Atomic Theory Development
What is Today’s Model?
Dense, Positively
Charged Nucleus
Mostly
Empty
Space
Composed of Protons,
Neutrons, and Electrons
Negatively
Charged
Electron Cloud
Most Probable
Location of the
Electrons
Timeline of Development of Current
Atomic Model
Discovery
of the
Neutron
Discovery
of the
Proton
450 BC
Democritus
proposed
the idea of
atomos.
1802
Beginning
of
Modern
Atomic
Theory
1897
1911 1913 1930
Discovery
of the
Nucleus
Discovery
of the
Electron
Introduction
of the wave
mechanical
model
The Idea
of Energy
Levels for
Electrons
was
Proposed.
Early Greeks
Matter
is made of indestructible
particles called “atomos”
Democritus (400 BC)
Summary for Dalton’s
Atomic Theory
(Father of the Modern Atomic Theory)
All atoms of a single element have the same mass
Atoms of different elements are different.
Atoms can’t be divided, created or destroyed.
Atoms of different elements combine in simple
whole-number ratios to form compounds.
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a
negatively charged particle.
Crookes Tube
Cathode ray tubes pass electricity through a gas that is contained at a very low
pressure.
Cathode Ray
J.J. Thomson
• He proved that atoms of
any element 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 positive that
balances the negative
charge.
J.J. Thomson
William Thomson’s (Sir Kelvin)
Atomic Model (1910)
Thomson believed that the electrons were like plums embedded in a positively
charged “pudding,” thus it was called the “plum pudding” model.
Ernest Rutherford’s (1871-1937)
Where exactly are those electrons?
Thomson’s Theory: “Plum Pudding”
 electrons embedded in a positive pudding.
Rutherford’s idea:
 Shoot something at them to see where they are.
Rutherford’s has an idea…
What if I shoot alpha radiation at
gold atoms in gold foil?
Discovery of the nucleus
Lead
block
Uranium
Flourescent
Screen
Gold Foil
He Expected
• The alpha particles to pass through without
changing direction very much.
• Because…
• The positive charges were spread out evenly.
Alone they were not enough to stop the alpha
particles.
What he expected
Because
Because, he thought the mass was
evenly distributed in the atom
Because, he thought
the mass was evenly
distributed in the
atom
What he got
How he explained it
• Atom is mostly empty.
• Small dense,
positive piece
center.
• Alpha particles
are deflected by
it if they get close
enough.
at
+
+
Rutherford’s Conclusion (1911)…
Small, dense, positive nucleus.
Equal amounts of (-) electrons at large distances
outside the nucleus.
Neils Bohr’s Atomic model (1913)
Small, dense, positive nucleus.
Equal amounts of (-) electrons at specific orbits
around the nucleus.
This incorrect version of the atom is often used to represented atoms
because it shows energy levels for electrons.
** James Chadwick
discovered neutrons in 1932.
-- n0 have no charge
and are hard to detect
-- purpose of n0 = stability of nucleus
photo from liquid H2
bubble chamber
Chadwick
And now we know of many
other subatomic particles:
quarks,
muons,
positrons,
neutrinos,
pions, etc.
Quantum Mechanical Model
-electron cloud model-charge cloud model-
Schroedinger, Pauli, Heisenberg, Dirac (up to 1940):
According to the QMM, we never know for certain
where the e– are in an atom, but the equations of the
QMM tell us the probability that we will find an
electron at a certain distance from the nucleus.
Quantum Mechanical Model
Modern atomic theory describes the electronic
structure of the atom as the probability of finding
electrons within certain regions of space (orbitals).
Modern Atomic Theory
 All matter is composed of atoms.
 Atoms of the same element are chemically alike
with a characteristic average mass which is unique to
that element.
 Atoms cannot be subdivided, created, or
destroyed in ordinary chemical reactions. However,
these changes CAN occur in nuclear reactions!
 Atoms of any one element differ in
properties from atoms of another element
 The exact path of electrons are unknown
and e-’s are found in the electron cloud.
The Atomic
Scale
 Most of the mass of the
atom is in the nucleus
(protons and neutrons)
 Electrons are found
outside of the nucleus (the
electron cloud)
 Most of the volume of the
atom is empty space
“q” is a particle called a “quark”
About Quarks…
Protons and neutrons are
NOT fundamental particles.
Protons are made of
two “up” quarks and
one “down” quark.
Neutrons are made of
one “up” quark and
two “down” quarks.
Quarks are held together
by “gluons”
Size of an atom
• Atoms are incredibly tiny.
• Measured in picometers (10-12 meters)
– Hydrogen atom, 32 pm radius
• Nucleus tiny compared to atom
– Radius of the nucleus near 10-15 m.
– Density near 1014 g/cm3
• IF the atom was the size of a stadium, the nucleus
would be the size of a marble.
California WEB
Models of the Atom
Dalton’s
Greek
modelmodel
(400 B.C.) (1803)
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1800
1805 ..................... 1895
Thomson’s plum-pudding
model (1897)
1897 J.J. Thomson, a British
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
1900
1905
1910
1915
1920
Rutherford’s model
(1909)
1911 New Zealander
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1925
1904 Hantaro Nagaoka, a
1930
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
1935
1940
Bohr’s model
(1913)
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
Charge-cloud model
(present)
1926 Erwin Schrödinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1945
1924 Frenchman Louis
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
1932 James
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
Match The Models
Billiard
Ball
Plum
Pudding
Nucleus
Energy
Levels
Neutrons