The Atom
Atomic Number and Mass Number
Isotopes
(Take out your Periodic Tables)
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EQ’s
1. What is the Atomic Theory?
2. What is an atom and how are its
components arranged?
3. What is an isotope and how are neutrons
related to isotopes?
4. How are isotopes and an element’s average
atomic mass related?
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Atomic Theory
 Atoms are building blocks of elements
 Each element has similar atoms
 Different from atoms of other elements
 Two or more different atoms bond in simple
ratios to form compounds
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Subatomic Particles
Particle
Symbol
Charge
Relative
Mass
Electron
e-
1-
0
Proton
p+
1+
1
Neutron
n
0
1
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Location of Subatomic Particles
10-13 cm
electrons
protons
nucleus
10-8 cm
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neutrons
5
Atomic Number
Represents the number
of
protons
in an atom
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Periodic Table
 Represents physical and chemical
behavior of elements
 Arranges elements by increasing atomic
number
 Repeats similar properties in columns
known as chemical families or groups
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Periodic Table
1
2
3
4
5
6
7
8
11
Na
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Atomic Number on the
Periodic Table
Atomic Number
Symbol
11
Na
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All atoms of an element have
the same number of protons
11 protons
Sodium
11
Na
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Learning Check AT 1
State the number of protons for atoms of
each of the following:
A. Nitrogen (N)
1) 5 protons
2) 7 protons 3) 14 protons
B. Sulfur (S)
1) 32 protons
2) 16 protons
3) 6 protons
C. Barium (Ba)
1) 137 protons 2) 81 protons
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3) 56 protons
11
Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 Atomic number = Number of electrons
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Mass Number
Represents the number
of
protons and neutrons
in an atom
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Isotopes
 Atoms with the same number of protons,
but different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35 LecturePLUS
chlorine
- 37
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ISOTOPIC NOTATION
A
Z
X
A = mass number
(the total number of protons + neutrons)
Z = atomic number
(the total number of protons)
X = element symbol
Atomic Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na
atomic number
sodium-23
11
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READING ISOTOPIC NOTATION
46
21
Sc
46 = mass number
(the total number of protons (21) + neutrons (25))
21 = atomic number
(the total number of protons (21))
Sc = element symbol
In a neutral atom, the number of electrons (21) is
equal to the number of protons.
More Atomic Symbols
16
O
31
P
65
8
15
30
15 p+
16 n
15
e
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30 p+
35 n
30 e-
8 p+
8n
8 e-
Zn
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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 _______
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Solution AT 2
12C
6
13C
14C
6
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
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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
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3) 67
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Learning Check AT 4
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17eC. 47p+, 60 n, 47 e-
16 O
8
___________
37 Cl
17
___________
107 Ag
47
___________
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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
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3)
36X
14
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Masses of Atoms
 A scale designed for atoms gives their small
atomic masses in atomic mass units (amu)
 An atom of 12C was assigned an exact mass of
12.00 amu
 Relative masses of all other atoms was
determined by comparing each to the mass of
12C
 An atom twice as heavy has a mass of 24.00
amu. An atom half
as heavy is 6.00 amu. 25
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Atomic Mass
Na
22.99
 Listed on the periodic table
 Gives the mass of “average” atom of each
element compared to 12C
 Average atom based on all the isotopes and their
percent abundance (% abundance)
 Atomic mass is not a whole number
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Learning Check AT6
Using the periodic table, specify the
atomic mass of each element (round to the
tenths place):
A. calcium
40.1 amu
__________
B. aluminum
27.0 amu
__________
C. lead
207.2 amu
__________
__________
137.3 amu
__________
55.8 amu
D. barium
E. iron
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Calculating Atomic Mass
 Percent abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
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Atomic Mass of Magnesium
Isotopes
Mass of Isotope
Abundance
24Mg
=
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
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Learning Check AT7
Gallium is a metallic element found in
small lasers used in compact disc players.
In a sample of gallium, there is 60.2% of
gallium-69 (68.9 amu) atoms and 39.8% of
gallium-71 (70.9 amu) atoms. What is the
atomic mass of gallium?
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Solution AT7
Ga-69 (%/100)
68.9 amu x
60.2
=
41.5 amu for
69Ga
28.2 amu for
71Ga
100
Ga-71 (%/100)
70.9 amu x 39.8
=
100
Atomic mass Ga =
69.7 amu
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Finding An Isotopic Mass
A sample of boron consists of 10B (mass
10.0 amu) and 11B (mass 11.0 amu). If
the average atomic mass of B is 10.8
amu, what is the percent abundance of
each boron isotope?
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Assign X and Y values:
X = % 10B
Y = % 11B
Determine Y in terms of X
X
+
Y
= 100
Y = 100 - X
Solve for X:
X (10.0) + (100 - X )(11.0)
100
100
= 10.8
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080
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Collect X terms
10.0 X - 11.0 X
=
1080 - 1100
- 1.0 X = -20
X
=
-20
- 1.0
=
Y = 100 - X
% 11B = 100 - 20% =
20 %
10B
80% 11B
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Learning Check AT8
Copper has two isotopes 63Cu (62.9
amu) and 65Cu (64.9 amu). What is the
percent abundance of each isotope?
(Hint: Check periodic table for atomic
mass)
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Solution AT8
2) 70%
Solution
62.9X + 6490 = 64.9X = 6350
-2.0 X = -140
X = 70%
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