Download chapt 4 early atomic theory

Chapter 4
Atoms and their structure
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History of the atom
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Not the history of atom, but really the idea of the atom
The original idea - Ancient Greece (400 B.C..)
Democritus and Leucippus Greek philosophers who
were debating whether matter was continuous or
discontinuous.
Continuous – divide matter forever and always have a
smaller piece of matter.
Discontinuous – divide matter only so far and can go
no farther.
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History of Atom
Start with a box of marble divide in half
eventually you get down to one marble
which if you divide again you no longer
have a marble.
The Greek word for “can not cut” is atomos – thus atom.
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Another Greek
Aristotle – another famous Greek philosopher
All substances are made of 4 elements
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
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Aristotle and others believed in 4 elements of
matter, combined in different proportions rather
than indivisible particles
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Who Was Right?
Greek society was slave based and so it was
beneath the famous to work with their hands.
The Greeks did not experiment, they settled
disagreements by argument (debates).
 Aristotle was more famous so his ideas carried
through to the middle ages.
 During the middle ages Alchemists tried to
change lead to gold
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Re-emergence of the Atomic Theory
Late 1700’s - John Dalton- England (a major contributor
to today’s Atomic Theory)
A teacher who summarized results of his
experiments and those of other’s
In Dalton’s Atomic Theory he combined ideas of
elements with that of atoms
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Dalton’s Atomic Theory
 All matter is made of tiny indivisible particles
called atoms.
 Atoms of the same element are identical, those
of different atoms are different.
 Atoms of different elements combine in whole
number ratios to form compounds
 Chemical reactions involve the rearrangement
of atoms. No new atoms are created or
destroyed.
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Law of Definite Proportions
• Each compound has a specific whole-number
ratio of elements; ratio is by mass [Definite
Proportions]
Water
H2O
Carbon dioxide
CO2
Methane
CH4
8.0 g oxygen reacts with 1.0 g hydrogen (H2O)
Ratio = 8:1 by mass
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Law of Multiple Proportions
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if two elements form more that one compound,
the ratio of the second element that combines
with 1 gram of the first element in each is a
simple whole number.
•In hydrogen peroxide 16.0 g oxygen reacts with 1.0 g
hydrogen (H2O2)
•Ratio of the masses of oxygen in hydrogen peroxide and
water is 16:8 = 2:1 Therefore H2O2 contains twice as many
oxygen atoms per hydrogen atom than H2O [Multiple
Proportions]
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What?
Water is 8 grams of oxygen per gram of hydrogen. (H2O)
Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen.
(H2O2)
16 to 8 is a 2 to 1 ratio
Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than
H2O [Multiple Proportions]
True because you have to add a whole atom, you can’t add a
piece of an atom.
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Parts of Atoms
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As scientists began to develop methods for more
detailed probing of the nature of matter, the atom
(supposedly indivisible) began to show signs of a
more complex structure
J. J. Thomson - English physicist. 1897
 Made a piece of equipment called a cathode
ray tube which was used to study the electrical
conductivity of gasses.
 The cathode ray tube is a vacuum tube - the air
has been pumped out.
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Thomson’s Experiment
Voltage source
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Vacuum tube
Metal Disks
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Thomson’s Experiment
Voltage source
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Thomson’s Experiment
Voltage source
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Thomson’s Experiment
Voltage source
-
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Thomson’s Experiment
Voltage source
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Passing an electric current makes a beam
appear to move from the negative to the
positive end
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Thomson’s Experiment
Voltage source
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Passing an electric current makes a beam
appear to move from the negative to the
positive end
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Thomson’s Experiment
Voltage source
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+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
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Thomson’s Experiment
Voltage source
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Passing an electric current makes a beam
appear to move from the negative to the
positive end
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Thomson’s Experiment
Voltage source
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By adding an electric field
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Thomson’s Experiment
Voltage source
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 By adding an electric field
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Thomson’s Experiment
Voltage source
+
 By adding an electric field
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Thomson’s Experiment
Voltage source
+
 By adding an electric field
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Thomson’s Experiment
Voltage source
+
 By adding an electric field
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Thomson’s Experiment
Voltage source
+
 By adding an electric field
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Thomson’s Experiment
Voltage source
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 By adding an electric field he found that the
moving pieces were negative because the
rays were attracted to the positive electrode
in the external field.
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Cathode Ray Tube
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• Thompson concluded that:
» Cathode rays consist of beams of particles
» The particles have a negative charge
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Based on his findings, a new fundamental
particle of matter was discovered – The
Electron!
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Thomsom’s Model
Couldn’t find positive (for a
while)
 Said the atom was like plum
pudding or (blueberry muffin)
 A bunch of positive stuff, with
the electrons embedded
(able to remove the
embedded electrons)
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Other pieces
Proton - positively charged pieces 1840
times heavier than the electron
 Neutron - no charge but the same mass
as a proton.
 Where are the pieces?
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Rutherford’s experiment
Ernest Rutherford English physicist. (1910)
 Believed in the plum pudding model of the
atom.
 Used radioactivity
 Alpha particles - positively charged pieces
given off by uranium
 Shot them at gold foil which can be made a few
atoms thick
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Rutherford’s experiment
When the alpha particles hit a florescent
screen, it glows.
