Download Average Atomic Mass 1213

Hydrogen
Deuterium
Tritium
Objectives
Explain
what an isotope is.
Compare and contrast two
different isotopes
Calculate the average atomic
mass of an element.
Isotopes and
Average Atomic Mass
Isotopes
Atoms
When you
change the
number of
electrons,, you
electrons
get an
ion of the
same element
Mass Number →
When you
When you
change the
change the
number of
number of
protons,, you get neutrons
protons
neutrons,, you
an
get
completely
isotopes of the
new element
same element
238
Can change!
Atomic Number →
NEVER Changes
92
U
Symbols
contain the
mass number
and the atomic
number.
Naming & Writing Isotopes
There are two ways we
can write isotopes. We
can write the mass as an
exponent after the
number.
Isotopes of Carbon
include:
C14 and C12
We can also put the mass
number after the name of
the element:
carbon
carbon--12
carbon
carbon--14
uranium
uranium--235
Atoms of the same element
can have different numbers of
neutrons..
neutrons
Atoms with the same number
of protons, but different mass
isotopes..
numbers are called isotopes
Isotopes can be natural like
carbon--12/carbon
carbon
12/carbon--14 or they
man--made like
can be man
Einsteinium--252/Einsteinium
Einsteinium
252/Einsteinium-251
Isotopes can be stable or
unstable. Unstable isotopes
will decay and are
radioactive.
How do we measure Atomic Mass?
We use grams to measure the mass of
most things in chemistry, but not for
atomic mass. Why?
Because the masses would be too
small if measured in grams.
grams.
Instead of grams, the unit we use is the
Atomic Mass Unit (amu
amu))
It is defined as one
one--twelfth the mass of a
carbon--12 atom.
carbon
Don’t worry about why we use it, just memorize this as a fact! It
is like 1 gallon = 4 quarts or why a dozen = 12. It just is.
Carbon--12 chosen because of its isotope purity.
Carbon
purity.
1
Why Average Atomic Mass?
The majority of the masses listed on the
periodic table are decimals. Why?
Because natural samples of elements are a
mixture of naturally occurring isotopes.
Ex: How heavy is an atom of cesium?
It depends on which isotope.
isotope. Most cesium
isotopes have a mass of 133, but some have
a mass of 132 and 134.
To account for the mixture of isotopes, we
report the masses of elements as the average
atomic mass.
This is based on the abundance (percentage)
of each isotope of that element found in nature.
Average Atomic
Mass
Decimal that is
reported in the
periodic table
Weighted average of
all naturally occurring
isotopes
What is the:
Average atomic
mass:
95.94
To calculate the Average
Atomic Mass:
How would you solve this
Example: A sample
problem?
of cesium is 75%
1. Convert the abundance
Cesium--133, 20%
Cesium
percentages into decimals
(divide by 100)
Cesium--132 and 5%
Cesium
Cesium--134. What
Cesium
2. Multiply the abundance (in
decimal form) by the mass of is its average atomic
the isotope for each isotope
mass?
3. Add the answers from step
2 together.
Mass
Number
Whole Number
It is the average atomic
mass rounded
Represents the most
common isotope
What is the:
Average atomic
mass:
40.08
Mass Number:
Mass of Most
Common Isotope:
96
40
2