- Catalyst

Monday, July 2
• Section 3.4 – 3.5
• Atomic # and mass #
• Isotopes and atomic mass
– Suggested Book Questions (Red indicates those covered in this lecture): • 3.1: 1,3,5; 3.2: 7‐13 odds; 3.3: 15,17,19; 3.4: 23‐29 odds; 3.5: 31‐39 odds; 3.6: 41,45,47 odds; 3.7: 49‐55 odds; 3.8: 69; EOC: 77‐95 odds, 101 (a,b,e), 103, 107, 109
• ALEKS Objective #1 Due Thursday, July 5th at 10:00PM
• Exam 1 is Friday, July 6th.
Exam 1 Info
• Equations and constants sheet was posted with a study guide. I also posted a practice test.
• A seating chart will be posted soon. Know your seat! We will check!
• Bring extra writing utensils.
• Bring a scientific calculator. No graphing calculators and no cellular telephones.
• Chapters 1 – 3
• Show all of your work for full credit. Partial credit can be awarded for an incorrect answer. Show your work!
What roles do the different particles play?
• # Protons = chemical identity of the atom (which element is it?)
– In an electrically‐neutral atom, the number of protons in the nucleus is exactly balanced by the number of electrons.
• # Electrons = ionic character of the atom. An ion has either more or fewer electrons than the electrically‐neutral atom.
– anion = more electrons, so ion is negatively‐charged
– cation = fewer electrons, so ion is positively‐charged
• # Neutrons = isotopic character of the atom
– an atom of an element usually comes in at least 2 or 3 different isotopes
(sometimes more) – usually there will be one isotope that is far more abundant than the others
– isotopes have the same number of protons, so they are chemically indistiguishable.
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Some Massive Definitions
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Atomic number (Z): the number of protons in the nucleus of an atom
Mass number (A): the sum of the numbers of protons and neutrons in the nucleus of an atom
Atomic Mass: an average of the atomic masses of the most common isotopes
Atomic Symbol
A
X
Z
For example:
16
O
8
or
16
O
8
In the periodic table...
O
16.00
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Let’s count some particles
# protons
Cobalt‐60
Uranium‐238
60
Co
238
U
Chlorine‐37 anion
37
Copper‐63 cation
63
Cu2+
Copper‐65 cation
65
Cu2+
Cl–
# electrons
# neutrons
Buddy Quiz: Counting Particles
Magnesium plays several critical roles in the functioning of human cells. However, this form of Magnesium is the ion made with 12 protons and 10 electrons. Therefore the ion would be…
1. positive and called an anion.
2. positive and called a cation.
3. negative and called an anion.
4. negative and called a cation.
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Isotopes and Atomic Mass
• Atoms of the same element that differ in mass (eg. 12C, 13C, 14C)
– isotopes are the ________________________________
– isotopes have the same ____________________________
– isotopes differ in the number of neutrons, and therefore
they ______________________________________(which is measured in amu).
• Many isotopes occur in nature. Most natural isotopes are not radioactive, nor are they necessarily harmful. • A sample of an element will contain some percentage of all its isotopes.
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Calculating Atomic Mass
• Two isotopes of copper are naturally occurring, with copper‐63 (62.93 amu) at 69.09% and copper‐65 (64.93 amu) at 30.91%. Calculate the atomic mass of copper.
• Indium (In, atomic mass = 114.8 amu) consists of two naturally‐occurring isotopes, indium‐113 and indium‐
115. If 4.30% of a sample of indium is indium‐113, which has a mass of 112.90 amu, what is the mass of indium‐115?
Buddy Quiz: Calculating Atomic Mass
Let’s say we have a sample of 1000 lithium (Li) atoms. Based on isotopic abundance: 925 weigh 7 amu (92.5%)
75 weigh 6 amu (7.5%)
What is the average mass of a lithium atom in this sample?
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Some Isotope Comparisons
Element
Isotope
Hydrogen
1H
2H
Carbon
12C
13C
Sulfur
32S
33S
34S
36S
Lithium
6Li
7Li
Mass of Isotope (amu)
Relative Abundance
Average Atomic Mass (amu)
1.00782
2.01410
99.9844%
0.0156%
1.0079
12 (exact)
13.00335
98.892%
1.108%
12.01115
31.972071
32.971458
33.967867
35.967080
95.06%
0.74%
32.064
4.18%
0.0136%
6.015123
7.016005
7.5%
92.5%
6.941
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Buddy Quiz
Name whether these are Different elements, Isotopes, or Neither:
1. Two atoms have the same mass number, but different atomic numbers.
2. Two atoms have the same atomic number, but different mass numbers.
3. 35Cl– and 35Cl
A Brief History of the amu
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•
•
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Stanislao Canizzaro (1826‐1910) proposed that the H atom be used as a standard of mass and set its atomic mass at 2.
Other chemists of the day wanted to use a more massive atom to reduce experimental error. Chemists eventually took the mass of naturally occurring oxygen (O) to be 16 amu.
Concurrently, physicists defined the oxygen‐16 isotope as 16 amu.
Are these two definitions that use oxygen the same?
NO. Naturally occurring oxygen is a combination of three stable isotopes, oxygen‐16, oxygen‐17, and oxygen‐18.
These two definitions resulted in conflicting values.
• Finally in the 1950s the carbon‐12 isotope was adapted as the standard…hence 1 amu = 1/12 the mass of one 12C atom.
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Don’t confuse “Atomic Mass” with the mass of one atom!!
• An atom can be only one isotope at a time.
•
•
12C: Z = 12, isotopic mass
= 12 amu (exactly) 13C: Z = 13, isotopic mass = 13.003354 amu
• The Atomic Mass (aka Atomic Weight or Average Atomic Mass) is the average of the atomic masses of all of the element's isotopes, weighted by isotopic abundance.
• Naturally occurring carbon has an atomic mass of 12.011 amu
• There is no carbon isotope that weighs 12.011 amu.
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