This document discusses isotopes and atomic mass. It defines isotopes as atoms of the same element that have different mass numbers due to varying numbers of neutrons. A nuclear symbol represents a particular atom and gives the mass number and atomic number. Examples are provided of writing nuclear symbols for atoms with given numbers of protons, neutrons, and electrons. The document also discusses that the atomic mass of an element is based on the relative abundances of its naturally occurring isotopes and gives weighted average masses.

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
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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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