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9/2/15 Isotopes,
Atomic Mass,
and Mass
Spectrometry
September 1st, 2015
Do Now:
›  Who
is considered the father of modern
chemistry?
›  What
›  Why
did he discover?
did JJ Thomson perform his experiment?
›  What
was his experiment?
›  What did he discover?
›  What
did Rutherford discover?
›  Why
was his finding so shocking?
1 9/2/15 Review: Isotopes
•  are atoms of the same element that have
different mass numbers.
•  have the same number of protons, but
different numbers of neutrons.
Learning Check
Naturally occurring carbon consists of three isotopes:
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of the following:
12C
6
13C
14C
6
6
protons
______
______
______
neutrons
______
______
______
electrons ______
______
______
2 9/2/15 Atomic Mass
The atomic mass of an element
•  is listed below the symbol of each element on the
periodic table.
•  gives the mass of an“average” atom of each
Na
22.99
element.
•  is not the same as the mass number.
•  You must take the weighted average of all
isotopes of that element.
Atomic Mass Unit
›  12C—Carbon
12—In 1961 it was agreed that this
isotope of carbon would serve as the standard used
to determine all other atomic masses and would be
defined to have a mass of EXACTLY 12 atomic mass
units (amu). All other atomic masses are measured
relative to this.
›  1 AMU (atomic mass unit) = 1/12 of a Carbon 12
atom
›  1 AMU = 1.66053892 × 10-27 kilograms
3 9/2/15 Calculating Atomic Mass
The calculation for atomic mass requires the
•  percent(%) abundance of each isotope.
•  atomic mass of each isotope of that element.
•  sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) +
100
100
Calculating Atomic Mass for Cl
35Cl
has atomic mass 34.97 amu (75.76%) and 37C
has atomic mass 36.97 amu (24.24%).
•  Use atomic mass and percent of each isotope to
calculate the contribution of each isotope to the
weighted average.
4 9/2/15 Isotopes of Magnesium
Learning Check
Gallium is an element found in lasers used in compact disc players. In a
sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms and
39.90% of 71Ga (atomic mass 70.925) atoms.
What is the atomic mass of gallium?
5 9/2/15 Solution
69Ga
68.926 amu x
60.10
100
= 41.42 amu (from 69Ga)
71Ga
70.925 amu x 39.90 = 28.30 amu (from 71Ga)
100
Atomic mass Ga
= 69.72 amu
AP* Chemistry
Stoichiometry
TOMIC MASSES
Mass Spectrometry
12
C—Carbon 12—In 1961 it was agreed that this isotope of carbon would serve as the standard used to
›  Mass
Spectrometeralso
a mass
determine all other atomic
masses and
would be defined to have
a masscalled
of EXACTLY
12 atomicspec
mass
units (amu). All other atomic masses are measured relative to this.
›  Measures the mass of an atom or molecules
mass spectrometer—a device for measuring the mass of atoms or molecules
o
o
o
o
o
o
o
o
atoms or molecules are passed into a beam of high-speed electrons
this knocks electrons OFF the atoms or molecules transforming them into cations
apply an electric field
this accelerates the cations since they are repelled from the (+) pole and attracted toward the (−)
pole
send the accelerated cations into a magnetic field
an accelerated cation creates it’s OWN magnetic field which perturbs the original magnetic field
this perturbation changes the path of the cation
the amount of deflection is proportional to the mass; heavy cations deflect little
6 9/2/15 Mass Spectrometry
›  How
1. 
2. 
Does it Work?
Ionize the Sample- in order for the mass spec to do its
job, the sample must be turned into an ionà knock
off an electron and make a cation
Accelerate the ions and send them through a
magnetic field. The positively charged electric field
will deflect the cations.
› 
3. 
Particles move through the magnetic field. The smaller the
particle, the more the particle will be deflected
A detector produces a signal from the ionsà output is
a mass spectrum (measures mass-to-charge ratio)
7 9/2/15 Mass Spectrometry
mass spectrometer to determine isotopic composition—load in a pure sample of natural neon or other
ss spectrometersubstance.
to determine
isotopic in
composition—load
in abars
pure
sample
of natural neon or other
20
›  Data
the from
of a
mass
spectrum
Theoutput
areas of the
“peaks”
or heights
of the
indicate
the relative abundances
of 10
Ne ,
20
stance. The areas21 of the
or heights
of the
bars indicateofthe
relative abundances of
22
›  “peaks”
Can
give
relative
abundance
isotopes:
e , and
22
10
Ne
10
Ne , and
10
Ne
10
Ne ,
termine isotopic composition—load in a pure sample of natural neon or other
20
he “peaks” or heights of the bars indicate the relative abundances of 10
Ne ,
Exercise 1
The Average Mass of an Element
When a sample of natural copper is vaporized and injected into a mass spectrometer, the results shown in the
Theare
Average
Mass
andata
Element
figure
obtained.
Use of
these
to calculate the average mass of natural copper. (The mass values for 63Cu
and 65Cu are 62.93 amu and 64.93 amu, respectively.)
mple of natural copper is vaporized and injected into a mass spectrometer, the results shown in the
obtained. Use these data to calculate the average mass of natural copper. (The mass values for 63Cu
Mass
re 62.93 amu and 64.93
amu, Spectrometry
respectively.)
Practice: Use the mass spectrum to determine the atomic
mass of copper (the mass of the 63Cu isotope is 62.93 amu
is vaporized and injected into a mass
spectrometer,
the results
shown65
inCu
the isotope is 64.93 amu)
and
the mass
of the
ta to calculate the average mass of natural copper. (The mass values for 63Cu
age Mass of an Element
› 
3 amu, respectively.)
63.55 amu/atom
THE MOLE
mole—the number of C atoms63.55
in exactly
12.0 grams of 12C; also a number, 6.02 × 1023 just as the word
amu/atom
“dozen” means 12 and “couple” means 2.
63.55 amu/atom
Avogadro’s number—6.02 × 1023, the number of particles in a mole of anything
LE
toms in exactly 12.0 grams of 12C; also a number, 6.02 × 1023 just as the word
ouple” means 2.DIMENSIONAL ANALYSIS
02 × 1023, the number of particles in a mole of anything
DISCLAIMER: I will show you some alternatives to dimensional analysis.
8 on
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APshow
Exam.
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23
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ogadro’s
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