 Here’s what it looked like
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Lead
block
Uranium
Florescent
Screen
Gold Foil
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He Expected
Rutherford believed that the a particles
would pass through unhindered. (The
alpha particles to pass through without
changing direction very much.)
 Because the positive charges were
spread out evenly within the atom. The
positive charge in the atom was not
enough to stop the alpha particles
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What he expected
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Because
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Because, he thought the mass was
evenly distributed in the atom
a particles
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Because, he thought
the mass was evenly
distributed in the atom
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What he got
A small percentage of the particles were being reflected at unexpected
angles, inconsistent with the “muffin model”
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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
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Rutherford explained his observations as follows:
•Atom is mostly empty space
• Small, dense, and positive at the center
• Alpha particles were deflected if they got close enough
a particles
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Modern View
The atom is mostly empty space
Two regions
Nucleus- protons and neutrons
Electron cloud- region where you might find an electron
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Density and the Atom
Since most of the alpha particles went through,
the atom is mostly empty.
 Because the alpha particles turned so much,
the positive region of the atom is heavy.
 Small volume, big mass, big density
 This small dense positive area is the nucleus
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Subatomic particles
Relative Actual
mass (g)
Name Symbol Charge mass
Electron
e-
-1
1/1840 9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
nº
0
1
1.67 x 10-24
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Structure of the Atom
There are two regions
1. The nucleus – with protons and neutrons so
that it has a Positive charge and almost all the
mass
2. Electron cloud- Most of the volume of an atom
and is the region where the electron can be
found (extra nuclear)
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Size of an atom
Atoms are small and are measured in
picometers, 10-12 meters
 Hydrogen atom, 32 pm radius
 Nucleus tiny compared to atom. If the atom was
the size of a stadium, the nucleus would be the
size of a marble.
 Radius of the nucleus near 10-15m.
 Density near 1014 g/cm3
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Counting the Pieces
Atomic Number = number of protons (p+)
The number of protons determines kind of atom –
2 protons in the nucleus means that this is a
Helium atom.
Chemists use Z as a symbol for atomic number.
In a neutral atom there is the same number of
electrons (e-) and protons (atomic number)
Mass Number = number of protons + neutrons
[Sum of p+ and nº (p+ + nº)] The symbol used
for mass number is A.
The neucleons (p+ and nº) make up the mass of
the atom.
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Isotopes
Dalton was wrong when he said that all atoms
of one element are the same.
 Atoms of the same element can have different
numbers of neutrons and therefore have
different mass numbers and different masses.
 The atoms of the same element that differ in the
number of neutrons are called isotopes of that
element.
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Nuclear Symbols
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Contain the symbol of the element, the
mass number and the atomic number
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Nuclear Symbols
Contain:
the mass number
A
E
the symbol of
the element
the atomic number
Z
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Symbols
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Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
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F
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Symbols
 Find
the
–number of protons
–number of neutrons
–number of electrons
–Atomic number
–Mass Number
80
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Br
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Symbols
 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 nuclear symbol
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Symbols
 if
an element has 91 protons and
140 neutrons what is the
–Atomic number
–Mass number
–number of electrons
–Complete nuclear symbol
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Symbols
 if
an element has 78 electrons and
117 neutrons what is the
–Atomic number
–Mass number
–number of protons
–Complete nuclear symbol
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Naming Isotopes
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
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Measuring Atomic Mass
Unit is the Atomic Mass Unit (amu)
 One twelfth the mass of a carbon-12 atom.
 Each isotope has its own atomic mass we
need the average from percent abundance.
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Calculating averages
You have five rocks, four with a mass of 50
g, and one with a mass of 60 g. What is the
average mass of the rocks?
 Total mass =
4 x 50 + 1 x 60 = 260 g
 Average mass = 4 x 50 + 1 x 60 = 260 g
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5
 Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
5
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Calculating averages
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Average mass = 4 x 50g + 1 x 60g = 260 g = 52g
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5
5
Average mass = .8 x 50g + .2 x 60g = 52g
 80% of the rocks were 50 grams
 20% of the rocks were 60 grams
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Average = % as decimal x mass1+ % as decimal x mass2 + % as decimal x mass3 +…
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Atomic Mass
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Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a mass
of 62.93 amu and the rest (30.9%) has a
mass of 64.93 amu.
0.691 x 62.93amu + 0.309 x 64.93amu =
43.48463amu + 20.06337amu = 63.548amu
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Atomic Mass
Magnesium has three isotopes. 78.99%
magnesium 24 with a mass of 23.9850
amu, 10.00% magnesium 25 with a mass of
24.9858 amu, and the rest magnesium 26
with a mass of 25.9826 amu. What is the
atomic mass of magnesium?
 If not told otherwise, the mass of the
isotope is the mass number in amu
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Atomic Mass
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is not a whole number because it is an average
(are the decimal numbers on the periodic table).
Isotopes - atoms of the same element can have
different numbers of neutrons and therefore have
different mass numbers
When naming, write the mass number after the name of
the element
1
1
H
Protium
2
1
H
Deuterium
3
1
H
Tritium
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