Unit 2 / Matter
Modern Atomic Theory
1850's
 Robert Bunsen conducted experiments in which he observed that different
elements, when heated in a flame, gave off a characteristic color.
Late 1800's
 J.J. Thomson and others were experimenting with gas discharge tubes.
Gaseous elements, when subjected to electric current at low pressure, gave
off a colorful glow.
1869
 Dmitri Mendeleev introduced the scientific world to the idea of periodicity and
that patterns of behavior within the elements were in accord with their atomic
mass. Could all of these patterns follow from one property, atomic mass?
Mendeleev’s periodic table began a fantastic era of scientific discovery.
Scientists began an intense period of tinkering and experimentation to try and
answer all the puzzling questions.
Becquerel, Curie's ---> radioactivity
Thomson, Rutherford, Chadwick, and others ---> subatomic particles
Rutherford's atomic model
Mosely ---> atomic number
Early 1900's
 two extraordinary scientists, Albert Einstein and Max Planck, contributed to a
significant discovery. They determined that Thomson had missed something
in his study of the photoelectric effect. Thomson had shown that the
negatively-charged particles emitted when a metal was struck by light were
indeed the same particles that he called “electrons” from his study of cathode
ray tubes.
 Einstein and Planck were interested in what caused the electrons to leave an
atom. They studied the phenomena of energy and light. What must happen to
cause electrons to leave an atom? Energy is required to pull an electron from
its attraction to the nucleus. Where does the energy come from?
Einstein - Theory of Relativity 
Planck - Quantum Theory of Light 
Light is a form of energy
Observations:
 energy in the form of heat - element gives off energy as light in a particular color.
 energy in the form of electricity - element gives off energy as light in a
particular color.
What is going on? Why are atoms giving off light energy?
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Light:
 travels in waves with a characteristic frequency, wavelength, and energy
 frequency and wavelength are inversely proportional but frequency and
energy are directly proportional
 what is the electromagnetic spectrum?
 in 1672, Sir Isaac Newton discovered that the diffraction of sunlight in a glass
prism would produce a continuous spectrum of colors. We now know that light
travels in waves so that each color travels at its own distinct wavelength.
 it was later found that when one looks through a diffraction grating at an
element absorbing energy and emitting light, one sees a pattern of colored
lines. Each element has its own characteristic “line spectrum” which acts as a
set of “fingerprints” to identify the element.
Why were elements only giving off light at certain wavelengths?
1911
 Neils Bohr, a young Danish scientist working together with Ernest Rutherford,
proposed a new model for the atom. The line spectrum of an element led
Bohr to believe that the atom was releasing energy in the form of light only at
certain “energy states.”

Bohr proposed that the electron of a hydrogen atom moves about the nucleus
in a circular path of a certain radius having a certain energy state. This has
been called the planetary view of the atom - electrons were found outside the
nucleus in orbits moving like planets around the sun.

the lowest energy state/level is called the “ground state.” Electrons absorb energy
and move from one allowed energy state to another. When electrons move to a
higher energy state, they are said to in an “excited state.” When electrons fall back
to lower energy states, they release energy in the form of light.

the observation that only certain wavelengths of light were absorbed or
emitted led Bohr to believe that only certain energy changes were possible.
if the electron could move up to any particular energy level, than we would
see a continous spectrum and not a line spectrum.
Bohr’s proposed atomic model was for the hydrogen atom (1 proton, 1
electron). His mathematical formulas and calculations for the model explained
the line spectrum of the hydrogen atom. However, Bohr’s model was not able
to accurately predict the line spectrum for atoms with more than 1 electron. It
appeared that Bohr’s model was an oversimplification. The search to solve
the mystery of the atom continued.


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1923
 Louis de Broglie, a French physicist, proposed that particles in motion do not
travel in straight lines. Particles travel in waves!
1927
 Werner Heisenberg proposed the “Uncertainty Principle.” Heisenberg
reasoned that if matter, including electrons, travel in a wave-like motion then it
is impossible to predict the exact path and position of an electron in the atom.
Therefore, it is not correct to say that electrons move in well-defined circular
orbits around the nucleus.
Late 1920's
 Erwin Schrodinger, an Austrian physicist, applied mathematics to the study of
an electron’s wave-like motion. This began a field of study called “wave
mechanics” or “quantum mechanics.” We will not look at the mathematics
involved due to its complexity, but we will look at his results and theories.
 Schrodinger used probability to predict where an electron would be found in
an atom at any given time. Using complex wave equations, he was able to
verify Bohr’s work for the H atom and establish predictions for multi-electron
atoms.
 the region of space where the electron would most likely be found was called
the “electron cloud.”
 within the electron cloud, Schrodinger defined regions of space outside the
nucleus where the electron would be found.
1. shell - main energy level
2. subshell - each main energy level is made up of 1 or more sublevels
3. orbital - each subshell is composed of 1 or more orbitals
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