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Figure 3-4: Cathode Ray Tube
Figure 3.5: Cathode Ray Experiment
1. An object in the tube casts a shadow
2. The wheel rolled from cathode to anode
3. Rays were deflected by a magnetic field like an
electric current, known to be negative
4. Rays were deflected from a negative charge.
the cathode ray is made of electrons!
Electrons have a negative charge
Electrons are present in atoms of all elements
Millikan Oil Drop Experiment
Millikan Oil Drop Experiment
Millikan was able to determine the exact
charge of an electron by suspending oil
droplets in an electric field
By determining the mass of the oil droplet he could see
the charge required to suspend the drops were discreet
multiples of a specific charge.
Confirmed electrons are negative
 Showed they have a tiny mass compared to
their large charge
J. J. Thomson (cathode ray)
1904 – Model of the
atom…figured since the
atom is neutral there must
be something positive to
balance out the negative
 Called the “Plum
Pudding Model”
 A positive cloud with
negative electrons
within it.
Figure 3-6: Rutherford’s Gold Foil
Figure 3-6: Rutherford’s Gold Foil
 Bombarded
thin gold foil with alpha
particles and expected the particles to
pass through.
Figure 3-6: Rutherford’s Gold Foil
Figure 3-6: Rutherford’s Gold Foil
 Particles
were deflected greatly and
some went straight back to the source
 Conclusion: the volume of the nucleus
is very small compared to the volume
of the atom
Figure 3-7: Rutherford’s Gold Foil
Ernest Rutherford
Rutherford determined there was a small, dense
nucleus in the atom because some of the
particles were deflected.
1911 – Model of
atom includes a
nucleus surrounded
by electrons
Section 3.2
Composition of the Atomic Nucleus
 Atomic nuclei are made of protons and neutrons
Rutherford is typically credited with the discovery of
the proton in 1919.
 James Chadwick is credited with the discovery of the
neutron in 1932
 Proton has a positive charge equal to the magnitude
of the negative charge of the electron.
Section 3.2
Simplest hydrogen atom is simply a proton with
an electron orbiting.
Proton mass is 1836 times greater than the mass of
the electron.
The number of protons in the nucleus
determines the identity of the element and is the
atomic number.
Section 3.2
Forces in the Nucleus
 Generally, particles with the same electric charge
repel each other
 However, when two protons are extremely close
to each other, there is a strong attraction
between them.
Nuclear forces: these short-range proton-neutron,
proton-proton, and neutron-neutron forces hold the
nucleus together.
Section 3.2
Forces in the Nucleus
Section 3.2
The Sizes of Atoms
 The region occupied by the electrons is an
electron cloud.
 Atomic radius is the distance from the
nucleus to the outer portion of the electron
 Atomic radii are expressed in picometers ( 1
pm = 10-12 m)
 Atomic range is 40 to 270 pm.
Robert Frost
American Poet (1874-1963)
 Typically wrote pastoral poems about rural
Fireflies in the Garden
Here come real stars to fill the upper skies,
And here on earth come emulating flies,
That though they never equal stars in size,
(And they were never really stars at heart)
Achieve at times a very star-like start.
Only, of course, they can’t sustain the part
Robert Frost
American Poet (1874-1963)
 Typically wrote pastoral poems about rural
 Very interested in science
 Curious about the structure of the atom
 Read Scientific American
 Attended lectures atomic structure and
quantum physics by the chemist Niels