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Understanding Atomic Mass and Isotopes for Chemistry Students

Learn about atomic number, mass number, isotopes, average atomic mass, and calculations for atomic mass of elements with examples.

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Understanding Atomic Mass and Isotopes for Chemistry Students

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  1. How Atoms Differ

  2. a. Properties of Subatomic Particles outside the nucleus e- 1 1840 9.10 x 10-28 g -1 1.673 x 10-24 g p+ in the nucleus +1 1 1 1.675 x 10-24 g n0 in the nucleus 0

  3. Elements on the Periodic Table

  4. Mass number X b. Atomic Number atomic number • the number of protons in an atom • Identifies element c. Mass Number • represents the total number of protons and neutrons in the nucleus A Z

  5. d. Isotopes • Atoms that have the same number of protons but have a different masses • Ex: 3 isotopes of carbon: 12 13 14 C C C 6 6 6

  6. e. Average Atomic Mass • the weighted average of the isotopes of that element. • Formula:

  7. Average Atomic Mass • The mass of an atom is so small it is difficult to work with, so chemists have developed an atomic standard to compare all the masses • The standard is the atomic mass unit (amu) • If the mass of an element is not close to a whole number, it is because the atom has several isotopes • The atomic mass is the weighted average of the isotopes of that element

  8. Example 1 Silver has two naturally occurring isotopes. Ag-107 has an abundance of 51.82% and mass of 106.9 amu. Ag-109 has a relative abundance of 48.18% and a mass of 108.9 amu. Calculate the average atomic mass of silver.

  9. Example 1 • Silver has two naturally occurring isotopes. Ag-107 has an abundance of 51.82% and mass of 106.9 amu. Ag-109 has a relative abundance of 48.18% and a mass of 108.9 amu. Calculate the atomic mass of silver. .5182(106.9 amu) + .4818(108.9 amu) (remember to round at the end with more than one operation) **Round to the hundredths for amu values = 107.87 amu

  10. Example 2 Rubidium is a soft, silvery-white metal that has two common isotopes, Rb and Rb. If the abundance of 85Rb is 72.2% with 84.911794 amuand the abundance of 87Rb is 27.8% with 86.909187 amu, what is the average atomic mass of rubidium? 85 37 87 37

  11. Example 2 85 37 87 37 Rubidium is a soft, silvery-white metal that has two common isotopes, Rb and Rb. If the abundance of 85Rb is 72.2% with 84.911794 amuand the abundance of 87Rb is 27.8% with 86.909187 amu, what is the average atomic mass of rubidium? .722(84.911794 amu) + .278(86.909187 amu) (remember to round at the end with more than one operation) **Round to the hundredths for amu values = 85.467069 amu ≈ 85.45 amu

  12. Honors Example 3 Boron has two naturally occurring isotopes. If the abundance of 11B is 80.10% with an amu of 11.0093, find the abundance of 10B.

  13. Honors Example 3 • Boron has two naturally occurring isotopes. If the abundance of 11B is 80.10% with an amu of 11.0093, find the amu of 10B. Hint: find the abundance first. 0.801(11.0093 amu) + 0.199(X amu) = 10.81 amu 8.8184493 amu + 0.199 (X amu) = 10.81 amu (remember to round at the end with more than one operation) 0.199 X amu = 1.981551 amu = 10.007793 amu for 10B ≈ 10.01 amu for 10B **Round to the hundredths for amu values

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