Structure of the Nuclear Atom
Atoms can be broken
down into smaller,
more fundamental
particles
• Electrons: negatively
charged subatomic
particles.
• Thomson (English
physicist) discovered
electrons in 1897.
How did he discover electrons?
• passed electric current through
gases at low pressure
• sealed the gases in glass
tubes fitted at both ends with
metal disks called electrodes
• electrodes were connected to
a source of high-voltage
electricity
• one electrode, the anode,
became positively charged
• other electrode, the cathode,
became negatively charged
• a glowing beam formed
between the electrodes, called
a cathode ray.
• Cathode rays are attracted to
metal plates that have a
positive electrical charge.
Plates that carry a negative
electrical charge repel the
rays.
Cathode Ray Tube
• Opposites attract and like
charges repel, Thomson
proposed that a cathode
ray is a stream of tiny
negatively charged
particles moving at a high
speed. These particles
were named electrons
• Thomson also showed
that cathode rays did not
depend on the kind of gas
in the tube or the type of
metal used. Electrons
must be parts of the
atoms of all elements
More Discoveries
Robert Millikan:
• carried out experiments
that allowed him to find
the quantity of charge
carried by an electron.
He also determined the
ratio of the charge to the
mass of an electron. He
used these values to
determine an accurate
value for the mass of an
electron (9.11 x10-28 g)
Goldstein :
• observed in a cathode ray
tube that there were rays
traveling in the opposite
direction than the cathode
rays. Concluded that they
were positive particles,
and called them protons
Chadwick:
• discovered another
subatomic particle, the
neutron; subatomic
particles with no charge
but with a mass nearly
equal to that of a proton
Millikan Oil Drop Experiment
• Virtual Chem Lab
Atomic Nucleus: After the discovery of subatomic particles, scientists
then wondered how the particles were put together in an atom
• Rutherford: tested a theory to
see how an atom is composed
• directed a narrow beam of
alpha particles at a very thin
sheet of gold foil. (Alpha
particle are helium atoms that
have lost their two electrons
and have a double positive
charge because of the two
remaining protons)
• Scientists thought that the
electrons were evenly
distributed throughout the atom
and uniformly filled with
positively charged material.
According to this, the alpha
particles should have passed
easily through the gold, with
only a slight deflection due to
the positive charge thought to
be spread out in the gold
atoms.
Rutherford Experiment
• But, to everyone’s surprise,
the great majority of alpha
particles passed straight
through the gold atoms,
without deflection. A small
fraction however bounced
off the gold foil at very large
angles, even straight back.
• Based on his results, he
proposed that the atom is
mostly empty space. He
concluded that all the
positive charge and almost
all mass are concentrated in
a small region that has
enough positive charge to
account for the great
deflection of some of the
alpha particles.
Rutherford’s Experiment
• http://www.mhhe.com/physsci/chemistry/e
ssentialchemistry/flash/ruther14.swf
Properties of Subatomic Particles
• Nucleus: the central core of an atom, composed of
protons and neutrons
• mass of atom is due mainly to the number of protons and
neutrons
• assigned charges of +1 and -1, they are relative charges
Absolute charge is the same
Properties of Subatomic Particles
Particle Symbol
Relative electrical
charge
Relative mass
(Mass of proton =1)
Actual mass
(g)
Electron
e-
1-
1/1840
9.11x10-28
Proton
P+
1+
1
1.67x10-24
Neutron
n◦
0
1
1.67x10-24
Review
• What are the charges
of the three main
subatomic particles?
• Electrons
• Protons
• Neutrons
• How small is the
nucleus compared to
the entire atom?
• Housefly sitting on
second base in a
baseball stadium
represents the
nucleus, the rest of
the atom would be the
size of the stadium
Review
• What must be true of the number of
electrons and protons in electrically neutral
atoms?
– The number of protons and electrons must be
the same
– The number of protons must be greater than
the number of neutrons
– The number of protons must be greater than
the number of electrons
– The number of protons must equal the
number of neutrons
Review
Describe Thomson’s, Millikan’s, and Rutherford’s
contributions to atomic theory.
• Passed electric current through sealed glass tubes
filled with gases. The resulting glowing beam was
described as a stream of tiny negatively charged
particles moving at high speeds. Concluded that
electrons must be parts of atoms of all elements. (T)
• Determined the charge and mass of the electron (M)
• Gold-foil experiments indicated that the atom had a
positively charged, dense nucleus which is tiny
compared to the atom as a whole (R)