Atomic Theory
Let’s Take a Trip Through Time!
Democritus vs. Aristotle
400BC
350BC
Democritus
460 – 370 B.C.
Matter composed of Atoms
• basic elements from
which all matter is made
• atoms move in a void
(empty space)
• atoms have different
shapes (round, pointy,
oily, have hooks, etc.)
Ideas rejected by leading
philosophers because
void = no existence
First Concept of an Atom
Atoms are solid, homogeneous, indivisible particles
Aristotle
350 BC
•Matter can be subdivided into smaller and smaller
particles forever
•Matter composed of combination of 4 elements:
air, earth, water and fire
•No such thing as empty space – had the “ether”
Aristotle won that debate!
For 2000 years most people believed
that the 4 elements were:
air, earth, water, and fire
There was no such thing as empty
space!
Phlogiston Theory
Around 1700’s:
• wood  calx (ash) + phlogiston (↑ air)
• iron  calx (rust) + phlogiston (↑ air)
Antoine LaVoisier
• Demonstrated importance of
measurements in chemistry
• Disproved the phlogiston theory
• First to state Law of Conservation of Mass
LaVoisier was
the father of
modern
chemistry, but
he lost his
head in 1794!
J. Proust – 1797
Law of Definite Proportions
In pure compound:
elements combine in
definite proportions to
each other
• H2O
• CO2
John Dalton
1766-1844
Dalton’s Model
Billard Ball model
Dalton’s Atomic Theory
• Matter is composed of very small
particles called atoms
All atoms of a given element are
identical
• Atoms of different elements are different
Atoms are indivisible and cannot be
created or destroyed
• Different atoms combine in whole
number ratios to form compounds
• In a reaction, atoms are rearranged
So Dalton’s atoms are kind of like
billiard balls
Atoms combine in
whole-number ratios
In a reaction, atoms are rearranged
+

+
+
Thomson’s Model
1904
Plum Pudding model
(aka:
chocolate chip cookie model)
J.J. Thomson
1856-1940
• Discovered electron
1897
– Cathode Ray Experiment
• Discovered isotopes
1913
Cathode Ray Tube Experiment
Thought cathode rays were streams of particles smaller than
atoms
•magnetic field will deflect beam of charged particles
Thomson’s discovery meant that
the atom WAS divisible!
JJ Thomson’s Ideas
matter is neutral!
Plum Pudding Model
Negative
electrons in a
soup of
positive
charges
positive charge
evenly spread
out while the
negative charge
is in bits – like
chocolate chips
in cookies
source
Proton – Discovered by 1920
• Thomson & Goldstein – 1907
- discovered heavy particle with positive charge
• Rutherford – 1918
- shot alpha particles at nitrogen gas and got
hydrogen
- hydrogen had to come from nitrogen
- suggested hydrogen nucleus was a
particle & named it the proton
Rutherford’s Model
1910
Nuclear model
Ernest Rutherford
1871-1937
• atom’s mass is mostly
in the center of atom
– Nucleus
• nucleus has a positive
charge
Rutherford’s Experiment - 1911
Rutherford’s exp’t: animation
Compared to electron, alpha particle
is massive & fast
If Thomson’s model correct, alpha
particle wouldn’t be affected
- electrons are tiny
- positive charge uniformly spread
What did
Rutherford conclude
from the particles
that went straight
through?
mostly empty space
What did
Rutherford conclude
from the particles
that bounced back?
hit something really heavy &
positively charged
source
Results of Rutherford’s Expt
Results DO NOT match Thomson’s model
Rutherford Model
NUCLEAR Model
Rutherford proposed nuclear atom :
dense central core (+ charged) surrounded
by mostly empty space where electrons
are found
Rutherford did not
speculate on how
the electrons were
arranged around the
nucleus
So how big is the nucleus
compared to the entire atom?
• If atom as big as football stadium
– nucleus smaller than flea on 50-yard line!
• If atom as big as period at end of sentence
in textbook
– it would have mass of 70 cars!
So how big is an atom?
• Most atoms are 1-2 angstroms across
• An angstrom = 1 X 10-10 m
• 6 BILLION Cu atoms in a line = less than 1
meter!
• Can we “see” an individual atom?
James Chadwick
1891-1974
• Worked with
Rutherford
• Discovered Neutron
1932
• Nobel Prize in
Physics 1935
Bohr’s Model
1913
Planetary model
Niels Bohr
1885-1962
• His Model:
– Nucleus surrounded
by orbiting electrons at
different energy levels
– Electrons have definite
orbits
• Worked on the
Manhattan Project
(US atomic bomb)
Bohr’s Model
•Electrons travel only in
specific orbits
•Each orbit has definite
energy
•Inner orbit: least energy
•Outer orbit: most energy
•Atoms emit radiation
when e- jumps from
outer orbit to inner orbit
•Outermost orbits
determine atom’s
chemical properties
Bohr Model for Nitrogen
Modern Model
1926
Quantum
Mechanical model
Ernst Schrödinger 1887-1961
Quantum Mechanical Model
– Electron’s energy is quantized
(has only certain values)
– Electrons in probability zones
called “orbitals”, not orbits
- location cannot be pinpointed
– Electrons are particles & waves
at same time
– Electrons move around
nucleus at speed of light
Orbitals
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
The development of
atomic theory
represents the work
of many scientists
over many years
Next Atomic Theory ?
Which one of you will develop
a better theory and win the
Nobel Prize?