The History of the Atom
By Mrs. Palaski
Introduction
What does an atom look like? It is so small
that it cannot be seen. Yet we know there
are particles like protons, neutrons and
electrons that make up the atom.
How did scientists discover these subatomic
particles? This presentation will take you
through the scientists who contributed to
the discovery of the make-up of the atom.
The Early Chemists
Greeks
Approximately 400 B.C., the
Greeks classified matter into four
substances.
Earth
Fire
Air
Water
Democritus
Greek
philosopher
 Matter consists of
small, indivisible
particles called
atoms.

Aristotle
Greek philosopher who
opposed Democritus’
beliefs
 Believed that all matter
was continuous and not
made up of smaller
particles
 His ideas were accepted
until the 17th century

Alchemists
Mystics and fakes who claimed that they
could turn cheap metals into gold
 Dominated the next 2000 years of
chemical history
 Discovered the elements mercury,
antimony, and sulfur
 Learned to prepare the mineral acids
hydrochloric, nitric, and sulfuric acids

Robert Boyle
The first person to
perform truly
quantitative, physical
experiments using gases
 Defined element –
something that could not
be broken down into
two or more simpler
substances

Antoine Lavoisier
 Father
of Modern
Chemistry
 Explained that
combustion involved
oxygen
 Law of Conservation
of Mass
Law of Conservation
of Mass
Matter
is neither destroyed or
created during ordinary
chemical reactions or physical
changes.
Mass of reactants (starting
materials) = mass of products
(end materials)
John Dalton

English school teacher
who was fascinated
with science with an
intense interest in
astronomy, which led
to an interest in the
gases of the air and
their ultimate
components, atoms
John Dalton
First
to recognize that atoms could
explain the laws of: conservation of
mass, definite composition and multiple
proportions
Proposed the Atomic Theory in 1803
Dalton’s Atomic Theory
1. All matter is composed of extremely
small particles called atoms.
2. Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
elements differ in size, mass, and
other properties.
Dalton’s Atomic Theory,
cont.
3. Atoms cannot be subdivided,
created, or destroyed.
4. Atoms of different elements can
combine in simple, whole-number
ratios to form chemical compounds.
5. In chemical reactions, atoms are
combined, separated, or rearranged.
Modern Atomic
Theory
Dalton’s atomic theory was accepted and
became the basis for further
experimentation.
 There are exceptions to the theory:
Atoms CAN be subdivided.
Atoms of a given element may have
different masses, they are called
isotopes.

Dalton’s Model of the Atom
 Dalton
developed a model of the atom
based on his atomic theory. He felt
the atom was an extremely small,
indivisible particle.
 His model of the atom is called the
homogeneous spheres or the Billiard
Ball Model.

Early
Experiments to
Characterize
the Atom
The Electron
Sir William Crookes
Developed the Crooke’s tube – a tube that
produces cathode rays
 Noticed that the rays moved from the
cathode to the anode
 Found that rays traveled in straight lines
and possessed energy
 Concluded that the stream of light was
made up of particles

J.J. Thomson
Experimented with a CRT
(cathode ray tube)
 A CRT is an evacuated
glass bulb containing two
ends: the cathode and the
anode.

Cathode Ray Tube
 An
electrical current passes through the
tube from the cathode (negative end) to
the anode (positive end).
Cathode Ray Tube


A magnet was applied to these rays and always
with the same results:
Negative end of magnet repelled cathode ray;
Positive end of magnet attracted cathode rays.
Thomson noticed that the cathode rays were the
same regardless of the element or metal used to
make-up the cathode.
Discovery from CRT
 Thomson
concluded that the
cathode ray was a stream of
negatively charged particles,
which he called electrons.
 Determined ratio of particle’s
electrical charge to mass (e/m).
Thomson’s Conclusion
Thomson concluded that all atoms of all
substances contain the same kind of
negative particles, and all electrons are
identical.
 Thomson discovered the ELECTRONS.
 Plum Pudding model is Thomson’s model
of the atom. It is called the plum pudding
model because he pictured the electrons
dispersed like raisins in a pudding.

Plum Pudding Model
Plum Pudding model is Thomson’s model
of the atom.
 The atom consists of a diffuse cloud of
positive charge with negative electrons
embedded randomly in it.
 It is called the plum pudding model
because he pictured the electrons dispersed
like raisins in a pudding.

