Atomic Theories- Part I
Chemistry
Mrs. Coyle
A) Early Atomic Theories
Atom
The word atom comes from the Greek and
means “indivisible”.
Atom
The smallest particle in an element that
retains its identity in a chemical reaction.
Democritus –
Greek philosopher 4th Century BC
First to come up with
“atom”
Matter is composed of
tiny particles called
atoms
These atoms are
invisible, indestructible
fundamental units of
matter
Democritus’s ideas were opposed by
Aristotle and Plato.
Antoine Lavoisier (France 1782)
Law of Conservation
of Mass
In a chemical reaction
mass is conserved.
Joseph Proust (France 1799)
Law of Definite
Proportions:
The elements that
comprise a compound
are always in a certain
proportion by mass.
John Dalton (England 1766-1844)
School teacher
Studied the ratios in
which elements combine
in chemical reactions
Formulated first
modern Atomic Theory
Dalton’s Atomic Theory
1. All matter is made of atoms.
2. Atoms of the same element are
identical. The atoms of any one
element are different from those of
any other element.
Dalton’s Atomic Theory
3. Atoms of different elements can
chemically combine in simple whole
number ratios to form compounds.
Example: CO2
Dalton’s Atomic Theory
4. Chemical reactions occur when
atoms are rearranged.
Chemical reactions do not change
atoms of one element to another.
Dalton’s Atomic Model
Atomic Theory
Much of Dalton’s theory still
holds today
However, we now know that
atoms are not indivisible
Scanning Tunneling Microscope
Scanning Tunneling Microscope
Iron Atom Arrangement - STM
The Size of the Atom
If you placed 100,000,000 Cu
atoms side by side they would form
a line only 1 cm long.
Radius of most atoms is about
5x10-11 to 2x10-10m.
B) The Discovery of the
Electron and the Proton
History
Electron means “amber” in Greek
Properties discovered by the Greek
Thales of Miletos 600 BC. Rubbed the
mineral amber with cat fur and attracted
feathers.
Benjamin Franklin (America 1740’s)
Law of conservation of
charge.
Saw electricity as a
flowing fluid and called
the flow direction
positive.
Law of Charges
Like charges repel
Opposite charges attract
J(oseph) J(ohn) Thomson
(England 1897)
He discovered the electron while
experimenting with cathode rays.
Cathode Ray
Deflection of Cathode Ray
Cathode Ray Tube
JJ Thomson with the CRT
Thomson’s Discovery
He determined that the cathode ray
was made of negatively charged
particles – electrons.
Cathode Rays
Thomson also was able to estimate
that the mass of the electron was
equal to about 1/1840 of the mass of
a hydrogen atom.
His discovery of the electron won
the Nobel Prize in 1906.
Cathode Rays
Thompson showed that the production
of the cathode ray was not dependent
on the type of gas in the tube, or the
type of metal used for the electrodes.
He concluded that these particles were
part of every atom.
Charge of the Electron
Charge of Electron
1.6 x 10-19 C (coulombs)
Mass of Electron
9.11 x 10-28 g
Atoms have no net electric
charge.
Ions
Positively charge atom (cation)
Atom lost electrons.
Negatively charged atom (anion)
Object gained electrons.
Electron is the basic quantity of charge.
 Electric charges always exist in
whole number multiples of a single
basic unit, the electron.
A particle with a positive charge must be
present in the atom to balance each
negatively charge electron.
Plum Pudding Model (Thomson)
Application of the CRT
Why is watching television potentially
unsafe?
The Discovery of the Proton
Discovered by Eugen
Goldstein (German) in 1886.
He observed “Canal rays”
and found that they are
composed of positive
particles – protons.
Canal Rays
Canal Rays passed through holes, or
channels, in the reverse direction as the
cathode ray.
Canal Rays
c) Discovery of the Nucleus
Ernest Rutherford (Born in New
Zealand 1871-1937)
 University of Manchester,
England
Tested Thomson’s theory of
atomic structure with the “gold
foil” experiment in 1910.
Gold Foil Experiment
Bombarded thin gold foil with a beam
of ‘alpha’ particles.
If the positive charge was evenly
spread out, the beam should have
easily passed through.
Rutherford's Gold Foil Experiment
Rutherford and coworkers aimed a
beam of alpha particles at a sheet
of gold foil surrounded by a
florescent screen.
Rutherford
Expected
Found
Rutherford's Experiment
Most particles passed through with no deflection, while some
were highly deflected
Rutherford concluded that most particles passed through
because the atom is mostly empty space.
Rutherford’s Conclusions
All of the positive charge, and most of
the mass of an atom are concentrated in
a small core, called the nucleus.
Size of Nucleus Compared to the Atom
is as a Ball Compares to a Football
Field.
The Discovery of the
Neutron
Discovered in 1932 by James
Chadwick (England 1891-1974).
The Discovery of the Neutron
Chadwick bombarded alpha
particles(helium nuclei) at Beryllium.
Neutrons were emitted and in turn hit
parafin and ejected protons from the
parafin.
Discovery of the Neutron
Neutrons
Neutrons have mass similar to
protons.
No electrical charge.
Properties of Subatomic Particles
Particle Symbol Relative Mass
Charge Relative to
the Proton
Mass (g)
Electro en
1-
1/1840
9.11 x 10-28 g
Proton
p+
1+
1
1.67 x 10-24g
Neutro
n
n0
0
1
1.67 x 10-24g
Progression of Models
Dalton’s Atomic Model
Plum Pudding Model (Thomson)