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Chapter 4
Atomic Structure
Warm – Up
We have not yet discussed this material, but
what do you know already??
• What is an atom?
• What are electron, neutrons, and protons?
• Draw a picture of an atom from what you
know today.
Today’s Agenda
• QOTD: How did we come to understand the
atom?
• Historical journey of the atom
• Scientists and experiments
• Atomic Structure
• No homework tonight, but warning, you’ll
have some this weekend…
History of the Atom
• 1. Democritus vs. Aristotle pg. 102-103
• 2. John Dalton and conservation of mass
pg. 104-105
• 3. Cathode ray tube and Sir William Crookes
pg. 107-108
• 4. Mass and charge of electron (J.J.
Thompson) and oil drop experiment pg. 108109
• 5. Plum pudding model vs. Rutherford’s
experiment pg. 110-112
Democritus
• Greek philosopher who asked questions about
matter.
– Can you divide matter infinitely?
• Democritus says no!
– Tiny particles called atoms, indivisible!
• Matter is composed of atoms,
which move through empty space.
• Atoms are solid homogeneous, indestructible,
and indivisible.
• Atoms have different sizes and shapes. These
properties, and movement determine properties
of matter
John Dalton
• Matter is composed of small particles called
atoms that are indivisible and indestructible.
• Atoms of a given element are identical in size,
mass, and chemical properties, and are
different from those of another element.
• Different atoms combine in simple whole
number ratios to form compounds.
• In a chemical reaction, atoms are separated,
combined or rearranged.
The Atom
• EXTREMELY small particle of an element that
retains the properties of that element is an
atom.
• If atom is the size of an orange, an orange
would be the size of the EARTH
Subatomic Particles - Electron
• Cathode Ray Tube
Cathode
Thin beam of electrons
Vacuum
Anode
travels from cathode to
anode!
• Cathode rays are a
stream of charged particles. Particles carry a
negative charge…now called electrons!
J.J. Thompson
• Determined that the mass of the charged
particle (electron) was much less than that of
the hydrogen atom.
– Dalton was WRONG about the atom being the
smallest particle!
Dalton’s Inaccuracies
• Atoms are not the smallest type of matter!
– Subatomic particles – electrons, protons, and
neutrons
• Atoms of the same element can have slightly
different masses! - isotopes
Millikan Oil-Drop Experiment
• Determined the charge of an electron.
Charge up the oil particles
with electrons.
Change the electric field
changes the rate of oil
droplets! (MOVIE!)
Charge of electron 1.602 x 10-19 coulombs
Mass of electron = 9.1 x 10-28 grams
Plum Pudding Model
• Matter isn’t all negatively charged, so how do
we have negatively charged subatomic
particles without positively charged ones??
• J.J. Thompson thought an atom was a
positively charged sphere with electrons
hanging out within.
Warm Up
• What were Dalton’s inaccurate theories. Why
are they wrong?
• What were the correct ones?
• What was JJ Thompsons model for the atom?
Agenda
• Question of the Day: What is an isotope and how
do we write an isotopic symbol?
• Finish history lesson
• Periodic table and isotopes
• Ions
• Homework is actually due tomorrow…I forgot
that we started on a Tues…sorry!
Rutherford and the Nucleus
• Experiment proved that plum pudding model was
incorrect!
• Atom is mostly empty
space through which ecan move. Almost all of
the positive charge and
atomic mass resides in
the center – NUCLEUS!
Nucleus is positively charged to deflect alpha
particles and to balance electron charge.
Subatomic Particles
• Electron – VERY tiny, negatively charged
• Proton – located in the nucleus, charge
opposite of an electron (positive!)
• Neutron – located in the nucleus, same mass
as a proton, neutral!
Atomic Theory Today
• Quantum Mechanical Model
• All atoms are made up of electrons, protons, and
neutrons. Electrons are located outside of the
nucleus, protons and neutrons are located inside
the nucleus.
• Electrons exist in a cloud surrounding the
nucleus. Attracted to the nucleus so they hang
around!
