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Atomic History: An Overview
• Early Greeks - Democritus vs Aristotle
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Atomic History: An Overview
• Early Greeks - Democritus vs. Aristotle
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Early Greeks
• What is the nature of matter?-Infinitely divisible pieces of
“stuff”-Earth, Air, Fire ,and Water
• No experiments
Democritus vs. Aristotle
• Democritus• New idea• “atomos” -
indivisible• Eventually,
can’t divide matter any more
• First “atomic theory”
• Aristotle• Didn’t buy it…• All things are
infinitely divisible
Guess who won?
Atomic History: An Overview
• Early Greeks - Democritus vs Aristotle
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Dalton’s Atomic Theory
• 1807• Idea of “atom”
– Solid spheres that were indestructible
– Unique to each element
– Combine evenly– Reactions are
rearrangements
John Dalton
Dalton’s Atomic Theory
• Atoms combine evenly in compounds– Small, whole number proportions– (e.g.) water - H20 (not H1O1/2 )
H
OH
H
OH
Atomic History: An Overview
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Thomson’s Discovery• Studied electricity not
atomic structure• His 1st observation was
that a magnet could deflect the current and realized that it was made of particles
• So to study current he pumped all the air out of a glass tube and applied voltage to two metal plates + (anode)/- (cathode)
J.J. Thomson 1897
Thomson’s Discovery (cont…)
• He then sent a ray of particles through the tube and saw that they came out of the cathode side and were attracted to the anode side and the tube glowed
• This told him that the particles were negative… he called his discovery a Cathode Ray Tube– Sends a “ray” of -
particles– Used in TVs and
Computers
J.J. Thomson 1897
Thomson’s Discovery
• The beam was attracted to the positive plate.• So, cathode rays are negative• He realized that these negative particle must be
ELECTRONS
Thomson’s Discovery
• Most books give Thomson credit for discovering the proton
• He and Millikan found the mass of an electron to be much smaller than an atom
• So, electrons are VERY VERY small• Protons must be large in
comparison– Plum Pudding Model
Plum Pudding Model
Preface: “Plum Pudding” atomic model
• atoms are solid• made of positively-charged
material• with negative “bits” scattered
throughout(like raisins in plum pudding)(or raisin bread)
Atomic History: An Overview
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Rutherford’s Discovery
Image courtesty ofhttp://www.nobel.se/chemistry/laureates/1908/rutherford-
bio.html
Ernest Rutherford
Gold Foil Experiment
To test the Plum Pudding Model (1907):
• Shot alpha rays at thin gold foil– (about 2000 atoms thick)
• EXPECTED to see the ray scatter as it hit all the solid positive atoms– Like spray from a nozzle
Gold Foil ExperimentIn Detail:
Click here to see an
animation.
Gold Foil ExperimentRESULTS
• Most of the particles were not deflected
• Some were minimally deflected• VERY few (1 in 20,000) bounced
back– “as if you had fired a 15-inch shell at
a piece of tissue paper and it came back and hit you.” -
Rutherford
Gold Foil ExperimentCONCLUSIONS
• Plum pudding model wrong• A “nucleus” exists
– It is tiny– It is densely-packed and positively-
charged (this is the only way a strong positive particle could be deflected)
• Empty spaces exist in atoms– LOTS of it!!!!
Gold Foil Experiment
How much empty space?• Use a billiard ball to represent a
nucleus• The electrons occupy a volume one
kilometer in ALL DIRECTIONS• Most of that space is EMPTY.
Atomic History: An Overview
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Chadwick’s Discovery (1932)
• PROBLEM• There was more
mass in nucleus than explained by protons alone
• Where did it come from?
• NEUTRONSChadwick
Atomic History: An Overview
• Dalton’s Atomic Theory• Thomson - discovery of electrons• Rutherford - discovery of nucleus• Chadwick - discovery of neutron• Bohr - planetary model of atom
Bohr’s Atomic Model
• Nucleus has + charge
• Electrons have - charge
• Why don’t electrons simply “fall into” the nucleus?
Bohr
Bohr’s Atomic Model
• Bohr pictured atoms as little solar systems
• Nucleus in center• Electrons “orbiting” in
circles–We now know this isn’t correct
–But it helps to explain many
things about atoms
What Theory Do We Currently Support?
• Electron Cloud Model– Nucleus is composed of Protons and
Neutrons– Electrons spin very quickly around the
nucleus forming a “giant cloud” of negative energy
– These electrons can move further away or closer to the nucleus at anytime
– Their distance and space is infinite!
Protons, Neutrons, and Electrons
• Protons– Located in nucleus– Positively charged (+1)– Mass = “1” mass unit (or 1
gram/mole)• Number of protons = atomic
number (Z)
Three Main Subatomic Particles
Protons, Neutrons, and Electrons
• Neutrons– Located in nucleus– no charge (0)– Mass = “1” mass unit (or 1
gram/mole)
Three Main Subatomic Particles
Protons, Neutrons, and Electrons
• Electrons– Located outside nucleus– negative charge (-1)– So small we assume mass = 0
• Actual mass of 0.000 5 g/mol
Three Main Subatomic Particles
Protons, Neutrons, and Electrons
Atomic number (Z)• shown in lower
left• ALWAYS equals
the number of protons
• Equals the number of electrons in a neutral atom
C6
Isotope Notation
Protons, Neutrons, and Electrons
Mass Number (A)• shown in upper left• Is the sum:
neutrons + protons• In this example,
carbon has 6 protons and 7 neutrons
C6
Isotope Notation
13
Protons, Neutrons, and Electrons
• Atoms with the same number of protons can have different numbers of neutrons.
• We call such atoms isotopes
Protons, Neutrons, and Electrons
• Atoms with the same number of protons can have different numbers of neutrons.
• We call such atoms isotopes
C612
C613
C614
carbon -12 carbon -13 carbon -14
Protons, Neutrons, and Electrons
C612
C613
C614
6 protons6 electrons6 neutrons
6 protons6 electrons7 neutrons
6 protons6 electrons8 neutrons
All three kinds of carbon atoms have the same chemistry!
Protons, Neutrons, and Electrons
H1
1
Pb82
208
F9
19
1 proton1 electron0 neutrons
Neutrons = A - Z1 - 1 = 0
82 protons80 electrons126 neutrons
Neutrons = A - Z208 - 82 = 126
9 protons9 electrons10 neutrons
Neutrons = A - Z19 - 9 = 10
+2
Protons, Neutrons, and Electrons
Li3
7
What element is this?
How do you know?
Lithium
the atomic number is 3
look on the periodic table
Protons, Neutrons, and Electrons
Pb?
207
?92
235
K19
?
# of protons (Z)
# of neutrons
# of electrons
mass number (A)
name
82
125
82
207
lead-207
92
143
92
235
uranium-235
19
21
19
40
potassium-40