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ISOTOPES 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 structure. 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 – 7. 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.