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Transcript
Atomic Theory
1
A HISTORY OF THE
STRUCTURE OF THE ATOM
2
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMA
(greek for indivisible)
History

Greek Philosopher Democritus (460-370 B.C.):
all matter composed of small atoms
atomos = indivisible

1803, John Dalton (brit.): atoms are the
fundamental building blocks of matter
4
Dalton's Postulates
Each element is composed of extremely small
particles called atoms.
5
Dalton's Postulates
All atoms of a given element are identical to one
another in mass and other properties, but the
atoms of one element are different from the
atoms of all other elements.
6
Dalton's Postulates
Atoms of an element are not
changed into atoms of a
different element by chemical
reactions; atoms are neither
created nor destroyed in
chemical reactions.
7
Dalton’s Postulates
Compounds are formed when atoms of more
than one element combine; a given
compound always has the same relative
number and kind of atoms.
8
John Dalton’s Atomic Theory
(ca 1803)
1.
Each element is composed of extremely small
particles called atoms.
2.
All atoms of a given element are identical. The atoms of
different elements are different and have different
properties (including different masses).
Atoms of an element are not changed into different
types of atoms by chemical reactions. Atoms are neither
created nor destroyed in chemical reactions. This is the
Law of Conservation of Mass.
Compounds are formed when atoms of more than one
element combine. A given compound always has the
same relative number and kind of atoms. This is the
Law of Constant Composition.
3.
4.
9
John Dalton’s Atomic Theory
Led him to deduce the Law of Multiple Proportions:
When two or more elements combine to form more than
one compound, the relative masses of the elements
which combine will be in in the ratio of small whole
numbers.
In carbon monoxide, CO, 12 g carbon combine with
16 g oxygen. C:O ratio is 12:16 or 3:4.
In carbon dioxide, CO2, 12 g carbon combine with
32 g oxygen. C:O ratio is 12:32 or 3:8.
10
John Dalton’s Atomic Theory
Almost right. A good start.
very small
Structure of the atom after Dalton
(ca. 1810)
11
1898
Joseph John Thompson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
J.J. Thomson (1897):
Cathode Rays
Atoms subjected to high voltages
give off cathode rays.
13
J.J. Thomson: Cathode Rays
Cathode rays can be deflected by a magnetic field.
Cathode rays are negatively charged particles (electrons).
Electrons are in atoms.
14
J.J. Thomson – The Electron
“Plum pudding”
model: Negative
electrons are
embedded in a
positively charged
mass.
Unlike electrical
charges attract, and
that is what holds the
atom together.
Electrons (-)
Positively charged
mass
Structure of the atom after
Thomson (ca. 1900)
15
Radioactivity




Radioactivity is the spontaneous emission of
radiation by an atom.
First observed by Henri Becquerel
(1852-1908).
Marie and Pierre Curie also studied it.
Nobel Prize in 1903 (physics).
16
Studies of Natural Radioactivity
Some atoms naturally emit one or more of the following types
of radiation:
alpha (α) radiation (later found to be He2+ - helium nucleus)
beta (β) radiation (later found to be electrons)
gamma (γ) radiation (high energy light)
Alpha particles
α
γ
Positively charged
mass
α
Electrons (-)
γ
Somehow gamma
radiation is in
there, too.
Structure of the atom after Becquerel (early 1900s)
17
1910
Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them passed
through. About 1 in 10,000 hit
Radioactivity

Three types of radiation were discovered by
Ernest Rutherford:
 particles (positive, charge 2+, mass 7400 times
of e-)
 particles (negative, charge 1-)
 rays (high energy light)

19
Ernest Rutherford (1910)
Scattering experiment: firing alpha particles at a gold foil
20
The Nuclear Atom
Some alpha particles
bounce off the gold foil.
This means the mass of the
atom must be concentrated
in the center and is
positively charged!
Thompson’s model could not
be correct.
21
Ernest Rutherford
The Nucleus and the Proton
The mass is not spread evenly throughout the atom, but is
concentrated in the center, the nucleus.
The positively charged
particles in the nucleus
are protons.
Electrons (-) are now
outside the nucleus.
Structure of the atom after
Rutherford (1910)
22
James Chadwick – The Neutron
In the nucleus with the protons are particles of similar mass but
no electrical charge called neutrons.
The positively charged
particles in the nucleus
are protons.
+ nn
Electrons (-) are now
outside the nucleus in
quantized energy
states called orbitals.
(From Niels Bohr and
quantum mechanics)
Structure of the atom after
Chadwick (1932)
23
1913
Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits. Rather
like planets orbiting the sun. With each
orbit only able to contain a set number of
electrons.
Bohr’s Atom
electrons in orbits
nucleus
Structure of the Atom
proton (+)
neutron
electrons responsible for the
volume and size of
the atom, negatively
charged
10-10 m
10-14
m
nucleus - responsible for the mass of
the atom, positively charged
26