Atomic Models

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Atomic Models

• Historical Development

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Ancient Greece

• In the fourth century B.C., Greek philosopher
  Democritus inferred that substances were made of
  invisible units called atoms.
• Atom is derived from the Greek word meaning
  unable to be divided.
• Democritus, though, could not provide evidence
  that atoms really existed.

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Centuries of Work

• Throughout the centuries that followed, other
  theories were proposed
• Emphasis was put on making careful and repeated
  measurements.
• More reliable data were collected.

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Centuries of Work

• Beginning in the 1800s, newer models were
  devised
• More discoveries about electrons, protons, and
  neutrons.
• The model of the atom was continuously refined
  and revised.

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Dalton Model

• In the early 1800's, John Dalton developed the
  first scientific model about atoms
• All matter composed of tiny particles
• Particles cant be divided into smaller
  particles.
• Atoms of each element are exactly alike.
• Atoms of different elements are different masses.
• Atoms of different elements join to form
  different compounds.
• Dalton imagined atoms to be solid spheres.

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Modern Atomic Theory

• Several changes have been made to Daltons
  theory
• Dalton said
• Atoms of a given element are identical in size,
  mass, and other properties Atoms of other
  elements differ in size, mass, and other
  properties.
• Modern Theory states
• Atoms of an element have a characteristic
  average mass which is unique to that element.

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Modern Atomic Theory

• Dalton said
• Atoms cannot be subdivided, created or destroyed
• Modern Theory states
• Atoms cannot be subdivided, created or destroyed
  in ordinary chemical reactions. However, these
  changes can occur in nuclear reactions.

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Thomson Model

• At the end of the 1800's J.J.Thomson discovered
  that atoms were not just solid spheres
• He discovered the electron in 1897
• This meant that atoms contained even smaller,
  subatomic particles.
• In 1903, he developed a model that imagined
  negative electrons in a ball of positive
  material.
• The model was called Thomsons
• Plum Pudding Model.

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Discovery of the Electron

• In 1897, Thomson used a cathode ray tube to
  deduce the presence of negatively charged
  particles.

Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
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Conclusions from the Study of the Electron

• Cathode rays have identical properties regardless
  of the element used to produce them. All
  elements must contain identically charged
  electrons.
• Atoms are neutral, so there must be a positive
  particle in the atom to balance the negative
  charge of the electrons.
• Electrons have so little mass that the atoms
  must contain other particles that account for
  most of the mass.

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Rutherford's Model

• Ernest Rutherford made the discovery of protons
  in 1908 In 1911 he did the Gold Foil
  Experiment
• He tested the theory that protons and electrons
  were evenly scattered throughout the atom.
• He aimed a positively charged particle beam at a
  sheet of gold foil.
• Most passed through, but some, surprisingly
  bounced back.
• The particles struck something larger than a
  single proton.

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Gold Foil Experiment
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Rutherfords model

• Rutherfords model further
• refined the atoms structure
• An atom is mostly empty space.
• The small positive nucleus is in the center
• Most of an atoms mass is contained in the small
  nucleus.
• The nucleus is surrounded by negative orbiting
  electrons

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James Chadwick

• Recognized that the presence of protons and
  electrons explained the neutral charge of the
  atom but did not account for total mass of an atom

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James Chadwick

• In 1932, Chadwick discovered another particle
  within the nucleus
• Its mass was approximately the same as a proton
• He named it the neutron

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Bohr Model

• In 1913, Danish scientist Niels Bohr explained
  that Electrons surround the nucleus in distinct
  energy levels
• Energy levels are arranged in concentric circles
  like layers of an onion.
• Each electron has a certain amount of energy,
  which keeps it moving in its level.
• Bohr's model has been called the planetary
  model. It compares electrons to planets and the
  nucleus to the sun

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Bohr Model

• Each energy level is further away from the
  nucleus.
• The farther a level is from the nucleus, the
  higher the energy an electron needs to stay in
  that level.
• By absorbing or releasing specific amounts of
  energy, an electron can move from one energy
  level to the next.
• An electron can't exist between energy levels.

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Modern Electron Cloud Model

• Although many of the basic ideas of the Bohr
  atomic model still hold true, scientists now know
  that electrons do not actually orbit the nucleus
  as in the planetary model.
• The electron cloud model is now used to describe
  atoms.
• Electrons dart about in a constantly changing
  path.
• Electron paths form a region called an electron
  cloud.

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Modern Electron Cloud Model

• Example The idea can be related to the blur of a
  fan as the blades move, they seem to fill in the
  spaces between them, just as fast-moving
  electrons fill the space around the nucleus.

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Modern Electron Cloud Model

• A.K.A. The Quantum Mechanics Model
• Electrons act more like an energy wave than a
  tiny particle like getting hit by a wave, not a
  rock
• Only the probable location of an electron can be
  determined, not the exact location.

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Electron Orbitals

• Like in the Bohr model, electrons are arranged in
  energy levels within an electron cloud.
• Electrons are subject to the laws of quantum
  mechanics. This means they carry only certain
  quantities of energy
• These strengths define the energy levels.
• Electrons with the lowest energy are in energy
  levels closest to the nucleus.
• Electrons with the highest energy are in energy
  levels farthest from the nucleus.
• Each energy level can be further broken down into
  sublevels, or orbitals.
• Orbitals help identify electron placement.

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• Orbitals form a number of simple to bizarre
• 3-dimensional shapes, depending on their energy.