CHAPTER 4

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CHAPTER 4 THE ATOM
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John Daltons Atomic Theory (19th Century)

• 1) All matter is composed of small particles
  called atoms
• 2) All atoms of an element are identical in size,
  mass, and chemical properties
• 3) Atoms cannot be created, divided into smaller
  particles or destroyed
• 4) Different atoms combine in simple whole number
  ratios to form compounds
• 5) In chemical reactions, atoms are separated,
  combined, or rearranged

FALSE
FALSE
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Rule 4

ELEMENT B
ELEMENT A
COMPOUND of A and B
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Rule 5

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What is an atom?

• Defn smallest particle of an element that
  retains properties of the element
• How many copper atoms can fit on a penny?
• 29,000,000,000,000,000,000,000 atoms
• (29 x 1021)

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Electron

• Discovered by J.J. Thomson (1909) using a cathode
  ray tube (CRT)
• Electron negative charged particle of matter
  (-1)
• - has no relative mass

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Nucleus

• Discovered by Ernest Rutherford (1911) using Gold
  Foil Experiment
• Nucleus dense, positively charged region in
  center of atom
• http//www.mhhe.com/physsci/chemistry/essentialche
  mistry/flash/ruther14.swf

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Composition of Nucleus

• Proton positive (1) charged particle of
  nucleus
• Neutron neutral (0) charged particle of
  nucleus

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Relative Mass
Particle Symbol Location
Charge
Proton p nucleus 1
1
Neutron n0 nucleus 0
1
Electron e- around nucleus -1
0
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What makes an atom of one element different from
an atom of another element ?
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Atomic Number

• Defn number of protons in an atom
• How is periodic table arranged?
• by increasing atomic number
• How many electrons do atoms have?
• electrons protons

atomic number identifies atoms
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reading the periodic table
Element
Hydrogen
1
Atomic number
H
Symbol
1.008
Atomic mass
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Mass Number

• Defn total number of protons and neutrons
• NOT SAME AS ATOMIC MASS!!!!!!

Cannot be found on periodic table
Hydrogen
1
NO NO NO NO NO!!!!
H
1.008
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Mass Number

• Remember this formula to find neutrons
• MAN
• M A N
• (Mass ) (Atomic ) Neutrons

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Refresh!!!!

• Ive given you 3 numbers
• Atomic number
• Atomic mass
• Mass number

The ones with the word ATOMIC is on the
periodic table
Not on periodic table
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Whats in a cheeseburger?
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Isotope

• Defn atoms with same number of protons, but
  different number of neutrons

HYDROGEN - 1
HYDROGEN - 2
HYDROGEN - 3
protons
neutrons
electrons
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3 Different Hydrogen Isotopes
HYDROGEN 1 (protium)
HYDROGEN 2 (deuterium)
HYDROGEN 3 (tritium)
1 proton 0 neutrons 1 electron
1 proton 1 neutron 1 electron
1 proton 2 neutrons 1 electron
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Ways to write isotopes

• 1) Hyphen notation (2 parts)
• name of element
• OR symbol
• ex carbon-14 OR C-14
• chlorine-35 OR Cl-35
• hydrogen-3 OR H-3

MASS NUMBER
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Ways to write isotopes

• 2) Nuclear symbol (3 parts)

14 6
mass number
C
element symbol
atomic number
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Ex problem
Potassium
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Potassium-39 Potassium-40
Potassium-41
K
protons neutrons electrons Nuclear sym
bol
19
19
19
39.10
22
20
21
19
19
19
39 19
41 19
40 19
K
K
K
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Mass of individual atoms

• Atoms have very very tiny masses to work
  with (10-27 kg)
• Instead we use a relative mass atomic mass unit
  (amu)
• Atomic Mass Unit (amu) 1 amu mass of 1 proton
  or 1 neutron

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amus and subatomic particles
particle
Mass (amu)
e- p n0
0 1 1
0.000549 1.007276 1.008665
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Atomic Mass

• Defn weighted average mass of all isotopes of
  an element
• Where can it be found?
• found on periodic table

Potassium
19
K
39.10
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Atomic Mass

• Recall all the hydrogen atoms in the world

Hydrogen-1 Hydrogen-2 Hydrogen-3
Each has their own percent abundance in the world
99
0.2
0.8
Take averages of all the weights of each to find
atomic mass
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Ex problem Calculate atomic mass of chlorine.
Isotope mass (amu) percent
abundance
35Cl 35 75.770 37Cl
37 24.230
What do you expect the average to be?
About 35 amu
Actual 35.453 amu
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Ex problem Calculate atomic mass of chlorine.
Isotope mass (amu) percent
abundance
35Cl 35 75.770 37Cl
37 24.230
Mass contribution (mass)(abundance)
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Which isotope is most abundant?Ar-38, Ar-40, or
Ar-41?
Argon
Ar-40
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Ar
39.948
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Ex problem Calculate atomic mass of chlorine.
Mass contribution (mass)(abundance)
For 35Cl
(35)(0.75770)
26.5195 amu
For 37Cl
(37)(0.24230)
8.9657 amu

35.4852 amu
Matches periodic table for Cl