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Atomic Model/Theory
Chemistry
Videos
Atomic Theory: Dalton, Democritus, Thomson,
Rutherford, and Quantum
History of the atom
History of the atom Video #2
The Development of the Atomic Theory
J.J. Thomson Contribution *****
https://www.youtube.com/watch?v=GzMh4q-2HjM
Timeline of Atomic Models
Rutherford's Gold Foil Experiment
Gold Foil Experiment Animation *****
Quantum mechanics: The structure of atoms **
Atomic
Models
Early Models of the Atom
Democritus
Greek philosopher
 Was the first to
propose that all things
are composed of
minute, invisible,
indestructible particles
of matter
 Experiments to support
idea: NONE

“To understand the very large,
we must understand the very small.”
Democritus Model
• Solid and
Indestructible.
•Uncuttable/indivisible
•No protons,
electrons or neutrons
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Democritus: 400 BC
Information
• He wondered what we were
made of
• 5 components of his theory:
1. Matter is composed of empty
space through which atoms
move.
2. Atoms are solid, homogeneous,
indestructible & indivisible.
3. Different kinds of atoms have
different sizes and shapes.
4.The Diff sizes/shapes cause the
different properties.
5. Changes in matter are due to
changes in groupings of atoms
• “atomos” refers to particles –
means “indivisible”
Experiment
NONE
Atom Model
• Solid &
Indestructible.
• Uncuttable/indivisible
• No protons,
electrons or neutrons
Dalton’s Model of the Atom
1803 - England
Dalton

Made 5 assumptions based on his research:
1. All matter is composed of tiny particles called
atoms.
2. Atoms cannot be divided, created, or destroyed.
3. All atoms of a given element are identical and
differ from atoms of another element.
4. During chemical reactions, atoms preserve their
identity and are only combined, separated, or
rearranged.
5. Atoms of different elements combine in definite
simple ratios to form molecules of compounds.
Dalton’s observations help explain
three laws.
Law of Conservation of Mass
Mass is neither destroyed nor created during ordinary
chemical reactions and physical changes.
Law of Definite Proportions
A chemical compound contains the same elements in
exactly the same proportions by mass regardless of the size
of the sample or source of the compound (i.e. the same
percent composition).
Law of Multiple Proportions
Two elements can combine in more than one fixed
proportion by mass to produce different compounds.
Dalton’s Model

Atom is an indivisible, hard, solid sphere
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These main points disagree with
present atomic theory.
1. Elements are made of
indivisible particles called atoms.
• Atoms can be split during
nuclear reactions
2. Atoms of the same element are exactly
alike
• He believed all atoms of the same
element have the same # of protons,
neutrons, and electrons.
• But there are IONS and ISOTOPES
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Dalton:1803
Information
Experiment
• His atomic theory:
• Quantitative
1. All matter is composed of tiny
experiments
particles called atoms.
where he was
2. Atoms cannot be divided,
able to
created, or destroyed.
3. All atoms of a given element are
discover mass
identical.
ratios of
4. During chemical rxns, atoms are
elements
only combined, separated, or
involved in
rearranged.
5. Atoms of different elements
reactions
combine in simple ratios to
form compounds.
• Has been disproved…
• Atoms are indivisible – they can
be split
• Atoms of the same element are
the same – there are ions +
isotopes
Atom Model
• Atom is an
indivisible,
hard, solid
sphere
Thomson’s Plum Pudding Model
of the Atom (1904)
J.J. Thomson’s Research/Experiment
Scientists began to make connections
between matter and electric charge
 Worked with cathode ray tubes

◦ Glass tube in which most air was removed,
metal electrodes (positive cathode and
negative anode) on opposite ends
◦ Scientists noticed some type of radiation
(cathode rays) in the tube that originated in
the cathode and traveled to the anode when
high voltage applied
J.J. Thomson’s Research/Experiment
Cathode rays were a stream of charged
particles
 Through experimentation, determined
that they had a negative charge

Cathode Ray
Experiment
Thomson
Observation: Cathode rays
were charged particles and
carried a negative charge
 Hypothesis: Atoms are
composed of positively and
negatively charged matter, and
the negatively charged matter
is less massive and more
mobile than positive charged.
 Discovers the electron

Thomson’s Plum-Pudding Model (1904)
• The atom is a
ball of
positively
charged matter
with tiny
negatively
charged
electrons
embedded in it
• First model to
include
subatomic
particles
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Thomson: 1890-1904
Information
Experiment
• 3 hypotheses:
1. Cathode rays are
charged particles (called
corpuscles)
2. These corpuscles are
constituents of the
atom.
3. These corpuscles are the
only constituents of the
atom.
• #3 was disproved
• He discovered the electron
• He disproved Dalton because
atoms were divisible into
smaller subatomic particles!
Cathode Ray
Experiment:
• Glass tube in
which air was
removed, metal
electrodes
(positive and
negative) on
opposite ends
• Cathode rays
were a stream of
charged particles
• They had a
negative charge
Atom Model
• Ball of positive
matter
• Electrons
embedded
within
Rutherford’s Nuclear
Model of the Atom (1911)
Rutherford’s Gold Foil Experiment



He fired a (alpha) particles at a very thin sample of gold
foil
According to the Thomson model the a particles would
only be slightly deflected
Rutherford discovered that they were deflected through
large angles and could even be reflected straight back to
the source
What he expected:
• The alpha particles would pass through without
changing direction (very much) because the positive
charge was evenly spread out
• Alone the positive charges were not enough to
stop/greatly deflect the alpha particles.
What he got…
• Most of the particles went through undeflected while a
few were deflected at very wide angles
Rutherford’s Conclusion
Since most of the particles went through, the
atom was mostly empty space.
 Because the alpha particles were deflected so
much, the positive pieces it was striking were
heavy
 The atom’s positive charge and almost all of
its mass were contained in a small, dense
region in the center of the atom (he called it
the nucleus)

Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
+
+
-
-
+
+
-
+
+
-
+
-
+
-
-
Plum-pudding atom
Thomson’s model
Thomson’s Model must
have been incorrect!
Rutherford developed a
model that helped to
explain the results
Nuclear atom
Rutherford’s model
Rutherford’s Model
• Almost all of the mass of an
atom is located in a region
of space at the center of the
atom (the nucleus).
-
-
-
• The nucleus has a positive
charge.
• The atom consists of mostly
empty space occupied by
orbiting electrons (the
electron cloud)
Positive nucleus with electrons on
the outside in the electron cloud
-
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Rutherford: 1911 - 1920
Information/Experiment
• The gold foil experiment
• He fired a (alpha) particles at a
very thin sample of gold foil
• He THOUGHT alpha particles
would pass through without
changing direction because the
positive charge was evenly spread
out
• Instead…Most of the particles
went through undeflected while a
few were deflected at very wide
angles
• Some of the particles were hitting
the positive nucleus and bouncing
back!
• He discovered the nucleus
Atom Model
• Nucleus in center –
with most mass &
positive charge
• Mostly empty space
• Electrons orbit
nucleus
Bohr Model of the Atom (1913)
Issues with Rutherford

Rutherford’s model DID NOT…
◦ explain how e- are arranged
◦ address why negative e- are not pulled into
the + nucleus
◦ account for differences in chemical behavior
of elements

Scientists wanted to determine why
elements had different chemical behaviors
Background

When heated, elements can emit visible light
◦ Atoms absorb energy and become excited (high
energy state)
◦ These excited atoms return to their stable state by
emitting light to release that energy
◦ If the light emitted is passed through a glass prism, the
element’s atomic emission spectrum is produced.
Bohr determined that a
line spectrum is produced
when an electron moves
from one energy level to
an energy level closer to
the nucleus (energy being
released as light)
Background


Atomic emission spectrum of an element is the set of
frequencies of the electromagnetic waves emitted by
atoms of the element
Bohr proposed that the electron revolves around the
nucleus of the atom with a definite fixed energy in a
fixed path, without emitting or absorbing energy
◦ Based on his calculations and research with the atomic emission
spectrum of Hydrogen
◦ His model of electron arrangement correctly predicted
Hydrogen’s atomic spectrum
Bohr’s Model of the Atom
Explained energy absorption and emission by
atoms in terms of electrons moving from one
orbit to another
 Explained atomic spectra

◦ BUT experiments showed it can only explain atomic
spectra for atoms with one electron (such as H, He+,
Li 2+)
Depicted electrons having only certain allowed
circular orbits around the nucleus
 Predicted that the energy of an electron
increases as it moves further from the nucleus
(goes from lower to higher energy orbits)

Bohr’s Model
Increasing energy
of orbits
e-
e-
e-
ee-
e-
ee-
His model did not fully
account for chemical
behaviors…
e-
Draw Model
e-
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Bohr: 1915
Atom Model
Information
• Planetary model - The electrons orbit the nucleus
•
•
like planets orbiting the sun
He studied the atomic emission spectrum of elements
Key to Bohr’s model:
• Orbits are quantized (specific distances from
nucleus)
• Each orbital has an energy level
• Electrons in the lowest orbital = ground state
• But if excited with energy can move to higher
orbitals
• The key to the properties of each type of atom is
the organization of electrons.
• His model explained energy absorption &
emission by atoms and atomic spectra
•
Electrons
orbiting around
the nucleus at
quantized levels
Graphic Organizer
Chadwick: 1932
Information
Experiment
Atom Model
• He worked in
Rutherford’s lab
• He studied atomic
nuclei
• He shot alpha
particles at
beryllium foil,
which caused an
uncharged
radiation to be
emitted
• He determined
the radiation was
not charged but it
had mass
• He discovered the
neutron
• The model now has
protons AND neutrons
in the nucleus
Quantum Mechanical Model
Quantum Model


Mathematically determined by Erwin
Schrodinger
Determines the allowed energies an
electron can have and how likely it is to find
the electron in various locations around the
nucleus
◦ atomic orbital - region of space in which there
is a high probability of finding an electron.

In this model, the electrons are treated as
waves and the probable locations of
electrons are described.
Quantum Model (modern view)


Atom is mostly
empty space
Two regions
◦ Nucleus
◦ Electron cloud
 An electron is
described by 4
quantum numbers
 Modern atomic
theory describes
the probability of
finding electrons
within certain
regions of space
(orbitals).
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Quantum: 1926
Information
Atom Model
• Atom is mostly empty
• The Bohr model continues to be helpful
space
to imagine the distribution of electrons
• Two regions
and some of the behaviors of electrons
1. Nucleus
• He set out to include more information
2. Electron cloud
about the orbitals and the dual wave• Describes the probability of
particle nature of the electron
finding electrons within
• His model is based on probability because
certain regions of space
we cannot know where an electron is
(orbitals).
exactly
• This model determines the allowed
energies an electron can have and how
likely it is to find the electron in various
locations around the nucleus
Dalton proposes the
indivisible unit of an
element is the atom.
Thomson discovers electrons, believed
to reside within a sphere of uniform
positive charge (“plum-pudding model”).
Model
Review
Rutherford demonstrates the existence
of a positively charged nucleus that
contains nearly all the mass of an atom.
Bohr proposes fixed
circular orbits around
the nucleus for electrons.
In the current model of the atom,
electrons occupy regions of space
(orbitals) around the nucleus
determined by their energies.