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Lesson 3.3:
Distinguishing Among
Atoms
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Lesson 3.3 How Atoms Differ
• Explain the role of atomic number in determining
the identity of an atom.
• Define an isotope.
• Explain why atomic masses are not whole
numbers.
• Calculate the number of electrons, protons, and
neutrons in an atom given its mass number and
atomic number.
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Lesson 3.3 How Atoms Differ
periodic table: a chart that organizes all known elements
into a grid of horizontal rows (periods) and vertical
columns (groups or families) arranged by increasing
atomic number
•Atomic mass unit (amu)
•Atomic mass
•Atomic number
•Isotopes
•Mass number
The number of protons and the mass
number define the type of atom.
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Atomic Number
• Atoms are composed of identical
protons, neutrons, and electrons
• So….
How are atoms of one element different
from another element?
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Atomic Number
• Each element contains a unique positive charge in
their nucleus.
• The number of protons in the nucleus of an atom
identifies the element and is known as the
element’s
element
s atomic number
number.
• All atoms of a particular
element have the same
number of protons and
electrons.
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Atomic number (Z) of an element is
the number of protons in the
nucleus of each atom of that
element.
Element
# of p
protons
Atomic # ((Z))
Carbon
6
6
Phosphorus
15
15
Gold
79
79
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Mass Number
Mass number is the number of protons
and neutrons in the nucleus of an
isotope:
Mass # = (# of p+)+ (# of n0)
p+
n0
8
10
Arsenic - 75
33
Phosphorus - 31
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Nuclide
Oxygen - 18
e- Mass #
8
18
42
33
75
16
15
31
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Complete Symbols
• Contain the symbol of the element,
the mass number and the atomic
number.
Mass
Superscript 
number
Subscript 
Atomic
number
X
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Or….
Atomic #
X
M
Mass
#
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Symbols

Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
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B
Br
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Symbols

If an element has an atomic
number of 34 and a mass
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
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Symbols

If an element has 91
protons and 140 neutrons
what is the
a) Atomic number
b) Mass number
c) number of electrons
d) complete symbol
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Symbols
 If an element has 78 electrons
and 117 neutrons what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol
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Isotopes and Mass Number
• Dalton was wrong about all elements of
the same type being identical
• Atoms of the same element can have
different numbers of neutrons.
• Thus, different mass numbers.
…….. These are called isotopes.
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Isotopes and Mass Number
• The relative abundance of each isotope is
usually constant.
• Isotopes containing more neutrons have a
greater
t mass.
• Isotopes have the same chemical behavior.
• The mass number which is the sum of the
protons and neutrons in the nucleus is different
in isotopes of the same element.
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Isotopes and Mass Number
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Naming Isotopes
• We can also put the mass number after
the name of the element:
carbon-12
carbon 14
carbon-14
uranium-235
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Isotope
Hydrogen–1
Hydrogen–
(protium
protium))
Hydrogen--2
Hydrogen
(deuterium)
Hydrogen-3
Hydrogen(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
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More on Isotopes
Because element’s have more than one
isotope, we list their masses as a weighted
average of all of the isotopic masses on the
Periodic Table
Here are the various isotopes of Iron:
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Measuring Atomic Mass
• Instead of grams, the unit we use is the Atomic
Mass Unit (amu)
•
1
• It is defined as one-twelfth the mass of a
carbon-12 atom.
– Carbon-12 chosen because of its isotope purity.
– One amu is nearly, but not exactly, equal to one
proton (1.007276 amu) and one neutron
(1.008665 amu).
• Each isotope has its own atomic mass, thus we
determine the average from percent abundance.
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Calculating Average Atomic Mass
We multiple the mass of each isotope by their
(decimalified) abundance and add it all up:
This formula is NOT in your Reference
Tables!!!
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Iron Helps Us Learn!
Compute the Average Atomic Mass for Iron
from the following Data:
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A couple of notes
1. Average Atomic Mass is closest to the mass
of the most abundant isotope.
2. Don’t be surprised if your calculated result
is not quite the same as it is on the PT
- There are other isotopes that we aren’t
paying attention to.
3. Even the AMU is an average.
- Neutrons weigh slightly more than
protons
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Boron Demands Your Attentions
Boron (B) has two naturally-occurring isotopes. B-10
has a mass of 10.013 amu and B-11 with a mass of
11.009 amu. What is the percent abundance of each
one of the isotopes of boron? (Hint: AAM of boron is
found on the PT).
PT)
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Atomic Masses
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Carbon--12
Carbon
12C
Carbon--13
Carbon
13C
Carbon--14
Carbon
14C
Composition of
the nucleus
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
% in nature
98.89%
1.11%
<0.01%
Carbon = 12.011
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Learn to Love The Periodic Table
Is the Periodic Table the Greatest Model
Ever Made?
Quite Possibly!
So Much Useful Information in Something
that fits on a tee-shirt.
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So Many Answers!
How many Protons does Fe have?
26
What’s the nuclear charge?
+26
What is the mass of the protons in Fe?
26 amu
How many electrons does Fe have?
26
What’s the most common isotope?
Fe-56
How many neutrons are in the most common
isotope?
30
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A Final Note on Models
NEVER confuse models of things for reality,
particularly things that can’t be observed:
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Ask a question
or I will stare at
you tonight...in
your dreams!
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