Plum Pudding Model
+
+
+
+
+
+
+
Negative electrons
Robert Millikan
Performed the Oil
Drop Experiment
(1909)
 Work contained
excellent precision
 Determined the exact
charge and exact mass
of an electron

Oil Drop Experiment
How the Oil Drop
Experiment Worked
A fine mist of oil is sprayed into the chamber.
 A few oil drops will fall through the hole in
the positively charged plate at the top.
 As the oil drops fall due to gravity, they
acquire extra electrons which are dislodged
from gases in the air by X rays.
 As the charged oil drops descend, the
electrically charged plates are turned on.

How the Oil Drop
Experiment Worked
The oil drops now have two forces acting on
them. Gravity and electrical charge.
 Using the microscope to observe the oil
drops, Millikan could determine the charge
needed to suspend the drops in mid-air.
 Millikan calculated the:
exact mass (9.109 x 10-31 kilograms) and
charge (-1.6 x 10-19 coulombs) of an electron.

Results of CRT and Oil
Drop Experiment
1. Proved that atoms are divisible.
2. Atoms are electrically neutral
therefore they must have a positive
charge equal to the negative charge.
3. Since electrons have such a small
mass, atoms must have additional
particles to account for most of their
mass.
Oil Drop Experiment
http://physics.wku.edu/~womble/phys2
60/millikan.html
Eugene Goldstein
In 1885, Goldstein used a canal ray tube
filled with hydrogen to discover protons.
 He called them “canal rays.”

Ernest Rutherford
Thought that the
atom was all
empty space
 In 1908, he carried
out an experiment
to test Thomson’s
plum pudding
model.

Gold Foil Experiment
Used alpha particles as bullets
 Shot the alpha particles at a thin sheet of gold
foil
 Predicted the alpha particles would pass
straight through

Results of Gold Foil
Experiment
 Almost
all of the particles passed
straight through.
 Some were deflected. (1 in 10,000)
 1 in 20,000 were reflected straight
back
 Results caused Rutherford to
formulate a new model of the atom.
Gold Foil Experiment




Rutherford said it was “as if you had fired a 15-inch
(artillery) shell at a piece of tissue paper and it came back
and hit you.”
Why did this happen?
Rutherford reasoned that the fast-moving particles must be
repelled by some powerful force within the atom. Also,
whatever caused this repulsion must occupy a very small
amount of space since only a very few particles ran into it.
Results caused Rutherford to formulate a new model of
the atom.
The Nucleus
So how small is the
nucleus?


How large is an atom’s volume compared to its
nucleus?
Think of a football field and place a dime in the
center of the 50 yard line.
Rutherford’s Model of the
Atom
Rutherford’s model of the atom is the
nuclear model.
 There is a very small, dense center of
concentrated charge, called the nucleus.
 Rutherford concluded that the atom is
mostly empty space.
 Electrons must be present in the empty
space about the nucleus to maintain the
neutrality of matter.

Nuclear Model of the Atom
-
Nucleus is
positively
charged.
-
-
-
Nucleus
Nucleus
-
-
-
- electrons
A Puzzle
Electrons must be present in the empty
space about the nucleus to maintain the
neutrality of matter.
 If an atom has a positive center and the
negative electrons are on the outside of the
atom, why don’t the electrons fall into the
center?
 Centripetal force

Niels Bohr



Studied under Ernest
Rutherford
In 1913, he developed his
own structure of the atom
based on Rutherford’s theory.
Rutherford had shown that
the atom consisted of a
positively charged nucleus
with negative electrons in
orbit around it.
Bohr


Bohr expanded this theory by suggesting that
electrons travel in specific successively larger
orbits or paths around the nucleus which he
called energy levels. These energy levels are
designated distances from the nucleus in which
electrons may be found.
He suggested that outer orbits could hold more
electrons than the inner ones, and that these outer
orbits determine the atom’s chemical properties.
Planetary Model of the Atom
ee-
eNucleus
e-
James Chadwick
Discovered the neutron
in 1932
 The neutron is a particle
in the nucleus that has
about the same mass as
a proton, but has no
charge.
