Calculating Average Atomic Mass

1
Calculating Average Atomic Mass

• Its all about the ISOTOPES!

2
How Do You Weigh an Atom?

• Atoms have masses far too small to be weighed, so
  we use the atomic mass scale to express the mass
  of atoms in atomic mass units (amu). 

3
Atomic Mass is RELATIVE

• Atomic masses are relative masses where the mass
  of one atom is compared to another.
• It is NOT an absolute scale of measure like
  grams.

4
Its All About Carbon

• The Carbon-12 atom is the basis for relationships
  between all other atoms.
• The mass of a carbon atom was divided into 12
  parts (6 protons, 6 neutrons) and the mass of
  each part is known as an atomic mass unit (amu).
  

5
So Whats 12.011 Mean?

• We define the atomic mass of 12C (or C-12) as
  exactly 12 amu.
• but thats NOT the number you see in the
  Periodic Table!

6
Think Weighted Average!

• The mass listed in the periodic table for each
  element is the weighted average of all of the
  isotope masses for that element.
• (As you might expect, different isotopes of the
  same element will have different masses.)

7
Which Isotope Counts Most?

• The mass in the Periodic Table will be closest to
  that of the most abundant isotope of that
  element.
• The weighted average factors in the percent
  abundance of each isotope as it occurs in nature.

8
Example, Please!

• If you analyzed a lump of pure carbon from the
  planet Earth, you would find that 98.9 of all
  carbon atoms on earth are 12C atoms, and 1.11
  of all carbon atoms on earth are 13C atoms.

9
Look at That PT Mass Again!

• So the weighted average mass of carbon is
  (0.989)(12.000000 amu 12C )(.0111)(13.0039 amu
  13C) 12.011 amu

10
One More Example

• The natural abundance of 63Cu is 69.09 and for
  65Cu is 30.19 .
• If the atomic weight of 63Cu is 62.93 amu and
  65Cu is 64.93 amu, what is the average atomic
  weight for natural copper?

(0.6909)(62.93 amu 63Cu ) (0.3091)(64.93 amu
65Cu ) 63.55 amu