• Nucleus accounts for 99.97% of the atomic mass,
and occupies a VERY small volume.
• A neutral atom has the same number of electrons
and protons!
Current
Atomic
Model
Neutral atom:
# Protons = # Electrons
Simulation!!
Warm Up
• Which of John Dalton’s 4 theorems are wrong
and why?
• In what experiment was the electron
discovered?
• What was Rutherford’s contribution?
• Describe the current model of the atom.
Today’s Agenda
• Question of the Day: What is an isotope and
how do we write an isotopic symbol?
• Periodic table and isotopes
• Ions
• Average atomic mass
• Homework Ch 4 58-74 evens due Monday
(they’re not too bad)
Properties of Atoms
Atomic #
# of Protons =
# of Electrons
(in neutral atom)
Atomic mass
# Protons + # neutrons
Practice
What is the isotopic symbol for each?
Isotopes and Ions
• Isotope – Atoms with the same number of protons but
different number of neutrons.
• Things to remember –
– The # of protons of an element NEVER changes,
and is ALWAYS the same as the Atomic #.
– If the # neutrons is different = ISOTOPE
– If the # electrons is different = ION
• + = cation Less electrons
• - = anion More electrons
70
Isotopic symbol : 32 Ge
Warm Up!
Element
Calcium
Oxygen
Mercury
Atomic #
20
8
80
Mass #
46
17
204
What is the number of protons, electrons, and
neutrons for each?
What is the isotope symbol (shorthand notation)
for each?
Today’s Agenda
• Question of the day: How do we calculate the
average mass of an element?
• Finish table from yesterday
• Shorthand notation
• Average atomic mass calcs!
• Ch 4 58-74 evens due tomorrow (they’re not
too bad) LEAVE LAB NTBKS tonight!! Al lab!
Fill in the table!
Name
Symbol Atomic #
K
Atomic
#
#
#
Mass protons neutrons electrons
40
Boron
Br
Sodium
Nitrogen
Atom/
cation/
anion
Atom
6
5
45
36
24
10
8
10
What is the shortand notation (isotopic symbol with charges if needed!)
Warm Up!
Element
Calcium
Oxygen
Mercury
Atomic #
20
8
80
Mass #
46
17
204
Charge
2 cation
2 anion
0 atom
What is the number of protons, electrons, and
neutrons for each?
What is the isotope symbol (shorthand notation)
for each?
Today’s Agenda
• Question of the day: How do we calculate the
average mass of an element?
• Average atomic mass calcs!
• Homework problems
• Homework: Quiz review worksheet. Quiz
Wednesday on Ch 4. SSW lab due Wednesday.
Mass of Atoms
• Mass of electron = 1/1840th of a proton
• Mass of proton ≈ mass of neutron
• 1 atomic mass unit (amu) ≈ mass of proton
Carbon 12 atom = 12 amu
Why aren’t the masses of elements in whole
numbers?
Atomic Mass = Average of Isotopes
• Weighted average mass – mass of each isotope
contributes to total mass according to how much of
that isotope exists.
Amu = ((Mass of Isotope1) x (Relative Abundance1)) +
((Mass of Isotope2) x (Relative Abundance2))…
Amu = ((M1) x (RA1)) + ((M2) x (RA2)) + ((M3) x (RA3))…
Atomic Mass = Average of Isotopes
• Given info:
K
Potassium
Three isotopes = 39
K
19
Percent Composition: 93.26%
K
19
40
0.01%
K
41
19
6.73%
What is the atomic mass (AMU)??
Calculate the Atomic Mass of K
1. Use % composition and convert to relative
abundance decimal (divide by 100)
93.26% composition = .9326 relative abundance
2. Amu = ((Mass of Isotope1)x(Relative Abundance1))
+ ((Mass of Isotope2)x(Relative Abundance2))…
((0.9326)x(39)) + ((0.0001)x(40)) + ((0.0673)x(41)) =
39.1347 amu
Practice
• What element is this?
Isotope Mass of Isotope
Percent abundance
6X
6.015 amu
7.59%
7X
7.016 amu
92.41%
Find atomic mass and ID the element (from table)!
