Defining the Atom

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Section 4.2

• Defining the Atom

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Objectives

• Define atom
• Distinguish between the subatomic particles in
  terms of relative charge and mass.
• Describe the structure of the atom, including
  locations of the subatomic particles.

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What is an atom??

• The smallest particle of an element that still
  retains the properties of the element is called
  an atom.
• The scanning
• tunneling micro-
• scope (STM)
• allows scientists
• to see individual
• atoms.

A cobalt atom
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The Electron

• Because of Daltons atomic theory, most
  scientists in the 1800s believed that the atom
  was like a tiny solid ball that could not be
  broken up into parts.
• In 1897, a British physicist, J.J. Thomson,
  discovered that this solid-ball model was not
  accurate.

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The Electron

• Thomsons experiments used a
  vacuum tube (a tube that has had all the
  gases pumped out of it) called the cathode ray
  tube.

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The Electron

• At each end of the tube is a metal piece called
  an electrode, which is connected through the
  glass to a metal terminal outside the tube.
• These electrodes become electrically charged when
  they are connected to a high-voltage electrical
  source.

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The Electron

• When the electrodes are charged, rays travel in
  the tube from the negative electrode, which is
  the cathode, to the positive electrode, the
  anode.

• Because these rays originate at the cathode, they
  are called cathode rays.

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The Electron

• Thomson found that the rays bent toward a
  positively charged plate (placed next to the
  glass tube) and away from a negatively charged
  plate.

• He also knew that objects with like charges repel
  each other, and objects with unlike charges
  attract each other.

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The Electron
Video clip
Video Clip II
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The Electron

• Thomson concluded that cathode rays are made up
  of invisible, negatively charged particles
  referred to as electrons.

• He also concluded that these electrons had to
  come from the matter of the negative electrode.
  That is, they had to have come from the atoms of
  the metal and so were subatomic.

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The Electron

• In Thomsons experiments, he determined the mass
  of the charged particle to be MUCH LESS than a
  hydrogen atom. Scientists had to conclude that
  atoms were not just solid spheres, but somehow
  were composed of smaller, negatively charged
  particles. (Dalton was wrong!)

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The Electron

• In addition, since atoms themselves are not
  negatively charged but neutral, there had to be
  other particles in the atom, especially
  positively charged ones.
• And, because atoms have a measurable mass and the
  negatively charged particles that atoms were
  found to contain were extremely light, scientists
  had to conclude that atoms must contain
  positively charged particles with a much greater
  mass than electrons.

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The Plum Pudding Model

• Thomson proposed a model of the atom called the
  plum pudding model.
• The atom contained uniformly distributed positive
  charge containing electrons.

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The Electron (cont.)

• Robert Millikan (1868-1953) determined the charge
  of the electron with an experiment called the
  oil drop experiment.
• The value of the charge (1.6 x 10-19 C) has since
  been equated to a charge of -1.
• He also calculated the mass of an electron at 9.1
  x 10-28 g ( 1/1840 of a hydrogen atom).

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The Nuclear Atom

• In 1911, a team of scientists led by Ernest
  Rutherford in England carried out the first of
  several important experiments that revealed an
  arrangement far different from the plum pudding
  model of the atom.

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The Nuclear Atom

• The experimenters set up a lead-shielded box
  containing radioactive polonium, which emitted a
  beam of positively charged subatomic particles
  through a small hole.
• Today, we know that the particles of the beam
  consisted of clusters containing two protons and
  two neutrons and are called alpha particles.

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The Nuclear Atom

• This beam of particles was then shot at a thin
  sheet of gold foil. The gold foil was surrounded
  by a screen coated with zinc sulfide (which glows
  when struck by the positively charged particles
  of the beam).

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The Nuclear Atom
This experiment was called the gold foil
experiment
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The Nuclear Atom

• Based on the plum pudding model, Rutherford
  predicted that the paths of the fast-moving alpha
  particles would only be slightly altered by a
  collision with an electron. They should pass
  straight through the foil.

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The Nuclear Atom
He was amazed to discover that some of the alpha
particles were deflected at wide angles. Video
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The Nuclear Atom

• To explain the results of the experiment,
  Rutherfords team proposed a new model of the
  atom called the nuclear model
• Because most of the particles passed through the
  foil, they concluded that the atom is nearly all
  empty space.

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The Nuclear Atom

• The Nuclear Model (cont.)
• 2. Because the deflections had to be caused by a
  repulsive force between the alpha particles and
  another structure, they proposed that the atom
  has a small, dense, positively charged central
  core, called a nucleus.
• The nucleus was centrally located within the atom
  and contained all the mass and positive charge of
  the atom.

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The Nuclear Atom

• The Nuclear Model (cont.)
• 3. Electrons move rapidly through the space
  surrounding the nucleus. They are held there by
  the positively-charged nucleus.

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Completing the Atom

• By 1920, Rutherford refined his concept of the
  nucleus. He concluded that the nucleus contained
  positively charged particles called protons.
• A proton is a subatomic particle carrying a
  charge equal to but opposite of that of the
  electron a charge of 1.

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Completing the Atom

• In 1935, James Chadwick showed that the nucleus
  contained another particle. Since it was
  neutral, it was called a neutron.
• A neutron is a subatomic particle that has the
  same mass of a proton but no electrical charge.

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The properties of the three (3) subatomic
particles that are found in all atoms are
summarized below.
Scientists have determined that protons and
neutrons are composed of subatomic particles
called quarks.