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Defining the Atom

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Title: Defining the Atom


1
Section 4.2
  • Defining the Atom

2
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.

3
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
4
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.

5
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.

6
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.

7
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.

8
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.

9
The Electron
Video clip
Video Clip II
10
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.

11
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!)

12
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.

13
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.

14
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).

15
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.

16
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.

17
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).

18
The Nuclear Atom
This experiment was called the gold foil
experiment
19
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.

20
The Nuclear Atom
He was amazed to discover that some of the alpha
particles were deflected at wide angles. Video
21
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.

22
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.

23
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.

24
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.

25
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.

26
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.
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