1. Atoms and Elements
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
12 12 12
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2. Isotopes
Isotopes
 Are atoms of the same element that have different
mass numbers.
 Have the same number of protons, but different
numbers of neutrons.
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3. Nuclear Symbol
A nuclear symbol
 Represents a particular atom of an element.
 Gives the mass number in the upper left corner and
the atomic number in the lower left corner.
Example: An atom of sodium with atomic number
11 and a mass number 23 has the following atomic
symbol:
mass number 23 Na
atomic number 11
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4. Nuclear Symbols
For an atom, the nuclear symbol gives the number of
 Protons (p+)
 Neutrons (n)
 Electrons (e-)
16 31 65
O P Zn
8 15 30
8 p+ 15 p+ 30 p+
8n 16 n 35 n
8 e- 15 e- 30 e-
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5. Learning Check
Naturally occurring carbon consists of three isotopes,
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of the following.
12C 13C 14C
6 6 6
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
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6. Solution
12C 13C 14C
6 6 6
protons 6 p+ 6 p+ 6 p+
neutrons 6n 7n 8n
electrons 6 e- 6 e- 6 e-
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7. Learning Check
Write the nuclear symbols for atoms with the
following subatomic particles:
A. 8 p+, 8n, 8e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________
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8. Solution
16O
A. 8p+, 8n, 8e- 8
B. 17p+, 20 n, 17e- 37Cl
17
C. 47p+, 60n, 47e- 107Ag
47
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9. Learning Check
1. Which of the following pairs are isotopes of the
same element?
2. In which of the following pairs do both atoms have
8 neutrons?
A. 15X 15X
8 7
B. 12X 14X
6 6
C. 15X 16X
7 8 9

10. Solution
B. 12X 14X
6 6
Both nuclear symbols represent isotopes of carbon
with six protons each, but one has 6 neutrons and the
other has 8.
C. 15X 16X
7 8
An atom of nitrogen (7) and an atom of oxygen (8)
each have 8 neutrons.
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11. Isotopes of Magnesium
In naturally occurring magnesium,
there are three isotopes.
Isotopes of Mg
Atomic Mass 23.99 24.98 25.98
% abundance 78.70 10.13 11.17
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24Mg 25Mg 26Mg
Publishing as Benjamin Cummings 12 12 12
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12. Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S 95.02
33S 0.75 32S, 33S, 34S, 36S
34S 16 16 16 16
4.21
36S 0.02
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13. Atomic Mass
The atomic mass of an element
 Is listed below the symbol of each
Na
element on the periodic table.
22.99
 Gives the mass of an “average” atom of
each element compared to 12C.
 Is not the same as the mass number.
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14. Some Elements and Their Atomic
Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
TABLE 2.9
32.07 amu
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15. Atomic Mass for Cl
The atomic mass of chlorine is
 Based on all naturally
occurring Cl isotopes.
 Not a whole number.
 The weighted average
of 35Cl and 37Cl.
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Publishing as Benjamin Cummings

16. Learning Check
Using the periodic table, give the atomic mass of
each element:
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________
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17. Solution
Using the periodic table, give the atomic mass of
each element:
A. calcium 40.08 amu
B. aluminum 26.98 amu
C. lead 207.2 amu
D. barium 137.3 amu
E. iron 55.85 amu
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18. The End
Thanking You
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