More about isotopes
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More about isotopes. Atomic mass vs average atomic mass or atomic weight. Atomic Mass. The mass of a specific isotope of an element expressed in atomic mass units (amu) The atomic mass is the total mass of the protons, neutrons and electrons in the atom Atomic mass = mass of an atom

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More about isotopes

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More about isotopes

Atomic mass vs average atomic mass or atomic weight


Atomic Mass

  • The mass of a specific isotope of an element expressed in atomic mass units (amu)

  • The atomic mass is the total mass of the protons, neutrons and electrons in the atom

  • Atomic mass = mass of an atom

  • This is what we refer to in our isotope expression, eg.


Average Atomic Mass (also called Atomic Weight)

Atomic Number

Symbol of element

Atomic Weight

1

H

1.008

This is the number that you see in the periodic table.

This is the weighted average of the all of the isotopes of Hydrogen


What is a Weighted Average?

  • The isotopes of an element do not occur with equal frequency or amounts

  • For example, the relative abundances for the three carbon isotopes are:

    • carbon-12 98.9%

    • carbon-13 1.1%,

    • carbon-14 <001%.

  • The "average" mass for the atoms of an element is dictated by the most abundant or common isotope

    • The atomic weights in the periodic table = weighted averages

    • Tabulated atomic weight value doesn't match any actual atom, but is closer to the weight for the most common isotope

    • We’ll follow the steps of how to calculate a weighted average next….


    Calculating Weighted Average

    weighted average = ( decimal fraction A) mass A + ( decimal fraction B) mass B

    • Carbon-12 atomic mass = 12 amu; abundance = 98.9%

    • Carbon-13 atomic mass = 13 amu; abundance = 1.1%

    • Carbon-14 atomic mass = 14 amu; abundance = 0.001%

    Atomic weight = (0.989 * 12) + (0.011 * 13) + (0.00001 * 14) = 12.011

    6

    C

    12.011


    Average Atomic Mass Worksheet

    Homework - I will post answers on website Monday. You will have to show ALL of your work.


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