Average Atomic Mass Chemistry Notes 1 Relative Atomic















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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
Is atomic mass and relative atomic mass the same
Isotope abundance formula
Atomic mass formula
How to calculate abundance of isotopes
Atomic mass 10
Average atomic mass def
Average atomic mass of lead
Grade point average
Average atomic mass of chlorine
Relative atomic mass of barium
Relative atomic mass of boron
Define relative atomic mass
Relative atomic mass
Atomic
What is this
Mole bridge