10/6/2014
Lesson# 3.2:
Structure of the Nuclear Atom
• OBJECTIVES:
Not Really Indivisible
Atoms are composed of three major “subatomic”
particles (even they are divisible further—but we
won’t really deal with that):
In the nucleus: Nucleons
– Identify three types of subatomic particles.
– Describe the structure of atoms, according
to the Rutherford atomic model.
protons: mass of 1 amu (1.66E-24g),
charge of +1
neutrons: mass of 1 amu, charge of 0
Outside the nucleus:
electrons: negligible mass (1/1836 amu),
charge of -1
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Discovery of the Electron
Modern Cathode Ray Tubes
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively
charged particle: the electron
Television
Computer Monitor
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Mass of the Electron
Mass of the
electron is
9.11 x 10-28 g
The oil drop apparatus
1916 – Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
Conclusions from the Study
of the Electron:
a) Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
b) Atoms are neutral, so there must be positive
particles in the atom to balance the negative
charge of the electrons
c) Electrons have so little mass that atoms
must contain other particles that account for
most of the mass
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 Eugen Goldstein in 1886 observed
what is now called the “proton” particles with a positive charge, and
a relative mass of 1 amu (or 1840
times that of an electron).
• 1932 – James Chadwick confirmed
the existence of the “neutron” – a
particle with no charge, but a mass
nearly equal to a proton.
Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum
pudding” model.
Electrons
Subatomic Particles
Discovered by J.J. Thomson
(1st)
Particle Charge
Found outside of the nucleus
Electron
(e-)
-1
9.11 x10-28 1840 ~
0
Any neutral atom has an equal
number of protons and
electrons.
Proton
(p+)
+1
1.67 x10-
1
Nucleus
0
1.67 x10-
1
Nucleus
Electrons can be exchanged
between atoms to make
bonds.
J. J. Tompson
(1856 – 1940)
Neutron
(no)
Location
amu
Electron
cloud
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Rutherford’s Gold Foil Experiment
Protons
 Discovered by Ernest
Rutherford (2nd)
 Every element has a different
number of protons
 Atomic Number = an
element’s number of protons
 The periodic table lists
elements by Atomic Number
 Nuclear charge = # of protons
in the nucleus
 ALWAYS POSITIVE!!!
Mass
(g)
• http://www.mhhe.com/physsci/chemistry/ess
entialchemistry/flash/ruther14.swf
Ernest Rutherford
(1871 - 1937)
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Ernest Rutherford’s
Gold Foil Experiment - 1911
Rutherford’s Findings
Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected

“Like howitzer shells bouncing off
of tissue paper!”

Alpha particles are helium nuclei - The alpha
particles were fired at a thin sheet of gold foil
Conclusions:
a) The nucleus is small
b) The nucleus is dense
c) The nucleus is positively
 Particles that hit on the detecting screen
(film) are recorded
charged
Neutrons
The Rutherford Atomic Model
• Based on his experimental evidence:
 The atom is mostly empty space
 All the positive charge, and almost all the
mass is concentrated in a small area in the
center. He called this a “nucleus”
 The nucleus is composed of protons and
neutrons (they make the nucleus!)
 The electrons distributed around the
nucleus, and occupy most of the volume
 His model was called a “nuclear model”
Discovered by James
Chadwick (3rd)
Contribute to an atom’s
mass as much as protons.
Isotopes:
Atoms of the same
element (same # of
protons), with different #’s
of neutrons
James Chadwick
(1891 - 1974)
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