Fundamentals of ChemistryDOE-HDBK-1015/1-93CHARACTERISTICS OF ATOMSRev. 0CH-01Page 9Figure 2 A Mole of Gold Compared to a Mole of CopperBecause the sulfur atom weighs twice as much as an oxygen atom, a one gram sample of oxygencontains twice as many atoms as a one gram sample of sulfur. Thus, a two gram sample of sulfurcontains the same number of atoms as a one gram sample of oxygen. From this previous example, one might suggest that a relationship exists between the weight ofa sample and the number of atoms in the sample. In fact, scientists have determined that thereis a definite relationship between the number of atoms in a sample and the sample's weight.Experimentation has shown that, for any element, a sample containing the atomic weight ingrams contains 6.022 x 10 atoms. Thus 15.999 grams of oxygen contains 6.022 x 10 atoms,2323and 32.06 grams of sulfur contains 6.022 x 10 atoms. This number (6.022 x 10 ) is known23 23as Avogadro's number. The importance of Avogadro's number to chemistry should be clear.It represents the number of atoms in X grams of any element, where X is the atomic weight ofthe element. It permits chemists to predict and use exact amounts of elements needed to causedesired chemical reactions to occur.TheMoleA single atom or a few atoms are rarely encountered. Instead, larger, macroscopic quantitiesare used to quantify or measure collections of atoms or molecules, such as a glass of water, agallon of alcohol, or two aspirin. Chemists have introduced a large unit of matter, the mole, todeal with macroscopic samples of matter. One mole represents a definite number of objects, substances, or particles. (For example, a moleof atoms, a mole of ions, a mole of molecules, and even, theoretically, a mole of elephants.) Amoleis defined as the quantity of a pure substance that contains 6.022 x 10 units (atoms, ions,23molecules, or elephants) of that substance. In other words, a mole is Avogadro's number ofanything.For any element, the mass of a mole of that element'satoms is the atomic mass expressed in units of grams.For example, to calculate the mass of a mole ofcopper atoms, simply express the atomic mass ofcopper in units of grams. Because the atomic mass ofcopper is 63.546 amu, a mole of copper has a mass of63.546 grams. The value for the atomic mass of goldis 196.967 amu. Therefore, a mole of gold has a massof 196.967 grams. The mass of a mole of atoms iscalled the gram atomic weight (GAW). The moleconcept allows the conversion of grams of a substanceto moles and vice versa.Figure 2 contains a ball of gold and a ball of copper.The two balls are of different masses and differentsizes, but each contains an identical number of atoms.

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