Transcript Document 472269

1
Early Thoughts
2
• The earliest models of the atom
were developed by the ancient
Greek philosophers.
Leucippus of Miletus (490-??? B.C.).
First to introduce the idea of the atom, an
indivisible unit of matter. This idea was later
extended by his student, Democritus.
Democritus (about 470-370 B.C.)
thought that all forms of matter were
made of tiny particles called “atoms”
from the Greek “atomos” indivisible.
3
According to Democritus atoms are:
• Unchangeable and
indivisible.
• Identical except
for their size and
shape.
• Always in motion.
4
Democritus imagined that atoms of iron were shaped like
coils, making iron rigid, strong, and malleable. Atoms of
fire were sharp, lightweight, and yellow.
5
• Aristotle (384-322 B.C.) rejected the
theory of Democritus and endorsed
that of Empedocles that stated that
matter was made of 4 elements: air,
earth, fire , and water.
6
• Empedocles (492-432 B.C.)
believed that these elements
have always existed in fixed
amounts, and that there two
major forces which act upon
these elements to both
create and destroy: Love
and Strife. According to
legend, he died by falling
into a volcano's crater after
failing to become a god as
he predicted.
7
– Aristotle’s influence dominated the
thinking of scientists and
philosophers until the beginning of
the 17th century
8
9
Alchemical Symbols
antimony
copper
arsenic
gold
bismuth
iron
10
Alchemical Symbols
magnesium
mercury
platinum
potassium
phosphorus
silver
11
Alchemical Symbols
sulfur
tin
zinc
lead
12
Dalton’s Model
of the Atom
13
2000 years after Aristotle, John Dalton,
an English schoolmaster, proposed his
model of the atom–which was based on
experimentation.
14
15
Dalton’s Atomic Theory
1. Elements are composed of minute
indivisible particles called atoms.
2. Atoms of the same element are alike in
mass and size.
3. Atoms of different elements have
different masses and sizes.
Modernunder
research
has circumstances
demonstrated that
Atoms
special
can
4.
Chemical
compounds
are
formed
by
atoms
are composed of subatomic
be decomposed.
the union of two or more atoms of
particles.
different elements.
16
Dalton’s Atomic Theory
5. Atoms combine to form compounds in
simple numerical ratios, such as one to
one, two to two, two to three, and so on.
6. Atoms of two elements may combine in
different ratios to form more than one
compound.
17
Dalton’s atoms were individual particles.
Atoms of each element are alike in
mass and size.
18
Dalton’s atoms were individual particles.
Atoms of different elements are not alike
in mass and size.
19
H 2
=
O 1
H 1
=
O 1
Daltons atoms combine in specific ratios
to form compounds.
20
Composition of
Compounds
21
The Law of Definite
Composition
A compound always contains two
or more elements combined in a
definite proportion by mass.
22
Composition of Water
• Water always contains the same two
elements: hydrogen and oxygen.
• The percent by mass of hydrogen in
water is 11.2%.
• The percent by mass of oxygen in
water is 88.8%.
• Water always has these percentages. If
the percentages were different the
compound would not be water.
23
Composition of Hydrogen Peroxide
• Hydrogen peroxide always contains the same
two elements: hydrogen and oxygen.
• The percent by mass of hydrogen in hydrogen
peroxide is 5.9%.
• The percent by mass of oxygen in hydrogen
peroxide is 94.1%.
• Hydrogen peroxide always has these
percentages. If the percentages were different
the compound would not be hydrogen
peroxide.
24
The Law of Multiple Proportions
Atoms of two or more elements may
combine in different ratios to produce
more than one compound.
25
Combining Masses of Hydrogen and Oxygen
Mass
Hydrogen(g)
Mass
Oxygen(g)
Water
1.0
8.0
Hydrogen
Peroxide
1.0
16.0
Hydrogen
peroxide
has peroxide
twice as much
mass
of oxygen
in hydrogen
16g 2
=
=26 ¹
oxygen
water.
mass(by
of mass)
oxygenasindoes
water
8g 1
Combining Ratios of Hydrogen and Oxygen
• Hydrogen peroxide has twice as many
oxygens per hydrogen atom as does
water.
• The formula for water is H2O.
• The formula for hydrogen peroxide is
H2O2.
27
Discovery of Ions
31
• Michael Faraday discovered that
certain substances, when dissolved in
water, conducted an electric current.
• He found that atoms of some elements
moved to the cathode (negative
electrode) and some moved to the
anode (positive electrode).
• He concluded they were electrically
charged and called them ions (Greek
wanderer).
