Post on 24-Feb-2018
10/20/2015
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Early Atomic TheoryChapter 3
Democritus 460 BC- ~
370 BCNothing exists except atoms and
empty space; everything else is
opinion.
Matter is composed of small
indivisible particles, atomos –
meaning “Indivisible”
Plato and Aristotle didn’t buy it…
(play-doe Vs legos)
The AtomAristotle
Matter was continuous. Did not believe in atoms
4 elements – fire, air, earth and water
Blend all 4 of these in different proportions to get all substances
Neither theories supported by experimental evidence
Alchemy 2000 years of Chemical History
Many were scientists & made
important contributions to modern
Chemistry
Some looked to turn metals into
gold
Important contributions:
Discover of Mercury, Sulfur and
Antimony; lab procedures and
preparing acids.
The atom
Atom
The smallest unit of an element that
retains all the properties of that
element
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Foundations of Atomic Theory
Law of conservation of mass (Matter)
Mass is neither created nor
destroyed during ordinary chemical
reactions or physical changes
(Antoine Lavoisier, 1774) 8 X2Y16 X 8 Y+
2.1
Foundations of Atomic Theory
Law of Definite Proportions (Proust)
Chemical compounds contain the same elements in exactly the same proportion regardless of sample size
NaCl Sodium Chloride
39% Sodium and 61% Chlorine
Water will always contain 8g of Oxygen for every 1g of Hydrogen
Dalton’s Atomic Theory (1808)1. Elements are composed of extremely small
particles called atoms.
2. All atoms of a given element are different from
the atoms of all other elements.
3. Compounds are composed of atoms of more
than one element in fixed ratios.
4. A chemical reaction involves only the
separation, combination, or rearrangement of
atoms; it does not result in their creation or
destruction of atoms.2.1
Law of Multiple Proportions
If two or more different compounds are
composed of the same two elements, the
ratio of the masses of elements are always
a ratio of small whole numbers.
CO, CO2, CO32
2.1
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Example of Law of Multiple Proportions
Carbon combines with oxygen to form
CO and CO2 .
Mass of
Carbon(g)
Mass of
Oxygen(g)
Ratio of O
in CO2 to O
in CO
CO 12.01 16.00
CO2 12.01 32.00 2:1
Mysterious world of
the atom
Let’s probe deeper…
Electron
-properties
Model of the atom
Experiments to probe atom:
+ Cathode ray tube
+ Mulliken Oil drop
+ Rutherford scattering
Crookes Tube
1869-1875
“Something” was
making a glowing
shadow
Parts of the Atom
Discovery of Electron J. J. Thomson – English Physicist (1897)
Made a piece of equipment called a
cathode ray tube
Adaptation of Crookes tube
Cathode Ray experiment
Discovery of the electron
Experiments confirmed that electron has
negative charge
J.J. Thomson, measured mass/charge of e-
(1906 Nobel Prize in Physics) 2.2
CATHODE RAY TUBE EXPERIMENT
Thomson’s Experiment
Voltage source
+-
Vacuum tube
Metal Disks
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n Passing an electric current makes a beam
appear to move from the negative to the
positive end
Thomson’s Experiment
Voltage source
+-Voltage source
Thomson’s Experiment
n By adding an electric field he found that the
moving pieces were negative
+
-
Thomson Summary
Found the electron!
Couldn’t find positive (for a
while)
Said the atom was like plum
pudding
A bunch of positive stuff, with
the electrons able to be removed
Robert Millikan
Robert Millikan
Oil Drop Experiment
Electron mass 9.109 * 10-31 kg
Electron Charge – 1.602 * 10-19
Coulomb
Relative charge (-1)
Measured charge and mass of e- (1923 Nobel Prize in Physics)2.2
Millikan’s Oil Drop Experiment
Rutherford’s experiment
Used radioactivity
Alpha particles - positively charged pieces given off by uranium (2 protons & 2 neutrons)
Shot them at gold foil which can be made a few atoms thick
When the alpha particles hit a florescent screen, it glows.
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Lead
blockUranium
Gold Foil
Florescent
Screen
THOMSON (AND KELVIN)’S PLUM PUDDING
MODEL OF THE ATOM
He Expected
The alpha particles to pass
through without changing direction
very much
Because (he thought) the positive
charges were spread out evenly.
