Determining Atomic Mass Practice with Answers Atomic Mass • Refers to the mass of an atom. • Number of protons and neutrons • Atoms are too small to mass so mass is determined by the relative mass of a standard atom. By international agreement (IUPAC), Carbon­12 is the chosen standard atom because of its abundance here on Earth. Atomic Mass Unit ­ defined is a mass exactly equal to 1/12th of one carbon­12 atom. 1 carbon­12 atom = 12 amu Determining Atomic Mass Practice with Answers Determine Atomic Mass Unit for Hydrogen Hydrogen ­ 1 as 1 proton and 0 neutrons 1/12 = .0833 x 100 = 8.33% So hydrogen is 8.33% of one atom of Carbon­12 Atomic Mass of Hydrogen = mass of 1 atom of Carbon ­12 x 8.33% = 12 x .0833 =.9996 = 1.0 amu Through different methods of experimental testing, chemists have more accurately determined the mass of hydrogen to be 1.008, which is more like 8.40%. Determining Atomic Mass Practice with Answers Determining Average Atomic Mass: Because element's usually have more than one isotope, which means their masses are different an average must be determine to reflect the mass of all an atom's isotopes. Example: A sample of cesium is 75% Cs-133, 20% Cs-132, and 5% Cs-134. What is the average atomic mass? ANSWER: .75 x 133 = 99.75 .20 x 132 = 26.4 .05 x 134 = 6.7 Total = 132.85 amu average atomic mass unit Determining Atomic Mass Practice with Answers Isotopes and Average Atomic Mass Iodine: 80% 127 I 17% 126 I 3% 128 I ANSWER: .80 x 127 = 101.6 .17 x 126 = 21.42 .03 x 128 = 3.84 Total = 126.86 amu Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Determining Atomic Mass Practice with Answers Bromine which exists as a dark, red gas is Br2. There are two naturally occurring isotopes of bromine with an average atomic mass of 79.90 amu : Bromine­79 (78.918 amu) and Bromine­ 81 (80.916 amu). What is percent abundance of each isotope? Determining Atomic Mass Practice with Answers.