32
Michael Faraday
33
• Svante Arrhenius reasoned that an ion
is an atom (or a group of atoms)
carrying a positive or negative electric
charge.
• Arrhenius accounted for the electrical
conduction of molten sodium chloride
(NaCl) by proposing that melted NaCl
dissociated into the charged ions Na+
and Cl-.
Δ
NaCl → Na+ + Cl34
NaCl → Na+ + Cl• In the melt the positive Na+ ions moved
to the cathode (negative electrode).
Thus positive ions are called cations.
• In the melt the negative Cl- ions moved
to the anode (positive electrode). Thus
negative ions are called anions.
35
Svante Arrhenius
36
Subatomic Parts
of the Atom
37
An atom is very Small
38
This
The diameter
is 1 to 5often
anbillionths
atom is 0.1oftoa
meter.
0.5 nm.
If
Even
the diameter
smaller particles
of this dot
thanis atoms
1 mm
exist.
then 10
These
million
are hydrogen
called subatomic
atoms
particles.
would form a line across the dot.
39
Subatomic Particles
40
Electron
41
In 1875 Sir William Crookes
invented the Crookes tube.
42
• Crookes tubes experiments led the way
to an understanding of the subatomic
structure of the atom.
43
• Crookes tube emissions are called cathode
rays.
• Below are Crookes cathode-ray tubes. The
cathode-rays (streams of electrons) can be
clearly seen.
44
"Maltese Cross" Crookes Tube
Demonstrates that radiant matter is
blocked by metal objects
45
Other Interesting Crookes Tubes
May Be Found At the Sites Below:
• http://www.sparkmuseum.com/GLASS.HTM
• http://www.oneillselectronicmuseum.com/page9.html
46
In 1897 Sir Joseph Thomson demonstrated
that cathode rays:
• travel in straight lines.
• are negative in charge.
• are deflected by electric
and magnetic fields.
• produce sharp shadows
• are capable of moving a
small paddle wheel.
47
Paddle Wheel
48
Thomson’s Apparatus
batteries
49
Thomson’s Lab
50
J.J. Thomson determined and is given credit
for finding:
• The charge to mass
(e/m) ratio of the
cathode ray.
• The cathode ray was renamed the “electron”.
• Thomson “discovered”
the electron.
http://www.aip.org/history/mod/fission/fission1/01.html
51
Can atoms be split apart? Does each atom have inner workings? Parts which
can be separated? Parts which can perhaps be put to some use? These
questions had already come to mind in 1898, when J. J. Thomson isolated the
electron. That was the first solid proof that atoms are indeed built of much tinier
pieces. Thomson speaks of the electron in this recorded passage...
Could anything at first sight seem more impractical than a body which
is so small that its mass is an insignificant fraction of the mass of an
atom of hydrogen, which itself is so small that a crowd of these atoms
equal in number to the population of the whole world would be too
small to have been detected by any means then known to science.
52
Robert Millikan
• Determined the
charge of the
electron.
• Experiment called
the Oil Drop
Experiment.
53
Oil Drop Apparatus
54
Apparatus Used by Millikan
55
Modern Apparatus
56
Proton
57
• Eugen Goldstein, a German physicist,
first observed protons in 1886:
• Thomson determined the
protons’ characteristics.
• Thomson showed that atoms
contained both positive and
negative charges.
• This disproved the Dalton
model of the atom which held
that atoms were indivisible.
58
Thomson’s Plum-Pudding Model of
the Atom
59
Neutron
60
• James Chadwick discovered the neutron
in 1932.
• Its actual mass is
slightly greater than
the mass of a
proton.
61
62
Ions
63
• Positive ions were explained by
assuming that a neutral atom loses
electrons.
• Negative ions were explained by
assuming that extra electrons can be
added to atoms.
64
When one or more electrons are lost
from an atom, a cation is formed.
65
When one or more electrons are added
to a neutral atom, an anion is formed.
66
The Nuclear Atom
67
X-rays were discovered by Wilhelm
Röentgen in 1895
68
• Röentgen observed that a vacuum
discharge tube enclosed in a thin,
black cardboard box had caused a
nearby piece of paper coated with
the salt barium platinocyanide to
phosphorescence.
• From this and other experiments he
concluded that certain rays, which he
called X-rays, were emitted from the
discharge tube, penetrated the box, and
caused the salt to glow.
69
• Radioactivity was discovered by Henri
Becquerel in 1896.
70
• Shortly after Röentgen’s discovery,
Antoine Henri Becquerel attempted to
show a relationship between X-rays
and the phosphorescence of uranium
salts.
• Becquerel wrapped a photographic
plate in black paper, sprinkled a sample
of a uranium salt on it, and exposed it
to sunlight.
