Transcript No Slide Title

ATOMIC THEORY
“MORPHS”
Development of an Atomic Theory
During Dalton’s time the first empirical evidence
(observational evidence) for the existence of atoms was
being collected. This evidence was summarized in the
Laws of Chemical change as follows:
Laws of Chemical Change:
1.
Law of Conservation of Matter
(Matter is not lost or gained in a chemical Reaction)
2.
Law of Definite Composition
(Elements combine in definite ratios by mass)
3.
Law of Multiple Proportions
(Element-masses combine in small whole number ratios)
John Dalton, proposed theory for the underlying
principles supporting the Laws of Chemical Change must
include the following postulates:
Dalton’s Atomic Theory
1
All matter is made up of indivisible particles called atoms
2
All atoms of one element are identical. The atoms of any one
element are different from those of all other elements.
3
Chemical change is the union or separation of atoms
4
Atoms combine in small whole number ratios to form
compounds.
Daltton’s model of the atom was therefore a Q-ball model.
Thompson’ s Atomic Research
Using cathode ray tubes Thompson discovered that
rays given off by metals were made of tiny negatively
charged particles, he called them electrons. Milliken
determined the charge of an electron to be (1-). The
atom is no longer considered indivisible however yet
invisible. This model of the atom was called the raisin
bun model. It had subatomic structure, a positive
sphere with negative embedded electrons.
This is the raisin-bun model ----->
Rutherford Performs the Gold Foil Experiment.
This provided evidence that the atom is mainly empty
space with all the positive charge and mass being located
in the center known as the nucleus.
4
2
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This is a diagram of Rutherford’s experiment
4
2

4
2
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This is what actually happened and why the model was
changed from the “raisin bun” model to, an atom with
a nucleus.
These spectra came from excited
gaseous samples of hydrogen.
BOHR'S MODEL
OF THE
HYDROGEN ATOM:
•According to Bohr's theory, the electron can orbit the proton
(nucleus) only in certain "ENERGY LEVELS" or principal
quantum levels, numbered n=1, n=2, n=3, etc., with n=1 being the
normal or GROUND STATE level for the electron closest to the
nucleus.
• When the electron is excited up to the n=2 or higher level, a
photon of light is absorbed, giving an absorption spectrum. Since
the energy levels have a definite spacing, this results in photons of a
specific colour or wavelength being absorbed.
•When the electrons "fall" from a higher to lower levels, light is
emitted, giving an emission spectrum.
The BALMER SERIES is the visible spectrum series of emission lines for the
hydrogen atom. These lines are the result of electron transitions from:
[n = 3 to shell 2] red, [n = 4 to n = 2] blue green, [n = 5 to n = 2] blue, [n = 6 to n = 2]
violet, all ending in n = 2 energy level. These transitions can be modeled as follows.
Bohr’s model of the atom was therefore one with a positive nucleus and “fixed
energy” orbits for electrons.
Visible Spectrum
• Brilliant colours that are seen in an emission spectrum are:
• Atomic: When light is emitted by atoms of an element in the
gaseous phase each element has unique colour.
– The excited state environment that is generated when either heat or
electric energy causes electrons to jump to higher energy levels in
atoms.
– When these “excited” electrons fall back to a lower energy level they
emit light of particular colours or wavelengths of ‘fixed energy”.
• Bohr’s model of the atom was therefore one with a positive
nucleus and “fixed energy” orbits for electrons.
Presented By
Gator-chem
ATOMIC THEORY
“MORPH”
Terry Knock
2000
Central Kings Rural High School
Thanks to Jan Gough, Horton High for her valuable contributions.