Development of our understanding
of the atom
Early Models
 Democritus was first to suggest atom in 4th century B.C.
 Believed matter was made of tiny individual particles
called atomos meaning indivisible.
 Believed that atoms could not be created, destroyed, or
further divided.
 Aristotle was one of the most respected Greek
philosophers.
 Disagreed with Democritus because it was against his
views.
 Matter is made of earth, fire, air and water.
 Did not believe that that atoms moved through empty
space.
John Dalton
 Dalton’s model was the “Billiard Ball”
 Atomic Theory
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Matter is composed of atoms
Atoms are indivisible
Atoms of a given element have same size, mass and chem
props and are different from those of another element
Different atoms combine in whole number ratios to form
compounds and are separated, combined and rearranged in
chem reactions
 Conservation of Mass-is the result of the separation,
combination, or rearrangement of atoms-atoms are not
created, destroyed, or divided in the process.
Dalton’s Theory had to be
revised because he …
 Dalton was incorrect about atoms being indivisible.
 Atoms can be divisible into several subatomic particles.
 Dalton was also incorrect about all atoms of a given
element having identical properties.
 Atoms of the same element can have slightly different
masses.
Early Models
 Thomson used the Cathode Ray – determined atoms
were made of particles
 Negative charges in atom
 “Plum Pudding” model
 Millikan used oil drop experiment
 Found mass and charge of an electron
Important discoveries
 Rutherford experimented with Gold Foil
 Found nucleus
 Discovered its positive charge and tiny size
 Electrons moving around the nucleus
 Came up with “Nuclear Model”
Moving Forward…
 Bohr said electrons move in orbits
 Found in energy levels
 Explains bright-line spectrum
 Called “Planetary Model”
 But Bohr knew that it was flawed
What we believe now
 Orbital proposed by Schrödinger
 Describes most likely where electrons were
 Quantum mechanics new theory
 “Electron Cloud” model