Atomic Mass
Isotopes Reminder
• Yesterday we learned that the mass of all
atoms of a certain element are not always
the same
– Some atoms may have more neutrons than
protons
– We call these atoms isotopes
Atomic Mass
• On your Periodic Tables, each element has
its own atomic mass
– This mass is the weighted average mass of the
atoms that make up that element
• The masses of the potential isotopes of that element
are taken into consideration when the atomic mass is
calculated
Percent Natural Abundance
• For almost all elements, the relative
amounts of each isotope in a naturally
occurring sample are the same
For example…
• In any natural sample of Neon atoms:
90.48% are Ne-20
0.27% are Ne-21
9.25% are Ne-22
These percentages are referred to as the percent natural
abundance of the isotopes
Mass Number vs. Atomic Mass
• The mass number can be calculated by
adding the number of protons and neutrons
The mass of a proton is ____________amu
1
1
The mass of a neutron is ___________amu
With that said, an atom’s mass number can be used to
determine the atomic mass of that atom
**If you are not given the mass of an isotope, you can
always use the mass number to figure out the atom’s mass
Mass Number vs. Atomic Mass
An atom’s mass number can be used to determine the
atomic mass of that atom
For example
If an isotope of carbon has 6 protons and 8 neutrons
the mass number of the isotope would be 14
The 6 protons would have a total mass of 6 amu: 6 protons x 1amu = 6 amu
1 proton
The 8 neutrons would have a total mass of 8 amu: 8 neutrons x 1 amu = 8amu
1 neutron
6 amu + 8 amu = 14 amu
Calculating Atomic Mass
• These percent natural abundances are
used when the atomic mass is calculated
Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) +
(Fraction of isotope 2 x Mass of isotope 2) +
(Fraction of isotope 3 x Mass of isotope 3) +
...
***As a way to check your calculations, the weighted average
atomic mass should be in the range of the masses of the
isotopes or the mass numbers used in the calculation
Calculating Atomic Mass
• These percent natural abundances are used when
the atomic mass is calculated
– *The percentages need to be converted to their decimal
value before they can be included into the calculation
54.3% = 54.3/100 = 0.543
Atomic mass = (Fraction of isotope* 1 x Mass of isotope 1) +
(Fraction of isotope* 2 x Mass of isotope 2) +
(Fraction of isotope* 3 x Mass of isotope 3) +
...
Calculating Atomic Mass
Example 1: (from p. 109 in your book!!)
Naturally occurring chlorine consists of 75.77%
chlorine-35 (mass 34.97 amu) and 24.23%
chlorine-37 (mass 36.97 amu). It’s atomic mass is:
Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) +
(Fraction of isotope 2 x Mass of isotope 2)
Atomic mass = (0.7577 x 34.97 amu) +
(0.2423 x 36.97 amu)
35.45 amu
Calculating Atomic Mass
Example 2:
A sample of Cesium is 75% 133Cs, 20% 132Cs, and
5% 134Cs. What is its average atomic mass?
Hint: For this problem, you need to use the mass number to find the mass of
each isotope
Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) +
(Fraction of isotope 2 x Mass of isotope 2) +
(Fraction of isotope 3 x Mass of isotope 3)
Atomic mass = (0.75 x 133 amu) +
(0.20 x 132 amu) +
(0.05 x 134 amu)
132.85 amu