? How has the
understanding of atoms impacted history?
2. Can you predict why the use of the Bohr models showing electrons be useful to scientists? What is
your prediction?
3. Looking at the Bohr models for the first eleven elements, do you notice any patterns? If so, what do
you think these patterns could tell you?
References:
http://nobeliefs.com/atom.htm