Transcript History of the Atom

Unit: Atomic
Structure
History of the Atom
Important Experiments
Leading to Atomic Theory
Democritus (400 B.C.)
•A Greek philosopher
•Was the first person to think
about an atom’s existence.
•Believed that matter was
composed of tiny indivisible
particles called atoms.
Hmmm…
atoms…
“atomos”
•He had no experimental
evidence to support his
thoughts.
John Dalton (1766-1844)
•A meteorologist
•Unlike Democritus, he
had experimental
evidence to support his
theory.
•Dalton had four major
points (postulates) to
his theory.
Dalton’s Theory
1.) All elements
are composed of
indivisible
particles called
atoms.
2.) Atoms of the
same element
are identical.
The atoms of any
one element are
different from
those of another.
Dalton’s Theory
3.) Atoms of
different elements
mix or combine in
whole number
ratios.
Example: Oxygen
combines with
hydrogen to form
water in a 2:1
ratio.
4.) Chemical
reactions occur
when atoms
separate, join, or
rearrange.
In a chemical
reaction, atoms of
one element NEVER
change into
another.
Cathode Ray Tube Experiment
• In the tube was an inert gas,
and two plates, a positive and
a negative.
• The particles in the gas were
attracted to the positive
plate.
• Therefore, the particles MUST
have a negative charge.
(Opposites attract.)
Joseph John (J.J.) Thompson (1859-1940)
J.J. Thompson
• Discovered the electron.
• From his experimental evidence,
he believed that the atom was a
solid positive sphere with electrons
shoved into the sides of it.
• His model was said to resemble a
popular English dessert called Plum
Pudding, and so his model was
deemed the “Plum Pudding
Model.”
Ernest Rutherford
*click*
• Used the gold foil experiment to
discover the nucleus.
• Shot high energy beam of alpha
particles into gold foil.
Gold Foil Experiment
Ernest Rutherford Conclusions
Observation
Conclusion
Most of the alpha
particles went through he
concluded…
The atom is mostly
empty space
Few particles were
deflected at small angles
he concluded…
The alpha particle came
close to something small
and positive (nucleus)
Very rarely particles were
deflected at large angles
he concluded…
The alpha particles hit a
small, very dense, and
positively charged center
(nucleus)
Eugene Goldstein (1850-1930)
• Goldstein discovered the
proton.
James Chadwick (1891-1974)
• Chadwick discovered the
neutron.
Side note… Not all of Dalton’s
postulates were correct.
•We now know that atoms are indeed
divisible – atoms can be broken down into
their subatomic particles, protons, neutrons,
and electrons (and these too can be broken
down even further!).
•We also know that not all atoms of the same
element are identical. Isotopes exist for
different elements. (We’ll talk about this
later.)
Properties of Subatomic Particles
Particle
Symbol Charge Relative Mass
p+
+1
1
Neutron
n
0
1
Electron
e-
-1
1/1840
Proton
Location of Particles
• Protons – In the nucleus
• Neutrons – In the nucleus
• Electrons – In the electron cloud
outside the nucleus in energy
levels
Neutral Atom
An atom is considered neutral
when it has the same number
of electrons and protons.
(p+ = e-)