B. Model 2: Isotopes.

Each element found in nature occurs as a mixture of isotopes. The isotopic abundance can vary appreciably on

an astronomical scale-in the Sun and on Earth, for example. On Earth, however, the abundance shows little

variation from place to place.

28cm

Isotope

¹H

2H

Table 1. Natural abundance and atomic masses for various isotopes

Natural Abundance on Earth (%)

12C

13C

35Cl

37 Cl

24MB

25MB

26MB

99.985

0.015

98.89

1.11

75.77

24.23

78.99

Atomic Mass (amu)

1.0078

2.0140

12.0000

13.0034

34.9689

36.9659

23.9850

24.9858

25.9826

ypaimeriu me

10.00

11.01

ne vedmu

200M

1. How many isotopes of magnesium occur naturally on Earth? 100

2. Describe what all isotopes of magnesium have in common and also how are they different.

They have the same number of protons but different numbers.

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of neutrons.

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3. If you select one carbon atom at random, what is the mass of that atom most likely to be in amu? 12.6000

4. What is the mass (in amu) of

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a. 1000 ¹C atoms? 12,000 amu

b. 1000 ¹3 Catoms? 13,000 ama

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5. If you select 1000 carbon atoms at random, the total mass will most likely be: (circle your answer below)

a. 12000.0 amu

b. slightly more than 12000.0 amu

c. slightly less than 12000.0 amu

d. 13003.4 amu

e. slightly less than 13003.4 amu

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