Transcript The Atomic Model

The Atomic Model
Chem 9
Atomic Model
Atoms are so small that, even today, direct visual
inspection is all but impossible. Our model of the
atom is based on indirect experimental data.
Because of this, our model of the atom changes as
our experimental ability improves
The Greek Model
• Democritus - A Greek Philosopher
around the year 400 BC.
• Democritus concluded that matter
could not be divided into smaller and
smaller pieces forever.
• Eventually, the smallest piece of
matter would be found. He used the
word "Atomos“ meaning indivisible to
describe the smallest possible piece of
matter.
The Dalton Model: Billiard Ball Model
•
John Dalton - The English chemist that proposed
first Atomic Theory in 1803.
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All elements are composed of indivisible particles.
Atoms of the same element are exactly alike.
Atoms of different elements are different.
Compounds are formed by joining atoms of two or more
elements
What did Dalton base his theory on?
– Law of Conservation of Mass (Antoine
Lavosier, 1789)
•Atoms are neither created or destroyed
(under normal chemical reactions)
– Law of Definite Proportion, (Proust's Law)
•Atom ratio is fixed, so mass must be
constant.
– Law of Multiple Proportions (Dalton)
•Applies where two elements, A and B,
form more than one compound.
How Big is an Atom
• Imagine a penny, made up of copper (Cu)
• If we ground up the penny into dust, the dust would
still have the same properties as copper
If you placed atoms of copper in a row,
100 000 000 atoms would make a line 1
centimeter long
1
2
3
4
5
How Big is an Atom?
• A penny contains around 2.4 x 1022 atoms
of copper
• Can we see atoms?
• To see atoms we need a scanning
tunneling electron microscope or an ion
beam microscope
• A micrograph of a Nickel-Molybdenum
(Ni4Mo) bi-metallic compound, each
dot is a single atom
• The specimen is in the form of a needle
that is over 1000 times sharper than an
ordinary household sewing needle. In
fact, the end of the needle is so sharp
that it cannot be seen by the naked
eye or even a standard optical
microscope
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http://www.ornl.gov
What’s Wrong with DAT?
• Most of Dalton’s theory is accepted
today
• However, We now know that atoms
ARE divisible
• Atoms contain sub-atomic particles
• ELECTRONS
• PROTONS
• NEUTRONS
The Thomson Model: Chocolate Chip
Cookie Model
• J. J. Thomson 1856 – 1940 - The English scientist who discovered
Electrons in 1887.
• Thomson determined the charge of an electron to be
negative in his cathode ray experiment
• Thompson initially called electrons CORPUSLES
Electrons
Positive
Negative
Vacuum
High Voltage
Robert A. Millikan (1868 – 1953)
1900 – Electron defined as a small
charged particle
approximately 1/2000 the
mass of a Hydrogen atom
1916 -determined the charge
carried by an electron
OIL DROP EXPERIMENT
Protons – E. Goldstein 1850 -1930
• 1866 Eugene Goldstein found
evidence of positively charged
particles traveling in the opposite
direction to electrons in a cathode
ray tube
• He named them CANAL RAYS
• He deduced that canal rays were
positively charged particles we
now know as PROTONS
Neutron
• Sir James Chadwick
1891 - 1974
– Discovered
the neutron in 1932
– Has no charge but a
mass close to that of
the proton
electron
neutron
proton
The Rutherford Model: Gold Foil
Experiment
Ernest Rutherford - The British physicist who, in 1908, proved
the atom had a small, dense, positively charged Nucleus.
Rutherford's model proposed that an atom is mostly empty
space. There is a small, positive nucleus with the negative
electrons scattered around the outside edge.
Vacuum
Flourescent Screen
Au Foil
a-source
c. Rutherford Appleton Laboratories
Gold Foil Experiment
Conclusions
• Rutherford concluded that the atom is mostly
empty space
• However, the atom contains a nucleus where
the mass and positive charge of the atom are
concentrated
• We now know that the nucleus contains
PROTONS and NEUTRONS
• Electrons ORBIT the nucleus but are not part of
it
The Bohr Model: The Planet Model
 Niels Bohr - The Danish scientist who, in
1913, proposed the Planetary Model of
the atom.
• Electrons move in definite orbits around
the nucleus, like planets moving around
the nucleus. Bohr proposed that each
electron moves in a specific energy
level.
Properties of Atoms
The Wave Model

Based on Wave Mechanics, this model proposes that electrons have
NO definite path in an atom. The probable location of an electron is
based on how much energy it has. The more energy an electron has,
the farther from the nucleus. The small, positively charged nucleus is
surrounded by a large space in which there are enough electrons to
make the atom neutral.
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