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ISOTOPES. Atoms of the same element that contain different numbers of neutrons and have different atomic masses. If mass is not given, always add protons and neutrons. Examples: Carbon-13 13 6 C Oxygen-18 18 8 O. Average Atomic Mass.

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isotopes
ISOTOPES
  • Atoms of the same element that contain different numbers of neutrons and have different atomic masses.

If mass is not given, always add protons and neutrons.

  • Examples:
  • Carbon-13 136C
  • Oxygen-18 188O
average atomic mass
Average Atomic Mass
  • Every element with more than one isotope will have an average atomic mass.
  • This, of course is the average of all of the isotopes’ masses.
  • Unfortunately…not all isotopes were created equally…
so what does that mean
So What Does THAT Mean?
  • It means that the aam is a weighted average as some isotopes are more abundant in this world.
  • Huh? If we have two isotopes, A and B, and we have more of isotope A than isotope B, the mass of A will count more (hold more weight) in the aam.
math please using fractional abundance
Math Please…Using Fractional Abundance.
  • This gives the percent or fractional amount each isotope contributes to the aam.
  • Mg-24 23.985amu 78.70%
  • Mg-25 24.986amu 10.13%
  • Mg-26 25.983amu 11.17%
a fraction or percent is a part of a whole
A fraction or percent is a part of a whole…
  • Mg-24 23.985amu x .7870= 18.88
  • Mg-25 24.986amu x .1013= 2.531
  • Mg-26 25.983amu x .1117= 2.902

24.31 amu

  • So to calculate the average atomic mass turn the percent into a decimal (divide by 100) and multiply each decimal by each isotopes mass…then add them all up.
what if i want to find the percent fractional abundance
What if I want to find the percent/ fractional abundance?
  • With a little bit of algebra…
  • A fraction is a part of a whole…right?
  • So if we have two isotopes, the whole is broken into two different parts. We can call these two x and (1-x).
  • The 1 is the whole…if one isotope takes out x amount of the whole, whatever is left has to be the other isotope.
so it looks like
So it looks like?

AAM= x(mass of Isotope A)+(1-x)(mass of isotope B).

Solve for x and (1-x) to get the decimal fractional abundance…to get percent multiply them by 100.