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Atomic Models

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Title: Atomic Models


1
Atomic Models
  • Historical Development

2
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.

3
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.

4
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.

5
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.

6
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.

7
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.

8
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.

9
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.
10
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.

11
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.

12
Gold Foil Experiment
13
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

14
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

15
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

16
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

17
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.

18
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.

19
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.

20
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.

21
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.

22
  • Orbitals form a number of simple to bizarre
  • 3-dimensional shapes, depending on their energy.
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