Transcript Isotopes and Average Atomic Mass

Isotopes and
Average Atomic Mass
Objectives
 Explain
what an isotope is.
 Compare and contrast two
different isotopes
 Calculate the average atomic
mass of an element.
Review: How to read symbols
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When you change the number of electrons,
you get an __________________________
ion
When you change the number of protons, you
get an _____________________________
completely new element
When you change the number of neutrons,
you get ____________________________
Isotopes of the same element
Symbols contain the mass number and the
atomic number.
Mass Number →
238
Can change!
Atomic Number →
NEVER Changes
92
U
Isotopes
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Atoms of the same
element can have
different numbers of
neutrons.
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Atoms with the same
number of protons,
but different mass
numbers are called
isotopes.
Naming & Writing Isotopes
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There are two ways we can write isotopes.
Isotopes of Carbon include:
14C
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and
12C
We can also put the mass number after the
name of the element:
 carbon-12
 carbon-14
 uranium-235
Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
Elements occur in nature as mixtures
of isotopes.
Check for Understanding
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14
7
N and
14
6
C isotopes?
Are
No. Isotopes must be the same element.
Are all isotopes man-made?
No. Isotopes occur in nature. Right now, every
living thing has 14 C in them.
6
Are all isotopes radioactive?
No. Both Carbon -12 and Carbon -14 are isotopes.
Only Carbon-14 is unstable. We will learn how to
predict when an isotope is radioactive or not,
later.
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Why Average Atomic Mass?
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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, because there are different kinds
of cesium atoms. Most 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.
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.
 Instead of grams, the unit we use is the
Atomic Mass Unit (amu)
 It is defined as one-twelfth the mass of a
carbon-12 atom.
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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.
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Carbon-12 chosen because of its isotope purity.
Atomic Masses
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Composition of
the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
6 protons
7 neutrons
1.11%
Carbon-14
14C
6 protons
8 neutrons
<0.01%
Carbon = 12.011
** This rounds to the major isotope
To calculate the Average
Atomic Mass:
1.
2.
3.
Convert the
Example: A sample
of cesium is 75%
percentages into
Cesium-133, 20%
decimals (divide by
Cesium-132 and 5%
100)
Cesium-134. What
Multiply the
is its average atomic
percentage (in
mass?
decimal form) by the
mass of the isotope
Add the masses from
step 2
Example 1:
A sample of cesium is 75% 133Cs, 20% 132Cs and
5% 134Cs. What is its average atomic mass?
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What are the three isotopes in this problem?
1. Convert percents to decimals (divide by 100)
2. Multiply the percent (in decimal form) by the mass
(0.75) x (133) = 99.75
(0.20) x (132) = 26.4
(0.05) x (134) = 6.7
Total
132.85 amu
3. Add the masses together to get the avg atomic
mass.
Example 2:
Boron has two naturally occuring isotopes, 19.8%
Boron-10 and 80.2% Boron-11. What is its
average atomic mass?
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What are the two isotopes in this problem?
1. Convert percents to decimals (divide by 100)
2. Multiply the percent (in decimal form) by the mass
(0.198) x (10) = 1.98
(0.802) x (11) = 8.82
Total
10.8 amu
3. Add the masses together to get the avg atomic
mass.
Atomic Mass vs. Mass #
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Mass Number = Total number of
particles in the nucleus (always a
whole number!)
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Atomic Mass – weighted average of
all the isotopes of an element (a
decimal number)