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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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