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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Fig: 1