Atoms, Atoms Everywhere!
The History of Atomic Models
Timeline of the Models*
460 BC 1800 1897 1911 Today
Democritus John Dalton JJ Rutherford Many
“Atoms” “Billiard Ball” Thompson “Planetary” scientists
“Plum 1912 “Modern”
Pudding” Niels Bohr
*There are many more models. These are the ones we‟ll cover in class.
Democritus (c. 460 BC)
Democritus asked: If you keep breaking
matter in half, how many breaks will you
have to make before you can’t break it
apart any further?
Democritus called the smallest possible bits
of matter atoms. (indivisible in Greek)
He had no experiments to support his
Democritus was supposedly known as „the laughing philosopher‟
because of his wry amusement at human foibles.
The Ancients – B.C.
theory that everything
was made up of the
Unfortunately Aristotle (the more
popular Greek philosopher) dismissed
the atomic idea of Democritus as
For more than 2000 years nobody did
anything to continue Democritus‟ work.
No surprise, we call these the
“Dark Ages” of atomic theory.
John Dalton (c. 1800 AD)
English chemist, John Dalton, performed
experiments and showed matter seemed to
consist of “indivisible” particles (atoms).
Though he didn‟t know about atoms‟
structure, he did know about the Law of
Conservation of Matter and based his theory
on this. This theory was the result of
alchemist experiments trying to turn lead
into gold. The mass of starting chemicals is
equal to the mass of the products.
Dalton was an avid weather watcher and discoverer of color blindness
among other things.
John Dalton Theory --1800
(a.k.a “the marble guy”)
Atoms are the smallest
particles of nature-indivisible
All atoms of the same
element are identical
Atoms of different kinds can
combine to form compounds
in whole number ratios.
Chemical reactions are atoms
recombining to form new
J.J. Thomson (c. 1897)
In 1897, English physicist J.J.
Thompson discovered the electron
and proposed the Plum Pudding model
of the atom.
Using a CATHODE RAY TUBE,
Thomson discovered electrons have
a negative charge and thought that the
rest of matter must have a positive
charge to offset the negative electron.
JJ Thompson‟s Model
Because the beam of light
traveled from to the positive end
of the tube he concluded that the
light had a negative charge
Because the beam could push a
paddle wheel he concluded that
the particle had mass.
Thompson's model says atoms
are positively charged spheres
with negatively charged electrons
randomly located throughout.
Side Trip…Alpha Particles!
Around this time scientists also
discovered alpha rays (particles),
which had a positive charge.
Some physicists thought these
alpha particles were made up
of the positive parts of JJ
Ernest Rutherford (1911)
Rutherford as a student worked
under J.J. Thompson
supervision at the famous
In 1911 Ernest Rutherford
bombarded atoms with alpha
rays to investigate the inside of
The results were, to say the
Gold Foil Experiment
Radium as the source
of the alpha particles
and shot them at a thin
gold foil like aluminum
foil but made of gold
A fluorescent screen
sat behind the gold foil
on which he could
the alpha particles‟
When the particles bounced
back or were deflected
Rutherford reasoned that it
hit something massive and
positive. This mass became
know as the nucleus.
When the alpha particles
went straight through it hit
This happened most often
so the atom is mostly empty
Rutherford‟s “Planetary Model
Rutherford‟s model said the
negative electrons orbited a
positive center (NUCLEUS)like
our planets orbit the sun.
The nucleus contained most of the
mass of the atom
And the distance between the
positive center (nucleus) and the
electrons was huge-like a marble
in the center of a football field.
The atom was mostly empty
One little problem…
The theory of electricity
and magnetism predicted
that opposite charges attract
each other and the electrons
should gradually lose energy
and spiral inward toward the nucleus.
(BOOM! No more atom.)
Niels Bohr (1912)
In 1912 a Danish physicist, Niels Bohr
came up with a theory that said the
electrons do not spiral into the nucleus
and came up with some rules for what
This was a pretty radical approach,
because for the first time rules had to fit
the observation regardless of how they
conflicted with the theories of the time!
(Aristotle would have been furious).
Previous experiments-White light
gives off all wavelengths of energy- all
Previous experiments-helium gas only gives
off certain colors thus the line emission
spectra for helium is produced.
An electron absorbs
energy it jumps
farther away form the
As the electron falls
back closer to the
nucleus it gives off
the energy as
How Light Relates to Electron Location
that only certain
only orbit at
from the nucleus
RULE 1: Electrons can orbit only at
certain allowed distances from the
RULE 2: An atom absorbs energy when
an electron gets boosted from a low-
energy orbit to a high-energy orbit.
Rule 3: Atoms radiate energy when an
electron jumps down from a higher-
energy orbit to a lower-energy orbit.
Bohr’s Model of the atom. Light can
excite electrons around atoms and
this gives rise to “quantum levels”.
Are We Done Yet?
Almost… Cliff Notes version is that
Niels Bohr came really close, and
when you add the works of Arnold
Sommerfeld, Wolfgang Pauli, Louis
de Broglie, Erwin Schrödinger,
Max Born, and Werner Heisenberg,
we arrive at today‟s model…
Today‟s Model!-Electron Cloud
are not confined
to fixed orbits.