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Chapter 5 Notes

Chapter 5 Notes. The atomic mass of an element is a weighted average mass of the atoms found in nature. If you were to mass an oxygen atom, would it weigh 15.9994 amu? NO! 15.999 amu – THIS IS ONLY AN AVERAGE OF ALL THE ISOTOPES OF OXYGEN 16 O, 17 O, 18 O.

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Chapter 5 Notes

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  1. Chapter 5 Notes

  2. The atomic mass of an element is a weighted average mass of the atoms found in nature. • If you were to mass an oxygen atom, would it weigh 15.9994 amu? • NO! 15.999 amu – THIS IS ONLY AN AVERAGE OF ALL THE ISOTOPES OF OXYGEN 16O, 17O, 18O

  3. The relative abundance of each isotope of an element determines its atomic mass.

  4. Three isotopes of oxygen occur in nature: oxygen-16, oxygen-17, and oxygen-18. The atomic mass of oxygen is 15.9994 because oxygen-16 is the most abundant.

  5. QUESTION • If element Z has two naturally occurring isotopes: Z-20 and Z-22 and the atomic mass of Z is 21.5 amu • Which isotope occurs most often in nature? • Z-22 • the relative abundance of each isotope in nature is one Z-20 to two Z-22.

  6. Thallium has two isotopes, thallium-203 and thallium-205. Thallium’s atomic number is 81 and its atomic mass is 204.38 amu. • So, is there more Thallium-203 or Thallium-205? • There is more Thallium-205 found in nature. • Atoms of both isotopes have 81 protons. Thallium-205 atoms have more neutrons.

  7. A sample of oxygen contains three naturally occurring isotopes:

  8. The relative abundances and atomic masses are: • 99.7759% (mass = 15.995 amu) • 0.037% (mass = 16.995 amu) • 0.204% (mass = 17.999 amu) • Calculate the average atomic mass of the oxygen.

  9. (percent x mass) + (percent x mass) and so on…. • 0.99759(15.99491) + • 0.00037(16.99913) + • 0.00204(17.99916) = • 15.999 amu

  10. Radioactivity (Ch 25)

  11. An alpha particle (radiation) is a helium nucleus.

  12. A beta particle (radiation) is an electron.

  13. Gamma Radiation • Gamma radiation is high-frequency electromagnetic radiation. Gamma radiation has no mass or charge so there is no change in the atomic number caused by the emission of gamma radiation. • Nuclear equations are balanced.

  14. Complete these nuclear equations.

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