Alone they were not enough to
stop the alpha particles
GOLD FOIL EXPERIMENT
What he expected:
Because, he thought the mass was
evenly distributed in the atom
What he got
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How he explained it…
+
Atom is mostly empty
Small dense,
positive piece
at center
Alpha particles
are deflected by
it if they get close enough
Gold Foil experiment
+
Discovery of Atomic
Nucleus
Rutherford’s Gold foil experiment
Concluded atom had a positively
charged nucleus
Volume of nucleus small compared
to volume of atom
Most of the mass of the atom
contained in nucleus (dense)
Composition of Atomic
NucleusProton – positive charge
Number of protons in nucleus determines atoms identity – Atomic Number
Mass 1.673 * 10-27 kg
Relative Charge (+1)
Neutron – “neutral” no charge
Mass 1.675 * 10-27 kg
Summary of Subatomic particles
Electron
Proton
Neutron
Name Symbol Charge
Relative
mass
Actual
mass (g)
e-
p+
n0
-1
+1
0
1/1840
1
1
9.11 x 10-28
1.67 x 10-24
1.67 x 10-24
Structure of the atom
Atoms composed of smaller particles
Two regions
Nucleus – Protons and Neutrons
Electron Cloud
Number and arrangement of these particles determine chemical properties
Neutral – charges add up to zero
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Size of an atom
Atoms are small.
Measured in picometers, 10-12 meters
ex: Hydrogen atom, 32 pm radius
Nucleus tiny compared to atom
If the atom were the size of a stadium, the
nucleus would be the size of a marble.
Radius of nucleus ~ 10-15 m
Density near 1014 g/cm
Counting the Pieces
Atomic Number = number of protons
# of protons determines kind of atom
the same as the number of electrons in the
neutral atom
Mass Number = the number of protons +
neutrons
All the things with significant mass
Isotopes
Atoms of the same element (same # of
protons) can have different numbers of
neutrons
different mass numbers
called isotopes
Ions
Elements can also gain or lose electrons.
Each electron has a -1 charge so;
Adding an electron = -1
Removing an electron = +1
An element with a charge (gained or lost
electrons) is an ion..
Cations & Anions
Cations are ions with positive charges
Lost one or more electrons
Ca+ion
Think cats are nice (positive) but always
looking around for their electron
Anions are ions with negative charges
Gained one or more electrons
Nuclear Symbols
Contain the symbol of the element, the mass
number and the atomic number
XMass
number
Atomic
number
Charge
(ion only)
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Nuclear Symbols
Find the
number of protons
number of neutrons
number of electrons
Atomic number
Mass Number
Cation, Anion or Neither?
F19
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Nuclear Symbols
n Find the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
–Cation, Anion or Neither?
Br80
35
-1
Symbols
n If an element has an atomic number of
34 and a mass number of 78 what is
the ?
–number of protons
–number of neutrons
–number of electrons
–Complete symbol
Symbolsn if an element has 91 protons and
140 neutrons and a +2 charge
what is the ?
–Atomic number
–Mass number
–Number of electrons
–Complete symbol
Symbols
n if an element has a +1 charge, 78
electrons and 117 neutrons what is the
?
–Atomic number
–Mass number
–number of protons
–Complete symbol
Naming Isotopes
Hyphen Notation
Put the mass number after the
name of the element
carbon- 12
carbon -14
uranium-235
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Atomic Mass
How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
More concerned with average atomic mass.
Based on abundance of each isotope in nature.
Don’t use grams because the numbers would be
too small
Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
Defined as one twelfth the mass of a carbon-
12 atom.
Each isotope has its own atomic mass we
need the average from percent abundance.
Calculating averages
A student has an 81 test average, a 89 quiz
average, a 90 lab average, and 100
homework average.
Average grade is…?
81+ 89 + 90 + 100 = 360
360 =
4
90 average!
Calculating weighted
averages
Tests = 50%; Quizzes 20%; Labs 15 %; Homework 15%
Weighted average = (.5 x 81) + (.2 x 89) + (.15 x 90) + (.15 x 100) =
Weighted average = 40.5 + 17.8 + 13.5 + 15
Weighted average = 86.8 (rounds to 87)
Atomic Mass
Boron has 2 isotopes. 20% is Boron-10,
and 80% is Boron-11. What is the
average atomic mass?
(.20 X 10)+(.80 X 11) = ?
10.8 amu
Check out the periodic table!
Atomic Mass
Calculate the atomic mass of lead if 1.37% is
lead-204, 26.26% is lead-206, 20.82% is lead-
207, and 51.55% is lead 208.
207.21 amu
If not told otherwise, the mass of the isotope
is the mass number in amu