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THE ATOMIC THEORY
EARLY MODELS OF THE ATOM
•What are atoms?
•Smallest particle of an element that retains
its identity
•Anything that is matter is made up on atoms.
•Democritus (460BC – 370BC)
•Believed that atoms were indivisible and
indestructible
•Smallest possible existing thing must be an
atom
JOHN DALTON
•Chemist and school teacher
•1766 – 1844
•Used experiments to challenge
Democritus
•Came up with something very close to
the modern atomic theory
Dalton’s postulates
•1. All elements are
composed of tiny
indivisible particles called
atoms.
Dalton’s postulates (cont.)
•2. Atoms of the same element
are identical to each other but
different from every other atom
of different elements.
Dalton’s postulates (cont.)
•3. Atoms of different elements
can chemical combine in whole
number ratios to create
compounds.
Dalton’s postulates (Cont.)
•4. Chemical reactions occur when
atoms are separated, joined, or
rearranged. Atoms of one element,
however, are never changed into atoms
of another element as a result of a
chemical reaction.
(kind of like the law of conservation)
Did Dalton get it right???
•Are all of Dalton’s postulates true as
we know science and chemistry
today?  you decide
•1) True/False
•2) True/False
•3) True/False
•4) True/False
Sizing up the Atom
• Parts of an atom: Nucleus and orbitals
• Relative size of an atom  marble in a
football field
Sizing up the Atom
•Copper penny contains about 2.4 x 1022
atoms
•Earth’s population is roughly 6 x 109
people.
•Lining up 100,000,000 copper atoms side
by side would produce a line 1 cm long
The Atomic Theory
Evolution of the Atomic Theory
Visualizing the ATOM
Democritus’ Model
•Reminder:
•Democritus only stated an atom
was the smallest indivisible particle of
matter
that retained its identity.
Dalton’s Model
•Dalton, years later had
increased our knowledge
of the atom but still his
model would have been
the same as Democritus.
Progress, At Last
•1897 -- J.J. Thomson (physicist)
•Thomson experimented by passing electricity
through gases at low pressures.
•This gas was trapped in a vacuum sealed glass
with two electrodes connected to an energy
source.
JJ Thomson (cont)
• CRT  Cathode Ray Tube
(–)
(+)
Cathode Ray Tubes
•Cathode Ray – beam of charged particles
that travel from cathode to anode.
•Thomson’s Magnet
•This experiment led him to declare that
there must be something smaller than just
the atom.
•Completely shattered Democritus atomic
theory.
JJ Thomson
•Evidence from Thomson’s Experiment
•1. the – pole of the magnet near the ray = the
ray deflected away
•2. the + pole of the magnet near the ray =
attracted towards the magnetic field.
•3. The cathode ray traveled from cathode to
anode.
• Cathode Ray Tubes and TV/Computers
Addition to the Model of the Atom
•Conclusions:
•There must be something with negative charge!!!
•Thomson named this a “corpuscle of electric
charge”.
• Later named the electron
•Thomson’s Model
•Plum Pudding Model
•“think chocolate chip cookie”