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Why are relative atomic masses decimals, and not simple
whole numbers? Dalton’s original model of an atom assumed that
all atoms of each element were the same. According to the model
of atomic structure we have been developing, this would mean
that each atom of an element would have the same number of
protons, electrons, and neutrons as every other atom of the
element. Thus the atomic mass of every atom of an element
would be the same.
In the early twentieth century, scientists studying
radioactivity found that some substances had different atomic
masses, yet they had identical chemical properties. Scientists
inferred that they must be different atomic forms of the same
element. This phenomenon was also found among much lighter,
non-radioactive elements. For example, there are two forms of
the element lithium. They have the same chemical properties, but
their physical properties differ, leading to the conclusion that
they are atoms of the same element, but somehow different in
These two forms of lithium atom – “light” and “heavy” – are
called ISOTOPES of lithium. “Isotope” comes from the Greek
words for “same place.” Both isotopes of lithium occupy the same
place in the periodic table. Both have the same atomic number
and same number of electrons. They do not, however, have the
same number of neutrons. As a result, the two isotopes differ in
mass number and in relative atomic mass.
DRAW FIGURE 7.23 Isotopes of Lithium from Page 248.
Lithium’s atomic mass in the periodic table is 6.941 u, or 6.9 u to
one decimal place. It is not 6.0 u – what it would be if every
lithium atom were “light” lithium, or lithium – 6. Nor is it 7.0 u –
what it would be if all lithium atoms were “heavy” form, lithium –
The atomic mass for lithium that is shown in the periodic
table is calculated on the basis of what is called a “weighted
average.” This average is closer to 7 u than to 6 u, reflecting the
fact that there is a very high percentage of the isotope lithium –
7 in nature and a much lower percentage of lithium – 6.