Transcript Cathode-Ray Tube

Atomic Structure: Basic Concepts
The Electron
• Because of Dalton’s 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.
• Thomson’s experiments used a vacuum tube.
Atomic Structure: Basic Concepts
The Electron
• A vacuum tube has
had all gases pumped
out of it.
• 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.
Atomic Structure: Basic Concepts
Cathode-Ray Tube
• 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.
Atomic Structure: Basic Concepts
Cathode-Ray Tube
• Thomson found that the rays bent toward a
positively charged plate and away from a
negatively charged plate.
• He knew that objects with like charges repel
each other, and objects with unlike charges
attract each other.
Atomic Structure: Basic Concepts
Cathode-Ray Tube
• Thomson concluded that cathode rays are
made up of invisible, negatively charged
particles referred to as electrons.
• These electrons had to come from the matter
(atoms) of the negative electrode.
Atomic Structure: Basic Concepts
Cathode-Ray Tube
• From Thomson’s experiments, scientists had
to conclude that atoms were not just neutral
spheres, but somehow were composed of
electrically charged particles.
• Reason should tell you that there must be a
lot more to the atom than electrons.
• Matter is not negatively charged, so atoms
can’t be negatively charged either.
Atomic Structure: Basic Concepts
Cathode-Ray Tube
• If atoms contained extremely light,
negatively charged particles, then they must
also contain positively charged particles—
probably with a much greater mass than
electrons.
Atomic Structure: Basic Concepts
Protons
• In 1886, scientists discovered that a cathoderay tube emitted rays not only from the
cathode but also from the positively charged
anode.
• These rays travel in a direction opposite to
that of cathode rays.
Atomic Structure: Basic Concepts
Protons
• Like cathode rays, they are deflected by
electrical and magnetic fields, but in
directions opposite to the way cathode rays
are deflected.
• Thomson was able to show that these rays
had a positive electrical charge.
• Years later, scientists determined that the rays
were composed of positively charged
subatomic particles called protons.
Millikan’s Oil Drop Exp.
•In 1909, Robert Millikan
performed a famous oil drop
experiment and determined the
size of the charge of an electron
and also it’s mass!
•What Millikan did was to put a
charge on a tiny drop of oil, and
measure how strong an applied
electric field had to be in order to
stop the oil drop from falling
Atomic Structure: Basic Concepts
Protons
• At this point, it seemed that atoms were made
up of equal numbers of electrons and protons.
• However, in 1910, Thomson discovered that
neon consisted
of atoms of
two different
masses.
Atomic Structure: Basic Concepts
Protons
• Atoms of an element that are chemically
alike but differ in mass are called isotopes
of the element.
• Today, chemists know that neon consists of
three naturally occurring isotopes.
• The third was too scarce for Thomson to
detect.
Atomic Structure: Basic Concepts
Neutrons
• Because of the discovery of isotopes,
scientists hypothesized that atoms contained
still a third type of particle that explained
these differences in mass.
• Calculations showed that such a particle
should have a mass equal to that of a proton
but no electrical charge.
• The existence of this neutral particle, called a
neutron, was confirmed in the early 1930s
by James Chadwick who bombarded a
nucleus with alpha particles (video).
Atomic Structure: Basic Concepts
Rutherford’s Gold Foil Experiment
• In 1909, 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
cookie-dough model of
the atom.
Atomic Structure: Basic Concepts
Rutherford’s Gold Foil Experiment
• The experimenters set up a lead-shielded box
containing radioactive polonium, which
emitted a beam of positively charged
subatomic particles through a small hole.
Atomic Structure: Basic Concepts
Rutherford’s Gold Foil Experiment
• Today, we know that the particles of the
beam consisted of clusters containing two
protons and two neutrons and are called alpha
particles.
• The sheet of gold foil was surrounded by a
screen coated with zinc sulfide, which glows
when struck by the positively charged
particles of the beam.
Atomic Structure: Basic Concepts
The Gold Foil Experiment
Atomic Structure: Basic Concepts
The Nuclear Model of the Atom
• To explain the results of the experiment,
Rutherford’s team proposed a new model
of the atom.
• Because most of the particles passed
through the foil, they concluded that the
atom is nearly all empty space.
Atomic Structure: Basic Concepts
The Nuclear Model of the Atom
• Because so few particles were deflected, they
proposed that the atom has a small, dense,
positively charged central core, called a
nucleus.
Atomic Structure: Basic Concepts
The Nuclear Model of the Atom
• The new model of the atom as pictured by
Rutherford’s group in 1911 is shown below.