Atoms: Development of the Atomic Theory

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AtomsDevelopment of the Atomic Theory
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Democritus

• 460 BC - Greek philosopher proposes the existence
  of the atom
• He pounded materials until he made them into
  smaller and smaller parts
• He called them atoma which is Greek for
  indivisible.

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Democritus

• His Theory
• All atoms
• Are small hard particles
• Are made of a single material formed into
  different shapes and sizes
• Are always moving, and they form different
  materials by joining together

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John Dalton

• 1803 - British chemist elements combine in
  specific proportions to form compounds

Solid Sphere Model or Bowling Ball Model Proposed
by John Dalton
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John Dalton

• His Theory
• All substances are made of atoms that cannot be
  created, divided, or destroyed.
• Atoms join with other atoms to make new
  substances.
• Atoms of the same element are exactly alike, and
  atoms of different elements are different in mass
  and size.

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J.J. Thomson

• 1897 - English chemist and physicist discovered
  1st subatomic particles

Plum Pudding Model or Raisin Bun Model Proposed
by J.J. Thomson
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J.J. Thomson

• His Theory
• Atoms contain negatively charged particles called
  electrons and positively charged matter.
• Created a model to describe the atom as a sphere
  filled with positive matter with negative
  particles mixed in
• Referred to it as the plum pudding model

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Ernest Rutherford

• 1912 - New Zealand physicist discovered the
  nucleus

Nuclear Model Proposed by Ernest Rutherford
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Ernest Rutherford

• His Theory
• Small, dense, positively charged particle present
  in nucleus called a proton
• Electrons travel around the nucleus, but their
  exact places cannot be described.

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Niels Bohr

• 1913 - Danish physicist discovered energy levels

Bohr Model or Planetary Model Proposed by Niels
Bohr
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Niels Bohr

• His Theory
• Electrons travel around the nucleus in definite
  paths and fixed distances.
• Electrons can jump from one level to a path in
  another level.

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Erwin Shrodinger

• 1924 - Austrian physicist developed the electron
  cloud model

Electron Cloud Model Proposed by Erwin
Schrodinger
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Erwin Shrodinger

• His Theory
• The exact path of electrons cannot be predicted.
• The region referred to as the electron cloud, is
  an area where electrons can likely be found.

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James Chadwick

• 1932 - English physicist discovered neutrons
• His Theory
• Neutrons have no electrical charge.
• Neutrons have a mass nearly equal to the mass of
  a proton.
• Unit of measurement for subatomic particles is
  the atomic mass unit (amu).

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Modern Theory of the Atom

• Atoms are composed of three main subatomic
  particles the electron, proton, and neutron.
• Most of the mass of the atom is concentrated in
  the nucleus of the atom.

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Modern Theory of the Atom

• The protons and neutrons are located within the
  nucleus, while the electrons exist outside of the
  nucleus.
• In stable atoms, the number of protons is equal
  to the number of electrons.

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Modern Theory of the Atom

• The type of atom is determined by the number of
  protons it has.
• The number of protons in an atom is equal to the
  atomic number.

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Modern Theory of the Atom

• The sum of the number of protons and neutrons in
  a particular atom is called the atomic mass.
• Valence electrons are the outermost electrons.