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Atomic History Timeline
Major Contributors
Main Discoveries
Timeline of Development of Atomic Theory
• 4.1.1) List the main points of Dalton’s Atomic Theory
and describe which parts are still believed to be true
• 4.1.2) Explain the atomic models of the following
scientists: Dalton, Thomson, Rutherford, Bohr,
• 4.1.3) Describe the experiments performed by
Thomson and Rutherford and what the observations
suggested about the structure of the atom
• Explain the contributions of the following scientists to
the development of the Atomic Theory: Democritus,
Crookes, Becquerel, Curie, Millikan, Chadwick
Democritus’s Theory
(Foundation of Atomic Theory)
• Thought that all things were made of tiny,
invisible, indestructible particles, called atoms
• Thought atoms varied in size, shape and
Aristotle's Theory
• Opposed the Atomic
• Thought matter was
made of air, water,
fire, and Earth
Dalton’s Atomic Theory
• Chemical elements are made of atoms
• Atoms are indestructible (can’t be divided into
smaller parts)
• Atoms have no electric charge
• Each atom of the same element is identical
• When elements react, their atoms combine in
simple, whole-number ratios
Crooke’s Tube
Henri Becquerel
• When Becquerel placed uranium salt in a desk
drawer with a photographic plate, radiation from
the uranium formed an image on the plate.
• This proved that radiation could occur without
outside energy such as the sun
• When he placed metal between the uranium and
photographic plates, the metal blocked the
• Discovered natural radiation
• Discovered that a magnetic field could block
Marie Curie
• Confirmed the work of Becquerel
• Invented the term “radioactivity”
• Discovered two new elements: polonium and
radium in 1898.
• Her research indicated that some atoms are
unstable and will disintegrate over time on
their own.
– If they fall apart, the must not be the smallest
piece of matter out there
JJ Thomson
• Discovered electrons using the Cathode Ray Tube
• Beam will bend towards a positive charge,
therefore, it must be made of negatively charged
Thomson’s Atomic Model
“Plum Pudding” Model
• He realized that
neutral atoms must
have equal amounts
of positive and
negative charge.
• His model shows
negative charges as
plums and positive
charges as the
Rutherford's Gold Foil Experiment
• Consisted of shooting alpha particles (positive
charge) at a very thin piece of gold.
• Most went straight through, some deflected at
an angle, and a small percentage bounced
straight back
Rutherford’s Contribution to Atomic
• Atoms are mostly empty space.
• Atoms have a tiny dense center called ‘the
• Nucleus contains most of the mass of an
• Electrons are found outside of the nucleus.
• Nucleus is positively charged.
Bohr’s Atomic Model
• Electrons exist in
orbits around the
• Electrons can
sometimes jump
from one orbit to
another by gaining
or releasing energy
Robert Millikan
• Discovered mass and electric charge of the
electron by using the “Falling Drop” Method
Erwin Schrödinger
• Stated that one could only predict the
probability of where an electron could be
(based on a mathematical formula), not its
exact location
• These probabilities form a region of space
around the nucleus that he called orbitals (the
most probable location of the electrons).
James Chadwick
• Discovered the neutron in 1932
• Neutrons have approximately the same mass
as protons
• Neutrons have no electric charge