Download HistoryWebactivityKey

History of Atomic Structure Key
People in Chemistry – An Historical Development of Atomic Theory
Answer the following questions concerning the development of Modern Atomic Theory.
Some of these are review. You are responsible for learning the names of these scientists
and what they did. You may use your book to find the answers to the questions below but
it will be easier to do on a computer as the underlined words are linked to sites.
1. First to the Greeks, who was Democritus and what did he have to say about atoms?
All things are composed of minute, invisible, indestructible particles of pure matter
(atoma,”indivisibles”), which move about eternally in infinite empty space (kenon,”the void”).
Although atoms are made up of precisely the same matter, they differ in shape, size, weight,
sequence, and position. Qualitative differences in what the senses perceive and the birth, decay,
and disappearance of things are the results not of characteristics inherent in atoms but of
quantitative arrangements of atoms. Democritus viewed the creation of worlds as the natural
consequence of the ceaseless whirling motion of atoms in space. Atoms collide and spin, forming
larger aggregations of matter.
2. What was John Dalton’s atomic theory?
He proposed the Atomic Theory in 1803 which stated that (1) all matter was composed of small
indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics
and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules),
and complex (complex molecules).
3. Who discovered the electron? J J Thomson
4. Briefly summarize his work.
First, in a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube
ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to
an electrometer, a device for catching and measuring electrical charge. Perrin had found that
cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a
magnet, he could separate the charge from the rays. He found that when the rays entered the slit
in the cylinders, the electrometer measured a large amount of negative charge. The electrometer
did not register much electric charge if the rays were bent so they would not enter the slit. As
Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you
cannot separate the charge from the rays.
All attempts had failed when physicists tried to bend cathode rays with an electric field. Now
Thomson thought of a new approach. A charged particle will normally curve as it moves through
an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example).
Thomson suspected that the traces of gas remaining in the tube were being turned into an
electrical conductor by the cathode rays themselves. To test this idea, he took great pains to
extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an
electric field after all.
Thomson's third experiment sought to determine the basic properties of the particles. Although he
couldn't measure directly the mass or the electric charge of such a particle, he could measure how
much the rays were bent by a magnetic field, and how much energy they carried. From this data
he could calculate the ratio of the mass of a particle to its electric charge (m/e). He collected data
using a variety of tubes and using different gases.
5. Listen to Thompson talk about the discovery.
6. What did Ernest Rutherford have to do with the atom? Rutherford was the first to
establish the theory of the nuclear atom. Using the simulation, briefly his experiment.
(Don’t forget to change the slit width to see what happens)
7. Who was Millikan? Try his experiment?
American physicist who determined the charge on an electron using charged oil drop experiments
in 1909.
8. How did Neils Bohr change atomic theory? Try this site.
Danish physicist who proposed a successful quantum model of the atom in 1913. His model
assumed that (1) the electron exists at precise distances from the nucleus, (2) as long as an
electron remains in one location, no energy is given off, (3) electrons have circular orbits
(this is only correct for s orbitals), and (4) the angular momenta associated with allowed electron
motion are integral multiples of .
9. Who was Dmitri Mendeleev? What did he do?
Mendeleev is best known for his work on the periodic table; arranging the 63 known elements
into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in
1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending
order of atomic weight and grouping them by similarity of properties. He predicted the existence
and properties of new elements and pointed out accepted atomic weights that were in error.
10. How did Henry Moseley change the arrangement? Why?
Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table
by closer determination of atomic numbers. When atoms were arranged according to increasing
atomic number, the few problems with Mendeleev's periodic table had disappeared. Because of
Moseley's work, the modern periodic table is based on the atomic numbers of the elements.
11. Who was Max Planck? What was his contribution to modern atomic theory?
German physicist who formulated an equation describing the blackbody spectrum in 1900.
Planck's spectrum was obtained by postulating that energy was directly proportional to frequency
(
). Planck believed that this quantization applied only to the absorption and emission of
energy by matter, not to electromagnetic waves themselves. However, it turned out to be much
more general than he could have imagined.
12. What did Heisenberg do to confuse everybody? What is the Heisenberg Uncertainty
Principle?
German physicist who in 1927 derived the uncertainty principle. In its most common form, it
states that it is not possible to simultaneously determine the position and momentum of a particle.
Moreover, the better position is known, the less well the momentum is known (and vice versa).
13. What about de Broglie and the electron? What is “wave-particle duality?
In 1923, about ten years after Bohr published his results, Louis de Broglie came up with a
fascinating idea to explain them: matter, he suggested, actually consists of waves.
Sometimes light displays particle-like behavior, and sometimes it acts like a wave; it all depends
on what sort of experiment you're doing. This is known as wave/particle duality, and, like it or
not, physicists have just been forced to accept it.
14. Briefly summarize what Schrödinger said about electrons?
A physicist named Erwin Schrödinger showed that electrons are really waves. Schrödinger
showed that these electrons don't even move. The waves are stationary. Each time you check
where an electron is you will find it in a different place, but that doesn't mean it's moving in
between checks. For some energy levels, if you check position enough times you may see an
"orbit-like" pattern, but don't be fooled into thinking that electrons are actually moving around in
little circles. Fortunately, for most physics it doesn't really matter where it actually is, we only
care about how much energy it has. We call this the Energy Level of the electron. Because the
idea of orbits is so misleading, physicists started using a picture of the atom which just showed
energy levels as relative heights.