1. Atomic Mass Na 22.99 The atomic mass of an element
is listed below the symbol of each element on the periodic table.
gives the mass of an “average” atom of each element compared to 12C.
is not the same as the mass number. Na
22.99

2. Isotopes of Some Elements and Their Atomic Mass
Most elements have two or more isotopes that contribute to the atomic mass of that element.

3. Atomic Mass of Magnesium
The atomic mass of Mg
is due to all the Mg isotopes.
is a weighted average.
is not a whole number.

4. Atomic Mass for Cl The atomic mass of chlorine is
due to all the Cl isotopes.
not a whole number.
the average of two isotopes: 35Cl and 37Cl.

5. Learning Check Using the periodic table, specify the atomic mass of
each element.
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________

6. Solution Using the periodic table, specify the atomic mass of
each element:
A. calcium amu
B. aluminum amu
C. lead amu
D. barium amu
E. iron amu