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)

This is the number that you see in the periodic table.
1
Atomic Number
H
Symbol of element
1.008

Atomic Weight
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

The atomic weights in the
periodic table = weighted
averages

For example, the relative
abundances for the three carbon
isotopes are:

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….




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
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.