Democritus 460 – 370 BCE• Democritus believed matter was made
up of “atoms” (Greek for indivisible)
• Democritus thought:– Each element had its own kind of atom. – Atoms of all elements had different shapes and
sizes. – All atoms moved around in space.
air
water
granite
copperwood
Aristotle 400 BCE
• claimed that there was no smallest part of matter
• different substances were made up of different proportions of fire, air, earth, and water.
• The four element theory
Four element example
• Wood: – contains “earth”
because it has weight– contains “fire” because
it can burn– contains “air” which is
released as smoke– contains water before
it dries out
• Copper:– More earth because it
is heavier– Less fire, but can still
be burned or melted– Very little air
– Very little water
John Dalton 1850• Dalton stated that all matter is made of
indivisible and indestructible atoms, which differ from element to element
• He also said atoms of different elements have different properties
• Different elements combine in specific ways to form new substances.
• The billiard ball theory
J.J. Thomson 1897
• Experimenting with vacuum tubes (cathode rays), scientists discovered that an electric current could pass through a tube as a bright light
What do you think that bright light might have been?
J.J. Thomson 1897
• Thomson found that the glowing stream would bend toward a positive charge, and bend away from a negative charge.– What could this mean?
• Thompson theorized that negatively charge particles were responsible and called them electrons.
Ernest Rutherford
• He found that most of the alpha particles passed right through the gold foil as he expected
• However, a small number of alpha particles were deflected or bounced right back.
• What can we conclude?
Rutherford’s Planetary model
• In 1911, Rutherford proposed a revolutionary view of the atom.
• Proposed that the atom consisted of a small, dense core of (+) particles in the nucleus of the atom.
• Electrons (-) floated around the nucleus like planets around the sun.
James Chadwick 1932
• The Chadwick problem:
• Positive charges repel positive charges• How then, can a nucleus not be pushed
apart as two magnets are?• Finally, if protons weigh 1amu, where is all
this extra mass coming from?
• What did he discover???
Point of Interest
This is why all of those giant elements at the bottom of the periodic table are
unstable or radioactive.
The Nuclei of the atoms have too many particles to stick together.
Too much charge!!
James Chadwick 1932 • Chadwick discovered the neutron.
• Neutrons reduce the repulsion between protons and stabilize the atom's nucleus.
• Neutrons are in the Nucleus and weigh 1amu
• Neutrons do not have any electrical charge, they are neutral
Bohr’s Model 1913• Bohr was studying electrons in atoms and
proposed the idea that electrons travel in specific shells around the atom's nucleus.
• Each of these shells can hold a certain amount of electrons which must be filled in a certain order
• We now know his theory is not entirely correct but it is still useful today.
The Quantum Mechanical Model
• This is the correct model used today.
• The electron cloud model uses the basic idea of Bohr’s model except that the electrons are not found in distinct orbits but their position can be thought of as in a 3D cloud that has a particular energy
• http://videos.howstuffworks.com/science/atoms-videos-playlist.htm#video-29292
• http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html
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