Atoms And Elements Worksheet Page 6
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182.4 Isotopes and Atomic Masses
A magnesium atom will always have 12 protons, however the number of neutrons can
vary. Some magnesium atoms found in nature have 12 neutrons, some have 13, and
some have 14.
Atoms with the same number of protons but different numbers of neutrons are called
isotopes. Isotopes are the same element with different atomic masses.
Most elements found in nature exist in isotopic forms. We often refer to an isotope by
stating the elements name followed by its mass number, for example, magnesium-24.
All naturally occurring magnesium atoms contain 78.99% magnesium-24, 10.00%
magnesium-25, and 11.01% magnesium-26. The atomic mass of an element reported on
the periodic table is the weighted average mass of all naturally occurring isotopes of the
elements.
Natural abundance and atomic masses of several isotopes are listed in the following table:
Isotope
Atomic mass
Percent abundance
(amu)
in nature
1
H
1.0078
99.985
1
2
H
2.0140
0.015
1
24
Mg
23.9850
78.99
12
25
Mg
24.9858
10.00
12
26
Mg
25.9826
11.01
12
58
Ni
57.9353
68.27
28
60
Ni
58.9302
26.10
28
61
Ni
60.9310
1.13
28
62
Ni
61.9283
3.59
28
64
Ni
63.9280
0.91
28
2-6
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