71
• When Becquerel attempted to repeat
the experiment the sunlight was
intermittent.
• He took the photographic plate
wrapped in black paper with the
uranium sample on it, and placed the
whole setup in a drawer.
72
• Several days later he developed the
film and was amazed to find an intense
image of the uranium salt on the plate.
• He repeated the experiment in total
darkness with the same result.
• This proved that the uranium salt
emitted rays that affected the
photographic plate, and that these rays
were not a result of phosphorescence
due to exposure to sunlight.
73
• Two years later, in 1898, Marie Curie
coined the name radioactivity.
Radioactivity is the spontaneous emission of
particles and/or rays from the nucleus of an
atom.
74
Marie Curie, in a classic experiment, proved
that alpha and beta particles are oppositely
charged.
three types of radiation are
Beta rays
areare
strongly
deflected
to
radiation
passes
between
Gamma
rays
not
detected
by deflected
a the
photographic
thethe
positive
pole.
poles
of
an
electromagnet
by
magnet.
plate
Alpha rays are less strongly
radioactive source
deflected to the negativeapole.
was placed inside a
lead block
75
The Rutherford Experiment
76
Ernest Rutherford
77
• In 1899 Rutherford began to investigate
the nature of the rays emitted by uranium.
• He found two particles in the rays. He
called them alpha and beta particles.
78
• Rutherford in 1911 performed experiments
that shot a stream of alpha particles at a
gold foil.
• Most of the alpha particles passed through
the foil with little or no deflection.
• He found that a few were deflected at large
angles and some alpha particles even
bounced back.
79
Rutherford’s alpha particle scattering experiment.
80
• An electron with a mass of 1/1837 amu
could not have deflected an alpha
particle with a mass of 4 amu.
• Rutherford knew that like charges
repel.
• Rutherford concluded that each gold
atom contained a positively charged
mass that occupied a tiny volume. He
called this mass the nucleus.
81
• If a positive alpha particle approached
close enough to the positive mass it
was deflected.
• Most of the alpha particles passed
through the gold foil.
This led
Rutherford to conclude that a gold
atom was mostly empty space.
82
• Because alpha particles have relatively
high masses, the extent of the
reflections led Rutherford to conclude
that the nucleus was very heavy and
dense.
83
Deflection
Scattering
Deflection and scattering of alpha particles by positive gold nuclei.
84
Ideas about the atom were refined by one of Thomson's students,
Ernest Rutherford. He showed that the mass in an atom is not
smeared out uniformly throughout the atom, but is concentrated in a
tiny, inner kernel: the nucleus. Rutherford wanted to understand the
nucleus, not for any practical purpose, but because he was attracted
to the beauty of its simplicity. Fundamental things should be simple
not complex. Here is how he explains himself in 1931...
The bother is that a nucleus, as you know, is a very small thing,
and we know very little about it. Now, I had the opinion for a long
time, that's a personal conviction, that if we knew more about the
nucleus, we'd find it was a much simpler thing than we suppose,
that these fundamental things I think have got to be fairly simple.
But it's the non-fundamental things that are very complex usually. I
am always a believer in simplicity being a simple person myself.
85
• The gamma ray, a third type of emission from
radioactive material, was discovered by Paul
Villard in 1900.
86
Alpha, Beta, and Gamma Radiation
Name
Alpha
Particle
Symbol
Mass
(amu)
Charge

4
+2
e

1
1837
–1


0
0
Nuclide
Symbol
4
2
He
Beta
0
-1
Gamma Ray
0
0
87
General
Arrangement of
Subatomic Particles
88
• Rutherford’s experiment showed that an
atom had a dense, positively charged
nucleus.
• Chadwick’s work in 1932 demonstrated
the atom contains neutrons.
• Rutherford also noted that light,
negatively charged electrons were
present in an atom and offset the positive
nuclear charge.
89
• Rutherford put forward a model of the
atom in which a dense, positively
charged nucleus is located at the
atom’s center.
• The negative electrons surround the
nucleus.
• The nucleus contains protons and
neutrons
90
91
Atomic Numbers of
the Elements
92
• The atomic number of an element is
equal to the number of protons in the
nucleus of that element.
• The atomic number of an atom
determines which element the atom is.
93
Every atom with an atomic
number of 1 is a hydrogen atom.
Every hydrogen atom contains 1
proton in its nucleus.
94
Every atom with an atomic
number of 6 is a carbon atom.
Every carbon atom contains 6
protons in its nucleus.
95
atomic
number
Every atom with an
atomic number of 1
is a hydrogen atom.
