Atoms - 1
HNRS 227
Chapter 8: The History of the Atom and Application of the Knowledge
Outline
I.
II.
III.
IV.
V.
VI.
Early History: 18th and 19th Century
Discovery of the Electron (1897)
Ernest Rutherford (1911)
Consternation in the Model
Bohr Atom (1913)
Applications - Laser
Take Home Message
1. Science is an evolutionary process of increasing insight and observation and the
process is oftentimes years to decades in development (e.g., evolution of our
understanding of the structure of the atom)
2. By understanding basic principles of how nature operates, science can harness
that understanding to offer new technologies that improve the quality of life (e.g.,
development of Laser technology)
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I.
Early History: 18th and 19th Century
A. Early 1800’s: Modern Atomic Theory of Dalton
Recognition of:
Atoms
Elements
Molecules/compounds (e.g., water or H2O)
Atoms as basis for atomic theory but without electrons, protons and
neutrons
B. Early to Late 1800’s: Discovery of Elements
30 elements known in early 1800’s
Dimitri Mendeleev and the Periodic Table (1869)
63 elements known
Elements ordered by
Mass from low to high (e.g.)
Chemical property
Columns of elements (all behave similarly in a
chemical sense)
Column I
Column VII (far right)
Hydrogen (H)
Helium (HE)
Lithium (Li)
Neon (Ne)
Sodium (Na)
Argon (Ar)
Potassium (K)
Krypton (Kr)
(reactive with chlorine
(reactive with chlorine
in 1:1 ratio)
in 2:1 ratio)
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What did Mendeleev do?
Arrange elements from light to heavy
Arranged elements from top to bottom in rows and columns
Elements in a column show similar behavior chemically
Product: Periodic Table of Elements:
Predictive tool
Immensely creative insight
Do example of missing elements and how table can be used
to predict chemistry of unknown elements
II.
Discovery of the Electron (remember existing Dalton model from above)
1897: Thompson (using vacuum tube methodology)
Particle that was charged and
Very small
Very light (w/o mass)
Inside the “Dalton” atom
Conclusion: the atom could not be the fundamental building block
III.
Ernest Rutherford –1911
Objective: How are atoms put together knowing there are negative
(Thompson) and probably positive charges
Model/Hypothesis: negative electrons distributed around the atom
Test of Model:
using an alpha (+) particle, shot the alpha at thin gold foil
(like a bullet)
Observation:
1. Almost all the alphas passed through unaffected
2. Very small number of alphas were deflected at small
angles
2. 1/1000 alphas were scattered at very large angles or
bounced back as if they struck something “head on”
Rutherford hypothesized the following:
Large part of the atom’s mass is located in a very small and compact
center called the nucleus and most of the remainder “houses”
nothing (nucleus later called the proton)
Explain all three observations (walk through each using the raisin in the
bun analogy)
New Model of the atom emerged
1. Small dense center that was positively charged
2. Light (w/o mass) negative charged electrons circling the
nucleus
Later Additions to the Model in 1932
3. Neutrons
4. Protons
5. Charge: most atoms are neutral since + and – cancel each
other out
Diagram of Atom (used by NRC)
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IV.
Consternation with the Rutherford Model
Electrons moving must be giving off energy (EM) so that electrons would
eventually collapse on nucleus (~1 second); and yet atoms are billions of
years old
Violated fundamental laws of physics
V.
Bohr Atom (1913)
Observation:
heat hydrogen gas and light is emitted: discrete
wavelengths
Other gases also produce light of different discrete
wavelengths when heated
Hypothesis:
orbits for electrons with specific distances in which
electrons could “circle” the nucleus and could give
off EM only in discrete ways
Diagram
Each electron dropped back and lost energy as a “packet”
as a function of distance from the nucleus
Packets called “photons”
Two Key ideas
1.
No space between the orbits so jump had to be a quantum one
2.
If electron was at an excited state (outer orbit), it would
a. Drop down to a lower orbit and emit a packet of energy
(photon) unique to that distance
b. Drop back to lowest level (ground state) and emit a photon
unique to that distance
These ideas were verified 2 decades later using quantum mechanics
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VI.
Applications
A. Spectroscopy: quantum differences in light emitted or absorbed results in a
unique signature for each elements (“fingerprint”)
B. Laser (Light Amplification by Stimulated Emission of Radiation or
LASER)
Explain how a laser operates (refer to text pp: 186-188)
Start with an excited electron in an outer orbit (potential energy)
Pass by this electron with photon that matches the energy from
outer orbit to inner orbit…outer electron drops back and
emits packet of light identical to the first in wavelength
Imagine a whole row of electrons in a series of atoms all “sitting”
in the same configuration so0 that the first photons triggers
a sequence of packets to be simultaneous released all of the
exact same wavelength and all at the same time (referred to
as “coherent light”)
Properties:
Very intense light
High or low energy
Very straight and regular
Applications: Surveying
Scanning in stores
Medical