Ch. 3: Atoms
Sec. 1: From Philosophical Idea to Scientific Theory
History of the atom
• Greeks were first to try to explain
chemical reactions
• 400 BC: thought all matter composed of:
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Fire
Earth
Water
Air
• Democritus first used word “atomos”,
meaning indivisible
Nothing exists but atoms and empty space;
everything else is opinion. (Demokritos)
Next 2000 years
• Alchemy gave basis for modern
chemistry
• Two ideas
– Turning cheap metals into gold and
silver
– Finding the “elixir of life”
• Did identify elements and learn how
to prepare acids
• Very secretive
Robert Boyle (1627-1691)
• Defined element: any substance that
cannot be broken down into simpler
substances
• Felt science supported religion
• End of four element theory
Robert Boyle (1627-1691)
(Not necessary to copy what is in italics)
“I now mean by elements, as these chymists
that speak plainest do by their principles,
certain primitive and simple or perfectly
unmingled bodies, which not being made of
any other bodies, or of one another, are the
ingredients of which all those called perfectly
mixt bodies are immediately compounded,
and into which they are ultimately resolved.”
Antoine Lavoisier
• Father of chemistry
• Lost his head during the French
Revolution
• Devised metric system in 1771
• Responsible for naming and listing
elements of the day
• Lavoisier’s
wife played a
big role in his
work,
unusual for
the times.
The Elements
• 110 known elements
• 88 (?) occur naturally
The elements form a plethora of
compounds, just as 26 letters of the
alphabet make a seemingly endless
number of words.
Elements in Earth’s Crust,
Oceans and Atmosphere*
•These eight elements account for
approximately 98.5% of the total
mass of the earth's crust.
oxygen (46.6%)
silicon (27.7%)
aluminum (8.1%)
iron (5.0%)
*Know the top two.
calcium (3.6%)
sodium (2.8%)
potassium (2.6%)
magnesium (2.1%)
Elements in the Human Body*
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Oxygen (65%)
Carbon (18%)
Hydrogen (10%)
Nitrogen (3%)
Calcium (1.5%)
Phosphorus (1.0%)
Potassium (0.35%)
Sulfur (0.25%)
Sodium (0.15%)
Magnesium (0.05%)
Copper, Zinc, Selenium, Molybdenum, Fluorine,
Chlorine, Iodine, Manganese, Cobalt, Iron (0.70%)
Lithium, Strontium, Aluminum, Silicon, Lead,
Vanadium, Arsenic, Bromine (trace amounts)
*Know the top four.
Symbols for the Elements
• Sources of element names come
from the Greek, Latin & German
languages
• Symbols simplify writing by using
abbreviations
– 1st letter always capitalized (C, F)
– 2nd letter always small case (Zn, Na)
Symbols from Latin Names
Element
Copper
Gold
Lead
Mercury
Potassium
Silver
Sodium
Tin
Symbol
Cu
Au
Pb
Hg
K
Ag
Na
Sn
Latin name
cuprum
aurum
plumbum
hydrargyrum
kalium
argentum
natrium
stannum
Symbols and Compounds
• Chemical formulas represent
compounds
– Symbols indicate the elements present
– Subscripts show the relative numbers
of atoms of each element
• KCl: potassium and chlorine, 1 of
each
• CCl4: One atom of carbon and 4
atoms of chlorine
• C6H12O6
• (NH4)2SO4
Atomic Theory
States that all matter is composed
of discrete units called atoms
Democritus
• Matter consists of basic particles
that cannot be divided (atoms)
• “Atomos” means uncut or indivisible
18th century Observations
• Natural materials are mixtures of
pure substances
• Pure substances can be either
elements or compounds (Water was
thought to be an element until
electrical current was used to
separate the hydrogen and oxygen
• Law of Conservation of Mass
– Mass is neither created nor destroyed
in a chemical or physical reaction
18th century Observations,
cont.