1 proton in the
nucleus
96
atomic
number
Every atom with an
atomic number of 6
is a carbon atom.
6 protons in the
nucleus
97
atomic
number
Every atom with an
atomic number of
92 is a uranium
atom.
92 protons
in the
nucleus
98
Isotopes of the
Elements
99
• Atoms of the same element can have
different masses.
• They always have the same number of
protons, but they can have different
numbers of neutrons in their nuclei.
• The difference in the number of neutrons
accounts for the difference in mass.
• These are isotopes of the same element.
100
Isotopes of the Same
Element Have
Equal numbers of protons
Different numbers of
neutrons
101
Isotopic Notation
Mass number is also the number
of nucleons in the nucleus.
Nucleons = protons and/or neutrons
102
Relationship Between Mass
Number and Atomic Number
103
The mass number minus the atomic
number equals the number of neutrons in
the nucleus.
mass
number
atomic
number
109
47
Ag
atomic
mass number number
109
47
=
=
number of
neutrons
62
104
Isotopic Notation
8 protons + 8 neutrons
16
O
8
8 protons
107
Isotopic Notation
8 protons + 9 neutrons
17
O
8
8 protons
108
Isotopic Notation
8 protons + 10 neutrons
18
O
8
8 protons
109
Hydrogen has three isotopes
1 proton
1 proton
1 proton
0 neutrons
1 neutron
2 neutrons
110
Examples of Isotopes
Element Protons
Electrons Neutrons Symbol
Hydrogen
Hydrogen
Hydrogen
1
1
1
1
1
1
0
1
2
1
1
2
1
3
1
Uranium
Uranium
92
92
92
92
143
146
235
92
U
238
92
U
Chlorine
Chlorine
17
17
17
17
18
20
H
H
H
35
17
37
111
17
Cl
Cl
Atomic Weight
112
• The mass of a single atom is too small to
measure on a balance.
• Using a mass spectrometer, the mass of
the hydrogen atom was determined.
113
A Modern Mass Spectrometer
Positive ions
formed from
sample.
Electrical field
From the intensity and positions
at slits
A mass
of the lines Deflection
on the mass
of
accelerates
spectrogram
spectrogram,
the different
ions
positive
ions. positive
is recorded.
isotopes and
their at
relative
occurs
amounts can
be determined.
magnetic
field.
114
A typical reading from a mass spectrometer. The two
principal isotopes of copper are shown with the
115
abundance (%) given.
Using a mass spectrometer, the mass of one
hydrogen atom was determined to be 1.673
x 10-24 g
116
This number is very small.
small
small
small
small
small
small
small
small
small
small
small
small
small
small
small
small
small
small
small
117
The mass of a hydrogen atom is
very small.
Numbers
of this
sizeproblem
are too small
for of
To overcome
this
a system
practical
use.
relative atomic
weights using “atomic
mass units” was devised to express the
masses of elements using simple
numbers.
1.673 x 10-24 g
118
The standard to which the masses of all
other atoms are compared to was chosen
to be the most abundant isotope of
carbon.
12
6
C
119
A mass of exactly 12 atomic mass units
(amu) was assigned to
12
6
C
120
1
1 amu is defined as exactly equal to
12
the mass of a carbon-12 atom
1 amu = 1.6606 x 10-24 g
12
6
C
121
Average atomic weight 1.00797 amu.
H
122
Average atomic weight 39.098 amu.
K
123
Average atomic weight 248.029 amu.
U
124
Average Relative
Atomic Weight
125
• Most elements occur as mixtures of
isotopes.
• Isotopes of the same element have
different masses.
• The listed atomic mass of an element is
the average relative mass of the isotopes
of that element compared to the mass of
carbon-12 (exactly 12.0000…amu)
126
To calculate the atomic mass multiply the
atomic mass of each isotope by its percent
abundance and add the results.
Isotope
63
29
Cu
65
29
Cu
Isotopic mass
(amu)
Abundance
(%)
62.9298
69.09
64.9278
30.91
Average
atomic mass
(amu)
63.55
(62.9298 amu) 0.6909 = 43.48 amu
(64.9278 amu) 0.3091 = 20.07 amu
63.55 amu
127
Isotope Practice
(Fill-in the Blanks)
symbol
O 2
16
8
Pt
195
Pt
78
P
30 3
30 –3
15 P
atomic no
mass no
#e
#n
8
16
10
8
78
195
78
30
18
53
15
117
15
#p
8
78
15
I
53
127
Kr
36
84
36
48
36
Se
34
79
34
45
34
20
20
127
53
84
36
79
34
Ca
40
2
40Ca+2
20
20
40
18
74
53
128
129