 # Average Atomic Mass Chemistry Notes 1 Relative Atomic

• Slides: 15 Average Atomic Mass Chemistry Notes 1 Relative Atomic Mass o Masses of atoms expressed in grams are very small, for example: One atom of Oxygen-16 has a mass of 2. 66 x 10 -23 g = 0. 00000000000266 g o The standard used to denote units of atomic mass is the carbon-12 nuclide. o It has been arbitrarily assigned a mass of exactly 12 atomic mass units 2 Relative Atomic Mass o One atomic mass unit, or amu, is exactly 1/12 of a carbon-12 atom. o Atomic Mass is determined by comparing it with the mass of a carbon-12 atom. o Therefore, atomic mass is a measure of an atom’s mass relative to carbon 12. 3 Average Atomic Mass o Most elements occur naturally as a mixture of isotopes. o The percentage at which each isotope occurs in nature is taken into account when calculating the element’s average atomic mass. o Average Atomic Mass- the weighted average of the atomic masses of the naturally occurring isotopes of that element. 4 o Compare average atomic mass to mass number n Mass number is the sum of the protons and neutrons of ONE isotope of an element (whole number) n Average atomic mass is the weighted average of ALL of the isotopes of an element (usually not a whole number b/c its an average) 5 How Do You Calculate a Weighted Average? o You have a box of 100 marbles. o 25 of the marbles have a mass of 2. 0 g each. o 75 of the marbles have a mass of 3. 0 g each. 6 Weighted Average Calculation o Calculate the total mass of the mixture & divide by 100. 25 x 2. 0 g = 50 g 75 x 3. 0 g = 225 g o Total mass = 275 g o 275 g ÷ 100 = 2. 75 g (average mass of marbles) 7 Calculate the Average Atomic Mass of an Element (amu) Isotope Mass Number % Natural Abundance Atomic Mass (amu) Copper-63 63 69. 17% 62. 93 (69. 17 copper atoms out of 100 are this isotope) Copper-65 65 30. 83% 64. 93 (30. 83 out of 100 are this isotope) 8 Calculate the Average Atomic Mass of an Element (amu) o Copper-63 69. 17 x 62. 93 amu = 4353 amu o Copper-65 30. 83 x 64. 93 amu = 2002 amu o Total Mass = 6355 amu o 6355 amu ÷ 100 = 63. 55 amu o What is Copper’s Atomic Mass on the periodic table? 9 Calculate the Average Atomic Mass of Oxygen Isotope Mass Number % Natural Atomic Mass Abundance Oxygen-16 16 99. 76 15. 995 Oxygen-17 17 0. 038 16. 999 Oxygen-18 18 0. 200 17. 999 (amu) 10 Answer o 16. 00 amu 11 Formula to calculate avg atomic mass o Multiply the mass of each isotope by its percent abundance and then add them all together; then divide the total by 100 (Mass of isotope 1 x % abundance) + (mass of isotope 2 x % abundance)+ 100 12 Which isotope is most abundant? o The average atomic mass of an element is always closest in value to the most abundant isotope. n For example, Carbon’s average atomic mass is 12. 011, so the most abundant isotope of carbon is carbon-12. n The average atomic mass of sulfur is 32. 06. The isotopes of sulfur are sulfur-32, sulfur-33, sulfur -34, and sulfur-36. What is the most abundant isotope of sulfur? 13 o Sulfur-32 14 Assignment o Pg 117 practice problems #23 -24 o Pg 124 problem #81 15