• Law of Constant Composition
(Proust)
– A given compound always has the
same composition, regardless of where
it comes from
• Law of Definite Proportions
– When elements combine to form
chemical compounds, they unite in
definite proportions by mass
• In 18 grams of water, there are 2 g
of hydrogen and 16 g of oxygen
• In 72 g of water, there are 8 g of
hydrogen and 64 g of oxygen
• Ratio of hydrogen to oxygen is
always 1:8
John Dalton (1766–1844)
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Englishman
Was a school teacher at age 12
Extremely interested in meteorology
Collected air samples all over Europe
Made daily weather observations for 57
years (Jeff Lyons would have loved him!)
• Wondered about the composition of the
air, which led to his atomic theory
Dalton’s Atomic Theory
(1808-1810)
1. All elements are made of tiny particles
called atoms
2. All atoms of an element are identical
3. All atoms of a given element are
different from those of any other
element
4. Atoms of 1 element can combine
with atoms of other elements to
form compounds.
5. Atoms are indivisible, and are
neither created or destroyed.
Reactions simply change the way
atoms are grouped together.
6. A given compound contains the
same relative numbers and types
of atoms
• We might as well attempt to
introduce a new planet into the solar
system, or to annihilate one already
in existence, as to create or destroy
a particle of hydrogen. John Dalton,
A New System of Chemical
Philosophy, 1808)
Dalton’s
Symbols
for the
Elements
• John Dalton was the first person to use ball-andstick models to represent molecules. He asked
his friend Peter Ewart, a Manchester engineer, to
make a series of them for him in about 1810.
These wooden balls were made to demonstrate
Dalton's theory of how atoms in solids, liquids
and gases are arranged. He used these models
in his lectures and for other demonstrations, but
failed to publish details of them until 1842, two
years before his death.
12 g C
16g O
12 g C
32 g O
For the same amount of carbon, the ratio of oxygen in
the 2 compounds is 1:2.
Dalton's Model
• Not accepted at first
• Law of Multiple Proportions
– Relates to two compounds made by
the same elements
– Carbon dioxide has twice the mass of
oxygen as carbon monoxide
Changes to Dalton’s Theory
• Took until early 1900’s
– Atom was divisible (nuclear physics)
– Isotopes existed
• We still use his model (with some
revisions) today
Modern Atomic Theory
Changes to Dalton’s theory:
• Atoms of an element are nearly
identical, but may have different
masses
• Atoms are not indivisible, but are
composed of subatomic particles
• Matter is neither created nor
destroyed in ordinary chemical
reactions
Pop Quiz
1. What are the 2 most abundant
elements in the human body?
2. Write 3 postulates of Dalton’s
atomic theory.
3. Describe the Law of Multiple
Proportions
Pop Quiz
1. Oxygen and carbon
2.
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3.
Elements are made of tiny particles called atoms
All atoms of an element are identical
All atoms of a given element are different from those of any other
element
Atoms of 1 element can combine with atoms of other elements to form
compounds.
Atoms are indivisible, and are neither created or destroyed.
Reactions simply change the way atoms are grouped together.
A given compound contains the same relative numbers and types of
atoms
Sec. 2: The Structure of the Atom
• J. J. Thomson (1897)
– Discovered the electron
– Plum Pudding Model
• Ernest Rutherford
– Showed existence of a nucleus
How J. J. Thomson used properties of
cathode rays to hypothesize properties of
the electron.
observation
ray properties are independent of the
cathode material
hypothesis
cathode ray stuff is a component of all
materials
How J. J. Thomson used properties of
cathode rays to hypothesize
properties of the electron.
observation
cathode rays bend near magnets
hypothesis
magnets bend the paths of moving charged
particles; maybe cathode rays are streams
of moving charged particles
How J. J. Thomson used properties of
cathode rays to hypothesize
properties of the electron.
observation
rays bend towards a positively charged
plate.
rays impart a negative charge to objects
they strike.
hypothesis
cathode rays are streams of negative
charges
How J. J. Thomson used properties of
cathode rays to hypothesize
properties of the electron.
observation
Cathode rays don't bend around small
obstacles,
cast sharp shadows,
can turn paddlewheels placed in their path,
and travel in straight lines
hypothesis
cathode rays behave like streams of particles
(subatomic particles means smaller than atoms)
Thomson’s Conclusions
• Cathode rays are negatively charged
subatomic particles
• Later named electrons
• Also determine the ratio of the
charge of electron to its mass to be
approx. 1 x 107 (BIG number!)
