Download Development of the Atomic Theory

Development of the
Atomic Theory
The atomic model
The Greek Model
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Democritus was the first to propose an atomic theory
around 440 B.C.
He theorized that if you cut, for example, a coin in half
and cut that half in half and that half in half and so on,
eventually you would end up with an “uncuttable”
particle.
He named this particle the atom from the Greek word
atomos meaning indivisible.
He claimed
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Atoms are small, hard particles made up of a single material.
Atoms are always moving
Atoms join together to form different materials
Dalton’s Model
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In the late 1700’s, John Dalton performed many
experiments and from them developed a theory.
 All
substances are made of atoms. Atoms are small
particles that cannot be created, divided, or
destroyed.
 Atoms of the same element are exactly alike, and
atoms of different elements are different.
 Atoms join with other atoms in specific proportions to
make new substances.
Thomson’s Model
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In 1887, J. J. Thomson performed an experiment
that identified an error in Dalton’s theory and
proposed the Plum-Pudding model.
 Atoms
can be divided into smaller pieces.
 Atoms contain negatively charged particles he called
corpuscles that are identical. Today they are called
electrons.
 Atoms also contain a positively charged substance
because atoms are neutral.
 Atoms are made up of a positively charged pudding
like material with electrons located throughout like
plum-pudding.
Thomson’s Experiment
Rutherford’s Model
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In 1909, Rutherford performs an
experiment changing Thomson’s model.
 The
atom has a small, dense, positively
charged center he called the nucleus.
 The atom is mostly empty space
 Electrons orbit the nucleus in definite energy
levels like planets orbit the sun.
Rutherford’s Experiment
The Bohr Model
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In 1913, Neils Bohr proposed a new
theory.
 While
electrons had to be in “special orbits””,
electrons could jump from one level to
another.
Today’s Atomic Model
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Many twentieth-century scientists have contributed to
today’s atomic model.
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The atom contains a small, positively charged center called the
nucleus.
The nucleus contains both positively charged protons and
neutrally charged neutrons.
Negatively charged electrons orbit the nucleus in an electron
cloud. The location is impossible to determine as they gain and
lose energy constantly.
As electrons gain energy they move away from the nucleus. As
they lose energy they move closer to the nucleus.
The atom is made up of mostly empty space.
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