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From Democritus to modern atomic theory
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Atoms:Atoms:Development of Development of
the Atomic the Atomic TheoryTheory
DemocritusDemocritus
460 BC - Greek philosopher proposes 460 BC - Greek philosopher proposes the existence of the the existence of the atom atom
He pounded materials until he made He pounded materials until he made them into smaller and smaller partsthem into smaller and smaller parts
He called them He called them atoma atoma which is which is Greek for “indivisible”.Greek for “indivisible”.
DemocritusDemocritus
His Theory:His Theory:All atoms: All atoms: Are small hard particles Are small hard particles
Are made of a single material formed Are made of a single material formed into different shapes and sizesinto different shapes and sizes
Are always moving, and they form Are always moving, and they form different materials by joining togetherdifferent materials by joining together
John DaltonJohn Dalton
1803 - British chemist; elements 1803 - British chemist; elements combine in specific proportions to form combine in specific proportions to form compoundscompounds
Solid Sphere Model or Bowling Ball Model
Proposed by John Dalton
John DaltonJohn Dalton His Theory:His Theory:
All substances are made of atoms that All substances are made of atoms that cannot be created, divided, or destroyed.cannot be created, divided, or destroyed.
Atoms join with other atoms to make new Atoms join with other atoms to make new substances.substances.
Atoms of the same element are exactly Atoms of the same element are exactly alike, and atoms of different elements are alike, and atoms of different elements are different in mass and size.different in mass and size.
J.J. ThomsonJ.J. Thomson
1897 - English chemist and physicist; 1897 - English chemist and physicist; discovered 1discovered 1stst subatomic particles subatomic particles
Plum Pudding Model or Raisin Bun Model
Proposed by J.J. Thomson
J.J. ThomsonJ.J. Thomson
His Theory:His Theory: Atoms contain negatively charged Atoms contain negatively charged
particles called particles called electrons electrons andand positively positively charged matter.charged matter.
Created a model to describe the atom as Created a model to describe the atom as a sphere filled with positive matter with a sphere filled with positive matter with negative particles mixed innegative particles mixed in
Referred to it as the plum pudding modelReferred to it as the plum pudding model
Ernest RutherfordErnest Rutherford
1912 - New Zealand physicist 1912 - New Zealand physicist discovered the nucleusdiscovered the nucleus
Nuclear ModelProposed by Ernest
Rutherford
Ernest RutherfordErnest Rutherford
His Theory:His Theory: Small, dense, positively charged particle Small, dense, positively charged particle
present in present in nucleusnucleus called a called a protonproton
ElectronsElectrons travel around the travel around the nucleus,nucleus, but but their exact places cannot be described.their exact places cannot be described.
Niels BohrNiels Bohr 1913 - Danish physicist; discovered 1913 - Danish physicist; discovered
energy levelsenergy levels
Bohr Model or Planetary Model
Proposed by Niels Bohr
Niels BohrNiels Bohr
His Theory:His Theory: ElectronsElectrons travel around the nucleus in travel around the nucleus in
definite paths and fixed distances.definite paths and fixed distances.
Electrons can jump from one level to a Electrons can jump from one level to a path in another level.path in another level.
Erwin ShrodingerErwin Shrodinger
1924 - Austrian physicist; developed 1924 - Austrian physicist; developed the the electron cloud electron cloud modelmodel
Electron Cloud Model
Proposed by Erwin Schrodinger
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Erwin ShrodingerErwin Shrodinger
His Theory:His Theory: The exact path of electrons cannot be The exact path of electrons cannot be
predicted.predicted.
The region referred to as the electron The region referred to as the electron cloud, is an area where electrons can cloud, is an area where electrons can likely be found. likely be found.
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James ChadwickJames Chadwick
1932 - English physicist; discovered 1932 - English physicist; discovered neutronsneutrons
His Theory:His Theory: NeutronsNeutrons have no electrical charge. have no electrical charge. Neutrons have a mass nearly equal to Neutrons have a mass nearly equal to
the mass of a proton.the mass of a proton. Unit of measurement for subatomic Unit of measurement for subatomic
particles is the atomic mass unit (particles is the atomic mass unit (amuamu).).
Modern Theory of the Modern Theory of the AtomAtom
AtomsAtoms are composed of three main are composed of three main subatomicsubatomic particlesparticles: the : the electronelectron, , protonproton, , and and neutronneutron. .
Most of the Most of the massmass of the atom is of the atom is concentrated in the concentrated in the nucleusnucleus of the atom. of the atom.
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Modern Theory of the Modern Theory of the AtomAtom
The The protons protons and and neutrons neutrons are located are located withinwithin the the nucleusnucleus, while the , while the electronselectrons exist exist outsideoutside of the nucleus. of the nucleus.
In stable atoms, the number of In stable atoms, the number of protonsprotons is is equalequal to the number of to the number of electronselectrons. .
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Modern Theory of the Modern Theory of the AtomAtom
The type of atom is determined by the The type of atom is determined by the number of number of protons protons it has. it has.
The number of protonsThe number of protons in an atom is equal in an atom is equal to the to the atomic numberatomic number..
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Modern Theory of the Modern Theory of the AtomAtom
The sum of the number of protonsThe sum of the number of protons and neutrons and neutrons in a particular atom is called the in a particular atom is called the atomic massatomic mass..
Valence electronsValence electrons are the outermost electrons.are the outermost electrons.
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