Where is atomic mass on the periodic table?

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Atomic mass on the periodic table is the decimal number in each element's tile, usually printed below the symbol (or at the bottom of the box), such as 12.011 for carbon or 35.45 for chlorine; the whole number at the top is the atomic number. It is the relative atomic mass (atomic weight): the weighted average mass of an element's naturally occurring isotopes, measured in atomic mass units (u), where one u is one twelfth of the mass of a carbon-12 atom. Because it averages isotopes, it is rarely a whole number. Elements with no stable isotope, such as technetium or polonium, show the mass number of their most stable or best-known isotope in brackets, like [98] for technetium. Atomic mass is numerically equal to the molar mass in grams per mole, which is how chemists convert between mass and moles. The atomic mass of an element is calculated from its isotopes. Chlorine, for example, is about 76 percent chlorine-35 and 24 percent chlorine-37, which gives a weighted average of 35.45 u. Carbon is almost all carbon-12 with a little carbon-13, so its atomic mass of 12.011 is close to 12. Heavy elements with a single stable isotope, such as gold at 196.967, have atomic masses very close to whole numbers. Some modern tables show a range for elements such as hydrogen, carbon, boron and nitrogen, since the isotope mix in natural samples varies, so you may see hydrogen listed as 1.008 or as an interval of about 1.0078 to 1.0082. Atomic mass is used to convert between grams and moles: one mole of carbon weighs about 12.011 g and contains about 6.022 × 10 23 atoms. It is not the same as the atomic number, which counts protons and sets the order of the table. To locate atomic mass quickly, look for the decimal number in the tile, usually under the symbol. Do not confuse it with mass number, which counts the protons plus neutrons in one isotope.