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History of the Model of the Atom
“HOTMOTA”
Chapter 4
Chem X
Early Theories
• Ancient Greeks –
• Democritus (400 BC) –
• Aristotle –
• Speculation until 18th century (1700’s)
when scientists began to develop
evidence.
John Dalton
• studied reactions and
•
•
made careful
observations &
measurements
determined mass ratios
of elements in various
reactions
this lead to his atomic
theory, which was
generally accepted
Dalton’s Atomic Theory (1803)
1. All matter is composed of extremely small indivisible
2.
3.
4.
5.
particles called atoms.
Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple
whole-number ratios to form chemical compounds.
In chemical reactions, atoms are combined,
separated, or rearranged.
• Dalton turned Democritus’s idea into a scientific
theory that could be tested by experiment.
• Dalton made three mistakes in his theory:
The Atom and Subatomic Particles
Cathode Ray Tube Experiment:
William Crookes - Cathode Ray Tube
JJ Thomson
The Atom and Subatomic Particles
Cathode Ray Tube Experiment:
Important Observations
Conclusions
1)
2)
The Atom and Subatomic Particles
JJ Thomson (1856-1940): English physicist
Inferences made:
• If atoms are electrically neutral then
they must…
• The ratio of the charge of cathode-ray
particles to their mass it was always the
same, so he concluded that…
This lead to the _____________ Model
Robert Milikan (1868 - 1953)
_____________ Experiment (1909)
• forced oil through a tiny hole forcing the drops to
gain/lose electrons and thus carry a charge.
•Robert Milikan found mass of electron as 1/1840th the mass of a hydrogen atom
•(mass electron = 9.1 x 10-31 kg; mass proton = 1.673 x 10-27 kg)
The Atom and Subatomic Particles
Ernest Rutherford (1911)
________________ experiment
The Atom and Subatomic Particles
Important Observations
Conclusions
1)
2)
• He discovered the “nucleus”! And suggested that
the electrons surrounded the positively charged
nucleus like planets around the sun.
The Atom and Subatomic Particles
• Discovery of the Proton: Ernest Rutherford (1920)
• Discovery of the Neutron: Sir James Chadwick (1932)
Particle
Symbol
Location
Actual
Electrical
Charge
Relative
Electrical
Charge
Actual Mass
electron
-1.6 x 10-19 C
proton
+1.6 x 10-19 C
1.673 x 10-24 g
neutron
0
1.675 x 10-24 g
9.11 x 10-28 g
Relative
Mass
(amu)
Nuclear Symbol and Atomic Mass
• Atomic Number –
• Nuclear Symbol
• Isotope –
Same element;
different #
neutrons
• Atomic Mass –
• C-12 is assigned to be exactly 12 a.m.u.
(thus 1 a.m.u. =
)
Nuclear Symbol and Atomic Mass
Isotope
Abundance
C-12
C-13
Mass of
Isotope
12
13.0034
C-14
13.9996
Trace
98.89%
1.11%
Nuclear Symbol and Atomic Mass
Isotope
Mass of
Isotope
Abundance
Cu-63
62.93
69.09%
Cu-65
?
?
Nuclear Symbol and Atomic Mass
Isotope
# Proton
#
Neutrons
#
Mass Charge
Electrons
#
23
11Na
1
?H
16
8O
18
?O
23
+
11Na
55
92
16
133
146
16
0
0
-2
Unstable Nuclei & Radioactive Decay
• Radioactivity –
• “ordinary” reactions vs. nuclear reactions
Radioactive decay
• Alpha Radiation
226 Ra
88
Æ
4 He
2
+ ______
• Beta Radiation
14 C
6
Æ
0 β
-1
+ ______
• Gamma Radiation
238 U
92
Æ
4 He
2
+ ___ + 002 γ
Radioactive decay - Practice
• 8636Kr Æ
87 Rb
37
+ ______
• 21082Pb Æ 42He + ____ + 0-1β
• _____ Æ
• 24395Am Æ
4 He
2
+
239 Np
93
236 Pu
94
+ ___
Radioactive decay - Practice
• What’s the daughter nuclide after Kr87 undergoes an alpha and two beta
decays?
• 8736Kr Æ