Download Honors Chemistry Unit 02

Index
Honors Chemistry Unit 02
Atoms and Elements
Based on the PowerPoints by Kevin Boudreux.
Honors Unit 02 Lesson 01
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Index
Index
• Atomos
• Law of Conservation of
Mass
• Law of Definite Proportions
• The Law of Multiple
Proportions
• Dalton’s Atomic Theory
• Brownian Motion
• The Electron (e-)
• Cathode Ray Tubes
• Millikan’s Oil Drop
Experiment
• So Where’s the Positive
Charge?
• The Discovery of the
Nucleus
• Rutherford’s Gold Foil
Experiment
• The Nuclear Atom Model
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Index
Lesson 1:
THE ROAD TO THE ATOMIC THEORY
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Index
Atomos
• The ancient Greek philosopher Democritus (c. 460 – 370
BC) reasoned that if you cut a lump of matter into smaller
and smaller pieces, you would eventually cut it down to a
particle which could not be subdivided any further. He
called these particles atoms (from the Greek atomos,
“uncuttable”)
• Aristotle (384-322 BC) believed that matter was continuous,
and elaborated the idea that everything was composed for
four elementary substances, assembled in varying
proportions – earth, air, fire, and water, which possessed
four properties – hot, dry, wet, and cold.
• The idea of atoms did not surface again until the 17th and
18th centuries.
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Law of Conservation of Mass
• In 1661, Robert Boyle redefined an elment as
a substance that cannot be chemically broken
down further.
• Law of Conservation of Mass – Mass is
neither created nor destroyed in chemical
reactions (i.e. the total mass of a system does
not change during a reaction).
Massreactants = Massproducts
#Atomsreactant= #Atomsproduct
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Conservation of mass demo
(Alka-Seltzer)
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Balancing Reactions by Mass

CaCO3( s ) 
 CaO( s )  CO2( g )
If 25g of calcium carbonate (CaCO3) is heated to
produce 14 g of calcium oxide (CaO), what is the mass
of the carbon dioxide (CO2) produced?
Index
Balancing Reactions by Mass
Guided Practice
Na + H2O  NaO + H2
If 23 g of sodium reacts with 18 g of
water to produce 39 g of sodium oxide
how much hydrogen gas is produced?
Index
Balancing Reactions by Mass
Guided Practice
6CO2 + 6H2O  C6H12O6 + 6O2
Photosynthesis
If 30. g of glucose and 32 g of oxygen
are produced from 44 g of carbon
dioxide, how much water is needed?
Index
Law of Definite Proportions
• Law of Definite Proportions – All samples of a
pure chemical substance, regardless of their
source or how they were prepared, have the same
proportions by mass of their constituent elements.
(Joseph Proust, 1754-1826)
– Calcium carbonate, which is found in coral, marble,
the white cliffs of Dover, seashells, chalk, limestone,
and Coleman tap water, is always 40.04% by mass
calcium, 12.00% carbon, and 47.96% oxygen. (We
now know that this results from the fact that calcium
carbonate is CaCO3.)
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Limestone
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Oolitic limestone
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Marble
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The Law of Multiple Proportions
• Law of Multiple Proportions – Elements can
combine in different ways to form different
substances, whose mass ratios are small
whole-number multiples of each other. (John
Dalton, 1804)
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The Law of Multiple Proportions
Compound
Sample Mass of
Size
Sulfur
Mass of
Oxygen
Sulfur oxide I
2.00 g
1.00 g
1.00 g
Sulfur oxide II
2.50 g
1.00 g
1.50 g
mass of oxygen in sulfur oxide II per gram of sulfur 1.50g 3
=
=
mass of oxygen in sulfur oxide I per gram of sulfur 1.00g 2
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Index
Dalton’s Atomic Theory
• John Dalton (1766-1844) explained these observations
in 1808 by proposing the atomic theory:
– Each element consists of tiny indivisible (not quite)
particles called atoms.
– All atoms of the same element have the same mass (not
quite), but atoms of different elements have different
masses.
– Atoms combine in simple, whole-number ratios to form
compounds.
– Atoms of one element cannot change into atoms of another
element (not quite). In a chemical reaction, atoms change
the way they are bound together with other atoms to form a
new substance.
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Index
Dalton’s Atomic Theory
• Dalton’s atomic hypothesis had an uphill
struggle – many scientists didn’t like the idea
of using small, invisible entities to explain
phenomena.
• Most (but not all) chemists had accepted the
existence of atoms by the early 20th century;
however, many influential physicists did not
accept the atomic theory until Einstein’s
landmark paper on Brownian motion (1905).
