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Atoms and Elements
Isotopes and Atomic Mass
24Mg 25Mg 26Mg12 12 12
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Isotopes
Are atoms of the same element that have different
mass numbers.
Have the same number of protons, but different
numbers of neutrons.
Isotopes
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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
Nuclear Symbol
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For an atom, the nuclear symbol gives the number of
Protons (p+)
Neutrons (n)
Electrons (e-)
16 31 65
O P Zn8 15 30
8 p+ 15 p+ 30 p+
8 n 16 n 35 n
8 e- 15 e- 30 e-
Nuclear Symbols
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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 14C6 6 6
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
Learning Check
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12C 13C 14C6 6 6
protons 6 p+ 6 p+ 6 p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-
Solution
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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- ___________
Learning Check
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16O
A. 8p+, 8n, 8e- 8
B. 17p+, 20 n, 17e- 37Cl17
C. 47p+, 60n, 47e- 107Ag47
Solution
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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 15X8 7
B. 12X 14X6 6
C. 15X 16X7 8
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Solution
B. 12X 14X6 6
Both nuclear symbols represent isotopes of carbon
with six protons each, but one has 6 neutrons and the
other has 8.
C. 15X 16X7 8
An atom of nitrogen (7) and an atom of oxygen (8)
each have 8 neutrons.
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Isotopes of Magnesium
In naturally occurring magnesium,
there are three isotopes.
24Mg 25Mg 26Mg12 12 12
Isotopes of Mg
Atomic Mass 23.99 24.98 25.98
% abundance 78.70 10.13 11.17
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
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Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance32S 95.02 33S 0.7534S 4.2136S 0.02
32S, 33S, 34S, 36S16 16 16 16
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Atomic Mass
The atomic mass of an element
Is listed below the symbol of each
element on the periodic table.
Gives the mass of an “average” atom of
each element compared to 12C.
Is not the same as the mass number.
Na
22.99
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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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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.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
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Using the periodic table, give the atomic mass of
each element:
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________
Learning Check
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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
Solution
The End
Thanking You
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