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Average Atomic Mass

Average Atomic Mass. Chemistry Notes. Relative Atomic Mass. Masses of atoms expressed in grams are very small, for example: One atom of Oxygen-16 has a mass of 2.66x10 -23 g = 0.0000000000000000000000266g The standard used to denote units of atomic mass is the carbon-12 nuclide.

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Average Atomic Mass

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  1. Average Atomic Mass Chemistry Notes

  2. Relative Atomic Mass • Masses of atoms expressed in grams are very small, for example: One atom of Oxygen-16 has a mass of 2.66x10-23g = 0.0000000000000000000000266g • The standard used to denote units of atomic mass is the carbon-12 nuclide. • It has been arbitrarily assigned a mass of exactly 12 atomic mass units

  3. Relative Atomic Mass • One atomic mass unit, or amu, is exactly 1/12 of a carbon-12 atom. • Atomic Mass is determined by comparing it with the mass of a carbon-12 atom. • Therefore, atomic mass is a measure of an atom’s mass relative to carbon-12.

  4. Average Atomic Mass • Most elements occur naturally as a mixture of isotopes. • The percentage at which each isotope occurs in nature is taken into account when calculating the element’s average atomic mass. • Average Atomic Mass- the weighted average of the atomic masses of the naturally occurring isotopes of that element.

  5. Compare average atomic mass to mass number • Mass number is the sum of the protons and neutrons of ONE isotope of an element (whole number) • 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)

  6. How Do You Calculate a Weighted Average? • You have a box of 100 marbles. • 25 of the marbles have a mass of 2.0g each. • 75 of the marbles have a mass of 3.0g each.

  7. Weighted Average Calculation • Calculate the total mass of the mixture & divide by 100. 25 x 2.0g = 50g 75 x 3.0g = 225g • Total mass = 275g • 275g ÷100 = 2.75g (average mass of marbles)

  8. Calculate the Average Atomic Mass of an Element (amu)

  9. Calculate the Average Atomic Mass of an Element (amu) • Copper-63 69.17 x 62.93 amu = 4353 amu • Copper-65 30.83 x 64.93 amu = 2002 amu • Total Mass = 6355 amu • 6355 amu ÷ 100 = 63.55 amu • What is Copper’s Atomic Mass on the periodic table?

  10. Calculate the Average Atomic Mass of Oxygen

  11. Answer • 16.00 amu

  12. Formula to calculate avg atomic mass • 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

  13. Which isotope is most abundant? • The average atomic mass of an element is always closest in value to the most abundant isotope. • For example, Carbon’s average atomic mass is 12.011, so the most abundant isotope of carbon is carbon-12. • 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?

  14. Sulfur-32

  15. Assignment • Pg 117 practice problems #23-24 • Pg 124 problem #81

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