MODELS OF THE ATOM:
A HISTORICAL PERSPECTIVE
EARLY GREEK THEORIES

Democritus
Democritus
He asked: Could matter be divided into smaller and smaller pieces forever, or
was there a limit to the number of times a piece of matter could be divided?

400 B.C. – His theory: thought matter could not be divided indefinitely

There was a limit to how far you could divide matter. You would eventually
end up with a piece of matter that could not be cut

He named the smallest piece of matter “atomos,” meaning “not to be cut”
EARLY GREEK THEORIES

Aristotle


Very famous Greek philosopher

Believed matter could be divided smaller and smaller pieces
forever

Believed that matter was made of a combination of four
“elements”: earth, fire, water, air
Aristotle was wrong. However, his theory persisted for 2000 years.
WHAT IS AN ATOM?
PIECES OF
CARBON


An atom is the smallest
particle that an element
can be divided and still be
that element.
For example the smallest
particle of carbon is a
single atom of carbon. If
you divide it is no longer
carbon anymore.
CARBON
ATOM
JOHN DALTON

1800 -Dalton proposed a modern atomic model based on
experimentation not on pure reason
•
•
•
•
All matter is made of atoms
Atoms of the same element are identical
Each element has different atoms
Atoms of different elements combine in constant
ratios to form compounds.
• Atoms are rearranged in reactions.

His ideas account for the law of conservation of mass (atoms are neither
created nor destroyed) and the law of constant composition (elements combine
in fixed ratios).
WILLIAM CROOKES
Accidental discovery of the cathode ray
 Discovery led to the development of the
television

HISTORY OF THE ATOM

Joseph John Thompson

What particle did Thompson discover?
found that atoms could sometimes eject a far smaller
negative particle which he called an
ELECTRON
HISTORY OF THE ATOM

Thompson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere

This sphere contained a soup of positive charge to balance the
electron's charge
electrons similar to plums in a pudding or
chocolate chips in ice cream.
PLUM PUDDING
MODEL
Negative electron
Positive charge spread over
sphere
ROBERT MILLIKAN

Developed the charge of an atom… e-
ERNEST RUTHERFORD

Rutherford shot alpha () particles at gold foil
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance
path of invisible particles
• Most particles passed through
• So, atoms are mostly empty space
• Some positive -particles deflected or bounced
back!
• Thus, a “nucleus” is positive & holds most of
an atom’s mass
BOHR’S MODEL
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher shell if hit by an electron or a
photon of light
WHAT IS IN AN ATOM?

There are 3 types of subatomic particles.
 electrons (e–)
 protons (p+)
 neutrons (n0) They have: no charge

Elements are often symbolized with their mass number and atomic number
E.g. Oxygen:


16
8O
Nuclear Symbol
The periodic table give you all this information
For now, round the mass # to a whole number


# of protons = # of electrons = atomic number
Mass number – atomic number = # of protons
ATOMIC STRUCTURE
Particle
Charge
Mass
Proton
+ charge
1
Neutron
No charge
1
Electron
-
nil
Charge
ATOMIC STRUCTURE EXAMPLE
Atomic mass
the number of protons
and neutrons in an atom
4
Atomic number
the number of protons
in an atom
2
He
All of these pieces of information (numbers) are obtained from your periodic
table
Atomic number = number of electrons = number of protons
Practice
Atomic
Ca
Ar
Br
Mass
p+
n0
e–
ASSIGNMENTCopy the table right below your notes and fill in the information that is
required of you. Use the periodic table to fill in this information.
Symbol
Calcium
Neon
Magnesium
Sodium
Lithium
Oxygen
Carbon
Potassium
Silicon
Chlorine
Atomic
number
Mass
p+
n0
e-
QUESTIONS??