Plum Pudding Model
• Atom has a spherical cloud of
positive charge (pudding) with
electrons (plums) scattered within it
that make the atom neutral
Robert Millikan
• Discovered the charge of an electron
• From that information and the
charge/mass ratio, the mass of an
electron was calculated to be about
1/2000 the mass of a hydrogen atom
Ernest Rutherford's
scattering experiment
hypothesis:
• If the plum pudding model of the
atom is correct, atoms have no
concentration of mass or charge
(atoms are 'soft' targets)
Experiment to test
hypothesis:
– fire massive alpha particles at the
atoms in thin metal foil
– alpha particles should pass like bullets
straight through soft plum pudding
atoms
observation:
a few alpha particles ricocheted!
http://micro.magnet.fsu.edu/electromag/java
/rutherford/
http://www.mhhe.com/physsci/chemistry/es
sentialchemistry/flash/ruther14.swf
New hypotheses:
– all of the positive charge and nearly all of the
mass of the atom is concentrated in a tiny,
incredibly dense 'nucleus', about 10-14 m in
diameter
– electrons roam empty space about 10-10 m
across, around the nucleus
Nucleus (Protons
& Neutrons)
Orbital (Space
Occupied by
Electrons)
Proton
Neutron
If nucleus = basketball
The circle = outside edge of atom
Discovery of the neutron
• James Chadwick (1932)
• Helped explain the discrepancy
between the charge and mass of an
atom
Subatomic Particle
Comparison
Subatomic
Particle
Relative
charge
Relative
mass
Location
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Electron
-1
.0005
Orbital
Nuclear Forces
• Like charges repel, but there can be
many positive protons in a nucleus
• Held by very strong but very short-ranged
attractions
• Forces are proton-proton, proton-neutron,
and neutron-neutron
Sizes of Atoms
• Atomic radii range from 40-270 pm
o 1pm = 10-12m (Compare to size of 1
cm compared to 1000 km, or 600 mi.
• Nuclear radii are about 0.001 pm
and have a density of 2x108 metric
tons/cm3
Counting Atoms
Atomic Number
• Counts the number of protons
• in an atom
• Protons determine the element’s
identity
Atomic Number on the
Periodic Table
Atomic Number
Symbol
11
Na
All atoms of an element have
the same number of protons
11 protons
11
Sodium
Na
Learning Check AT 1
State the number of protons for atoms of
each of the following:
A. Nitrogen
1) 5 protons
2) 7 protons 3) 14 protons
B. Sulfur
1) 32 protons
2) 16 protons
3) 6 protons
C. Barium
1) 137 protons 2) 81 protons
3) 56 protons
Solution AT 1
State the number of protons for atoms of each
of the following:
A. Nitrogen
2) 7 protons
B. Sulfur
2) 16 protons
C. Barium
3) 56 protons
Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 In a neutral atom, the atomic number gives us
the number of electrons as well
 It is the electrons that determine the chemical
properties of an atom.
Isotopes
• Isotopes are atoms of the same element that
have different masses.
• The isotopes of a particular element all have the
same number of protons and electrons but
different numbers of neutrons.
• Most of the elements consist of mixtures of
isotopes.
Mass Number
Counts the number of protons
and neutrons in an atom
Designating isotopes
• Hyphen notation: The mass number
is written with a hyphen after the
name of the element.
• uranium-235
• Nuclear symbol: The superscript
indicates the mass number and the
subscript indicates the atomic
number.
235
92
U
Isotope Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na
atomic number
11
A
Z
X
sodium-23
More Atomic Symbols
16
O
31
P
65
8
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
8 p+
8n
8 e-
Zn
Learning Check AT 2
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
13C
14C
6
6
6
#P _______
_______
_______
#N _______
_______
_______
#E _______
_______
_______
Solution AT 2
12C
6
13C
14C
6
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
Learning Check AT 3
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) 65
3) 67
Solution AT 3
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
Learning Check AT 4
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution AT 4
16O
A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17e-
8
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Learning Check AT 5
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1)
34X
16
2)
34X
14
3)
36X
14
Solution AT 5
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
C. The element is
1) Si
D. Another isotope of this element would be
3) 36X
14
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