Unit: Atoms, Molecules, and Ions
Module: Atomic Structure
[page 1 of 2]
Examining Atomic Structure
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Isotopes of an element have the same number of protons, but different numbers
of neutrons.
The atomic mass unit (amu) is a convenient unit for describing the masses of
atoms.
The relative atomic mass expresses the weighted average of the masses of all
naturally occurring isotopes of an element.
The difference in mass between a nucleus and its constituent nucleons is
2
released as the binding energy, E = mc .
Isotopes of an element have the same number of
protons, but different numbers of neutrons.
The number of protons in the nucleus of an atom
determines the type of element. For example, every
isotope of carbon has 6 protons in the nucleus. The
number of protons is designated by a subscript
number before the element symbol.
The total number of protons and neutrons (the
number of nucleons) in the nucleus of an atom is
designated by a superscripted number before the
12
element symbol. For example, carbon-12 ( C) has
a total of 12 nucleons. Since 6 of these are protons,
the other 6 have to be neutrons. Similarly, carbon13
13 ( C) has a total of 13 nucleons—6 protons and 7
neutrons.
The atomic mass unit (amu) is a convenient unit for
describing the masses of atoms.
The kilogram is too large of a unit to be convenient
when dealing with atomic masses. For example, a
single atom of carbon-12 has a mass of 1.9924 x
–26
10 kg.
The atomic mass unit (amu) is defined as one
twelfth of the mass of an atom of carbon-12.
The mass of a particle in atomic mass units is
approximately the same as the total number of
nucleons. Carbon-12 has a mass of 12 amu (by
1
definition), while hydrogen-1 ( H) has a mass of
approximately 1 amu.
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Copyright  2003, Thinkwell Corp. All Rights Reserved.
6951 –rev 06/17/2003
Unit: Atoms, Molecules, and Ions
Module: Atomic Structure
[page 2 of 2]
Examining Atomic Structure
The relative atomic mass expresses the weighted
average of the masses of all naturally occurring
isotopes of an element. The relative atomic mass is
useful, because it takes into account the fact that a
single sample of an element is likely to contain
different isotopes of that element.
Three isotopes of neon exist in nature—neon-20
20
21
22
( Ne), neon-21 ( Ne), and neon-22 ( Ne). The
relative atomic mass of neon is found by multiplying
the mass of each isotope of neon (in amu) by the
relative abundance of that isotope, and then adding
these numbers together.
The mass of an atom is slightly lower than the
masses of the subatomic particles of which it is
4
composed. For example, helium-4 ( He) has a
–27
mass of 6.646482 x 10 kg, while the sum of the
masses of the subatomic particles of which it is
–27
composed is 6.696926 x 10 kg. The actual mass
–29
is 5.044599 x 10 kg less than the mass of its
components!
Einstein explained this mystery by showing that
matter and energy are interconvertible. The
difference in mass between a nucleus and its
constituent nucleons is released as the binding
2
energy, E = mc , where E is the binding energy, m
is the change in mass, and c is the speed of light.
However, during a chemical reaction, the mass of
a given nucleus will remain constant.
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info@thinkwell.com
Copyright  2003, Thinkwell Corp. All Rights Reserved.
6951 –rev 06/17/2003