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

More about isotopes

Atomic mass vs average atomic mass or atomic weight


Atomic mass

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

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

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

    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

    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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