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Development of the Atom A brief review of the history… 20map.htm.
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Transcript of Development of the Atom A brief review of the history… 20map.htm.
Development of the Atom
A brief review of the history…
http://www.unit5.org/christjs/site%20map.htm
John Dalton(1766-1844)
• English school teacher who began teaching at the age of 12 years old.
• He combined previous knowledge from other scientists with experimental observations to formulate a set of empirical laws known as the Atomic Theory of Matter.
http://www.kjemi.uio.no/software/dalton/graphics/john_dalton.gif
Dalton’s Atomic Theory All matter is made of tiny indivisible particles
called atoms.
Thomson Model of the Atom
• J. J. Thomson - English physicist. 1897• Experimented using a piece of equipment
called a cathode ray tube (CRT), following Crookes’ design.
http://dbhs.wvusd.k12.ca.us/webdocs/Gallery/JJ&CRT.GIF
Thomson’s Experiment
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By adding an electric field…
he found that the moving pieces were negative.
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J.J. Thomson• He proved that atoms of
many elements can be made to emit tiny negative particles.
• From this he concluded that ALL atoms must contain these negative particles.
• He knew that atoms did not have a net negative charge and so there must be something balancing the negative charge.
J.J. Thomson
http://www.unit5.org/christjs/site%20map.htmhttp://www.haverford.edu/physics-astro/songs/thomsonsm.jpg
Plum-Pudding Model(aka Raisin Bun)
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56
• In 1910, Thomson proposed the Plum Pudding model– Negative electrons
were embedded into a positively charged spherical cloud.
Ernest Rutherford (1871-1937)
• Noticed that ‘alpha’ particles were sometimes deflected by something in the air.
• Awarded the Nobel Prize in Chemistry in 1908 for his Gold-Foil Experiment
http://www.windows.ucar.edu/people/images/rutherford.gif
Rutherford’s Apparatus
beam of alpha particles
radioactive substance
gold foil
circular ZnS - coated
fluorescent screen
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
What he expected…
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Because, he thought the mass was evenly distributed in the atom
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What he got…
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Explanation of Alpha-Scattering Results
Raisin Bun atom
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Alpha particles
Nuclear atom
Nucleus
Thomson’s model Rutherford’s model
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Actual Results of Gold-Leaf Experiment
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
Atom’s Nucleus
• Since most of the particles went through, the atom was mostly empty.
• Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.
• This small dense positive area is the nucleus.
California WEB
Bohr’s Model
• Bohr says: Electrons orbit the nucleus.
Only certain orbits are allowed.
Each orbit has a specific energy.
Nucleus
Possible electron orbits
e
Electron energy
The energy of electrons is quantized.
Electons must possess specific
amounts of energy at each energy
level.
When the electron absorbs light that has a specific amount of energy, it jumps to a higher orbit. It is said to be in an excited state.
When the electron falls to a lower position, it releases a specific amount of energy.
When the electron is in the lowest possible energy level, it is said to be in ground state.
Light is emitted when electrons return to ground state from an excited state.
The colour of that light depends on the amount of energy that is being released.
Formation of Light
Nucleus Nucleus Nucleus Nucleus
e e
e
Lithium Atom
+
Ground State
e e e e e e
e
Excited State
e
Electron Returns to Ground State
Light is given offe
Ion is formedLi e + Li1+
hv
n = 1
n = 2
n = 3
n = 4
n = 5
n = 6
n = 7
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An Excited Lithium Atom
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 326
Photon ofred lightemitted
Li atom inlower energy state
Excited Li atom
Ene
rgy
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Excitation of Hydrogen Atoms
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 328http://www.unit5.org/christjs/site%20map.htm
Return to Ground State
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Bohr’s Experiment
A spectroscope separates light from a source into its component colours.
1 nm = 1 x 10-9 m = “a billionth of a meter”
410 nm 434 nm 486 nm 656 nm
These distinct lines exist because the electron is restricted to certain energy levels.
The energy of the electron is quantized.
http://jersey.uoregon.edu/vlab/elements/Elements.html
Summary…
• When an electric current passes through the gas, it excites or energizes the atoms.
http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/images/excited.gif
Summary…
• As atoms return to a lower energy state, the gas releases this energy in the form of light.
http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/images/excited.gif
Summary…
• When we pass this light through a prism or spectrometer, we see a series of individual lines called the element’s “atomic spectrum”.
1 nm = 1 x 10-9 m = “a billionth of a meter”
410 nm 434 nm 486 nm 656 nm
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