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The Development of
Modern Atomic Theory
Democritus, 460 BC
Greek philosopher
who described
matter as consisting
of small particles
that were indivisible
called “atomos”
“Nothing exists except atoms and empty
space; everything else is opinion.”
• Gathered evidence by measuring the
masses of elements that combine when
compounds form.
• Observed that compounds have a fixed
composition called the “Law of Definite
Proportions.” (i.e. 2 H and 1 O in every
water)
Dalton’s Atomic Theory, 1803
Proposed a theory to explain the Law of
Definite Proportions. Main points of the
theory:
1. Elements are composed of atoms.
2. All atoms of a given element are
identical.
3. Atoms of a given element are different
from those of any other element.
4. Atoms of one element combine with
atoms of other elements to form
compounds.
5. Atoms are indivisible in a chemical
process.
Dalton’s Atomic Theory, 1803
Dalton’s Atom Model
Tiny, solid sphere with
no internal structure
Called “Billiard Ball”
model
PROBLEM: DISCOVERY OF ELECTRONS BY JJ THOMSON
Thomson’s Atom Model, 1897
–J.J. Thomson using a cathode
ray tube through experiments, discovered
negative (-) particles and called them
“electrons.”
Proposed the “plum-pudding” model from
his work. Electrons were like pieces of
negative charge stuck in a positive
“pudding.”
PLEASE CORRECT THE YEAR
Thomson’s Atom Model, 1897
Positively
Charged
Field
Negatively
Charged
Electrons
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Thomson’s Atom Model, 1897
Provided first evidence that atoms are made of even smaller
particles.
Thomson's Atomic Theory:
1. Atom is “breakable.”
2. Atom has structure.
3. Electrons suspended in a positively charged electric
field.
(must have positive charge to balance negative charge of
electrons and make the atom neutral)
PROBLEM: DISCOVERY OF NUCLEAR PROTONS BY
RUTHERFORD
Rutherford’s Atom Model, 1908
• Hypothesized that alpha particles (+ charge) would
move through a gold atom without its path being
changed.
• Performed the gold foil experiment.
Observations
1. Most of the alpha particles pass straight
through the gold foil.
2. Some of the alpha particles get deflected
by very small amounts.
3. A very few get deflected greatly.
4. Even fewer get bounced off the foil and
back to the left.
Electrons
(Negative)
Nucleus
(Positive)
Rutherford's Conclusions:
1. The atom is 99.99% empty space.
2. The nucleus contains a positive charge and most of
the mass of the atom.
3. The nucleus is approximately 100,000 times smaller
than the atom.
4. The atom contains a tiny dense center called the
nucleus
5. The electrons move around in the empty space of the
atom surrounding the nucleus
PROBLEM: WHY DON’T ELECTRONS FALL INTO
NUCLEUS SINCE OPPOSITES ATTRACT?
Bohr’s Atom Model, 1913
Niels Bohr
• Observed light spectrum
of hydrogen atom and
proposed that electrons
orbit around the nucleus
just like planets orbit
around the sun.
• Called
Bohr’s Atom Model, 1913
Electrons
(Negative)
Nucleus
(Positive)
PROBLEM: MODEL ONLY EXPLAINED HYDROGEN
ATOM, BUT DIDN’T WORK FOR OTHER ATOMS.
Erwin Schrodinger’s model describes the
probability that an electron can be found in
a given region of space at a given time.
This model no longer tells us where the
electron is; it only tells us where it might
be. He introduced “wave mechanics” as a
mathematical model.
Electron Cloud (Schrodinger)
Atom Model, 1923