Section 4.2
Defining the Atom
Objectives
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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.
What is an atom??
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The smallest particle of an
element that still retains the
properties of the element is
called an atom.
The scanning
tunneling microscope (STM)
allows scientists
to see individual
A cobalt atom
atoms.
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.
The Electron
•Thomson’s experiments used a
vacuum tube (a tube that has had
all the gases pumped out of it)
called the cathode ray tube.
The Electron
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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.
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.
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.
The Electron
Video clip
Video Clip II
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.
The Electron
• In Thomson’s 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!)
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.
The Plum Pudding Model
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Thomson proposed
a model of the atom
called the plum
pudding model.
The atom contained
uniformly
distributed positive
charge containing
electrons.
The Electron (cont.)
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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).
The Nuclear Atom
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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.
The Nuclear Atom
• The experimenters set up a leadshielded 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.
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).
The Nuclear Atom
This experiment was called the “gold foil” experiment
The Nuclear Atom
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Based on the plum pudding
model, Rutherford predicted
that the paths of the fastmoving alpha particles would
only be slightly altered by a
collision with an electron. They
should pass straight through the
foil.
The Nuclear Atom
He was
amazed to
discover that
some of the
alpha particles
were deflected
at wide angles.
Video
The Nuclear Atom
• To explain the results of the
experiment, Rutherford’s team
proposed a new model of the
atom called the nuclear model
1. Because most of the particles
passed through the foil, they
concluded that the atom is
nearly all empty space.
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.
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.
Completing the Atom
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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.
Completing the Atom
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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.
The properties of the three (3)
subatomic particles that are found in
all atoms are summarized below.
Particle
electron
Symbol Location
e-
Actual
Relative
Relative
electric
mass
mass
charge
(g)
Outside
nucleus
-1
1/1840
9.11 x
10-28
proton
P or p+
In
nucleus
+1
1
1.673 x
10-24
neutron
N or n0
In
nucleus
0
1
1.675 x
10-24
** Scientists have determined that protons and neutrons are composed of subatomic
particles called quarks.