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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 13 6 C Oxygen-18 18 8 O. Average Atomic Mass.

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ISOTOPES

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  1. 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

  2. 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…

  3. 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.

  4. 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%

  5. 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.

  6. 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.

  7. 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.

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