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The History of the
Atom
Discovery of Small Particles
•The Greeks
•John Dalton
Democritus
460 BC - 370 BC
• Coined the term ‘atomos’
• All matter is composed
of small, indivisible
particles (atoms)
• No experimentation
Aristotle
Aristotle emphasized that nature consisted of four
elements: air, earth, fire, and water.
John Dalton’s Solid Sphere
Model
• In 1803, John Dalton was the first to show
laboratory evidence of the existence of atoms.
• Dalton’s model of the atom was a solid sphere with no
subatomic particles…the atom was indivisible!
• Dalton also proposed the first atomic theory which
stated that all things are made up of atoms and these
atoms can combine in simple whole number ratios to
form compounds.
John Dalton
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms
are indivisible and indestructible.
2) All atoms of a given element are
identical in mass and properties
3) Compounds are formed by a
combination of two or more different
kinds of atoms.
Discovery of the Electron
•William Crookes
•J.J Thomson
•Robert Millikan
William Crookes ~ Discharger
tube
Test with a magnet
The paddle wheel experiment
J.J. Thomson: Cathode Rays
Cathode rays can be deflected by a
magnetic field.
J.J. Thomson: Cathode Rays
The beam was composed of negatively charged fastmoving particles called ‘electrons’.
What were these particles???
• Thomson reasoned that since the beam came
from the cathode (a negative electrode), the
particles may have a negative charge.
• Studied the deflection of the cathode ray by an
electric field to determine if this was the case
• By studying factors that effected the deflection
of the beam, Thomson calculated the charge to
mass ratio of the particle
Robert A. Millikan
Established the charge of
an electron and
Determined the atomic
structure of electricity
Millikan’s Oil Drop
Experiment
• When he sprayed oil droplets into a chamber and bombarded
them with X-rays to place a negative charge on them, the
charged droplets were attracted to the positive plate. Changing
the strength of the electrical field offset the attraction and
allowed Millikan to determine the charge of the electrons.
• He measured the charge of the electrons (1.60 X 10-19 coulombs)
and later used this to calculate their extremely small mass (9.11
x 10-28 grams).
2.2
What did it look like inside?
Discover of the Nucleus
•Ernest Rutherford
J. J. Thompson
Thomson’s Plum Pudding Model (1896ish)
• Thomson concluded that the beam was made up of negatively
charged subatomic particles called electrons (e-)
• No matter what metal the electrodes were made of, the
same charge to mass ratio was calculated.
• Since atoms are neutral - there must be some positively
charged aspect as well…..
ball of positive charge
electrons
2.2
Ernest Rutherford
In Simpler terms…
Earnest Rutherford’s Gold Foil
Experiment
His expected results
The actual results
•
He shot alpha particles (+ charged) through the atoms of gold in a
thin sheet of foil, and studied how they were scattered by the atoms.
Oddly, some of the particles “bounced” back off the foil but most
when straight through.
•
The circles are atoms of gold, the dots are the nucleus and the arrows
are the alpha particles (positively charged electromagnetic radiation).
Rutherford’s Gold Foil
Experiment
ITS Chemistry
Interpretation of the experimental results
- Most of the atom is empty space.
- The mass concentrated at the
center of an atom called ‘nucleus’.
- The nucleus is positively charged.
- The positively charged particle is
called ‘proton’.
Discovery of the neutron
•James Chadwick
James Chadwick
ITS Chemistry
The Atom as we know it
today
Niels Bohr