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Index
Dalton’s Atomic Theory
• Dalton’s original formulation of atoms as
miniature billiard balls, however, could not
explain how atoms combined to form
compounds, or anything about their internal
structure. The theory was modified greatly
once charge particles coming form inside the
atom were discovered in the late 19th and early
20th century.
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Brownian Motion
Source: http://protonsforbreakfast.wordpress.com/2012/04/11/brownian-motion-observedin-milk/
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The Electron (e-)
• In 1897, J.J. Thomson (1856-1940) discovered that cathode
rays, which are produced by passing an electric current
through two electrodes within a vacuum tube, were
composed of negatively charge particles, which have a very
low mass. These particles were called electrons.
• His experiments showed that electrons were emitted by
many types of metals, so electrons must be present in most
if not all samples of matter.
• Although Thomson was unable to measure the mass of the
electron directly, he was able to determine that the chargeto-mass ration, e/m, was -1.758820x108 C/g. This meant
that the electron was about 2000 times lighter than
hydrogen, and that atoms were not the smallest unit of
matter.
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Cathode Ray Tubes
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High voltage (+)
High voltage (-)
Cathode
Anode
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This is how J.J. Thomson discovered that the electron was negatively charged.
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High Voltage (+) on
N
End View
High Voltage (-) on
Cathode
Anode
S
This is how J.J. Thomson discovered that the electron had mass.
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Index
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Source: http://www.youtube.com/watch?v=0pWBXcnP5Po
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_
N
S
+
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Inversely Prop
to the mass
2.37
North
South
_
1.96
Prop to the charge
+
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Plate Charge
(Conversion Factors)
Magnetic Field
Charge of Electron e
=
=
Mass of Electron
m
e
8 coulombs
8 c
= -1.76 x10 gram = -1.76 x10 g
m
Cathode Tube Demo
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The Mass of the Electron
• In 1909, Robert Millikan (1868-1953) measured
the charge on the electron by observing the
movement of tiny ionized droplets of oil passing
between two electrically charge plates. Knowing
the e/m ratio from Thomson’s work, the mass of
the electron could then be determined:
Charge of an electron:
e = - 1.60218 x 10-19 C
Charge to Mass Ratio:
e/m = -1.758820 x 108 C/g
Mass of an electron:
me = 9.1093897 x 10-28 g
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Index
Millikan’s Oil Drop Experiment
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Millikan’s Oil Drop Experiment
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Index
Source:http://www.youtube.com/watch?v=XMfYHag7Liw
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So Where’s the Positive Charge?
• If there is a negatively charged thing somewhere inside an
electrically neutral atom, then there must also be
positively charged portion.
• The model for the atom that Thomson proposed was of a
diffuse, positively charged lump of matter with electrons
embedded in it like “raisins in a plum pudding” (seeds in
a watermelon, blueberries in a muffin, or chocolate chips
in a cookie might be more familiar analogies)
J.J. Thomson’s Plum Pudding Model of the Atom
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Index
The Discovery of the Nucleus
• In 1910, Ernest Rutherford tested the “plum pudding”
model of the atom by firing a stream of alpha (α) particles at
a thin sheet of gold foil (about 2000 atoms thick).
• Alpha particles are a type of radiation given off by many
naturally-occurring radioactive elements (Ra, Rn, Po, U,
etc.). They are 7000 times more massive than electrons and
have a positive charge that is twice the magnitude of the
electron’s charge.
• It the “plum-pudding” model were correct, the mass of the
atom would be spread out evenly through the entire volume
of the atom, and all the alpha particles should have sailed
right through the foil – but that’s not what happened…
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The Discovery of the Nucleus
• …instead, while most of the alpha-particles
sailed through the gold foil, some were
deflected at large angles, as if they had hit
something massive, and some even bounced
back toward the emitter.
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The Discovery of the Nucleus
Source:http://www.youtube.com/watch?v=5pZj0u_XMbc
Other Resources:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
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Index
Rutherford’s Gold Foil Experiment
Source:http://www.youtube.com/watch?v=XBqHkraf8iE
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The Nuclear Atom Model
• Rutherford concluded that all of the positive charge and
almost all of the mass (≈99.9%) of the atom was
concentrated in the center, called the nucleus. Most of
the volume of the atom was empty space, through
which the electrons were dispersed in some fashion.
• The positively charged particles within the nucleus are
called protons; there must be one electron for each
proton for an atom to be electrically neutral.
• This did not account for all of the mass of the atom, or
the existence of isotopes (more later); the inventory of
subatomic particles was “completed” (for the moment)
by James Chadwick in 1932, who discovered the
neutron, and uncharged particle with about the same
mass as the proton, which also resides in the nucleus.
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