• Boron has two isotopes: Boron-10 (% abundance –
19.8%, mass = 10.013 amu) and Boron-11 (%
abundance – 80.2%, mass – 11.009 amu). Calculate
the atomic mass of Boron.
Warm Up
• What element is this?
Isotope Mass of Isotope
Percent abundance
6X
6.015 amu
7.59%
7X
7.016 amu
92.41%
Find atomic mass and ID the element (from table)!
• Boron has two isotopes: Boron-10 (% abundance –
19.8%, mass = 10.013 amu) and Boron-11 (%
abundance – 80.2%, mass – 11.009 amu). Calculate
the atomic mass of Boron.
Today’s Agenda
• Question for the day: How do we calculate
average atomic mass and relative abundances?
• Average Atomic mass calcs
• Prelab Sand, Salt, Water
• Prelab for Monday!
• Bromine has two isotopes with the first having a
mass of 78.918336 amu and occupying 50.69% and
the second isotope having a mass of 80.916289
amu and occupying 49.31%. What is the average
atomic mass of bromine?
• Verify the atomic mass of Magnesium:
24Mg = 23.985042 amu and percent abundance of
78.99% , 25Mg = 24.985837 amu and percent
abundance of 10.00%, 26Mg = 25.982593 amu and
percent abundance of 11.01%.
Warm Up
• Boron has an atomic mass of 10.81 amu
according to the periodic table. However, no
single atom of boron has a mass of 10.81 amu.
How can you explain this difference?
• Write the shorthand notation for the
following:
– A particle with 34 protons, 36 neutrons, and 36
electrons
– A particle with 44 protons, 54 neutrons, and 41
electrons
Agenda!
• Review!
• Review Ch 4
• Homework questions
• Nuclear chemistry intro
• Worksheet collected tomorrow before quiz!
Block 3 – leave notebooks!
Calculate % abundance
• Copper has two naturally occurring isotopes,
Cu-63 and Cu-65. The atomic mass of Cu is
63.55 amu. Calculate the percent abundances
of the two isotopes.
• Silver has two isotopes, 107Ag and 109Ag. The
average mass of the isotopes is 108.90470.
What is the percent abundance of each?
Warm Up
• What does it mean to be radioactive?
• What is an isotope?
• What is a radioisotope?
Today’s Agenda
• Question of the day: What is radioactivity?
• Radioisotopes
• Types of decay
• Writing Reactions
• Homework: Ch 4: 80, 82, 83, 85, 86, 88 Due
tomorrow!
Radioactivity – emit radiation
• Nuclear reactions – change an element into a
new element!! Lots of energy involved!
– Unlike a chemical reaction because we are doing
more than rearranging – we CHANGE the identity.
• UNSTABLE nuclei are unhappy and lose energy
by emitting radiation – radioactive decay.
• They form STABLE atoms of a different
element.
Radioisotopes
• Isotopes of atoms with unstable nuclei.
• Undergo radioactive decay to attain stability.
Emit 3 types of radiation
– alpha,
– beta,
– gamma,
What are the charges on
radioactive particles?
Types of Radiation
• Alpha radiation –made up of POSITIVE alpha
particles.
• particle – helium cation (2 protons and two
neutrons, no electrons!)
4He2+
2
or
Alpha decay
238
4
92
2
U
226
4
88
2
247
4
Ra
96Cm
2
He
+
He
+
He
+
234
Th
90
222
Rn
86
243
Pu
94
Types of Radiation
• Beta radiation – negatively charged beta
particles
• Unstable neutron turns into a proton and
ejects 1 electron
e- or
Beta Decay Reactions
137
0
55
-1
14
0
Cs
6
C
+
+
-1
260
0
104
-1
Rf
+
137
Ba
56
14
7
N
260
105
Db
Types of Radiation
• Gamma radiation – emits gamma rays, high
energy photon that has no mass nor charge.
• Gamma rays almost always accompany alpha
and beta radiation and account for the energy
lost in the nucleus.
Usually omitted from nuclear equations.
238
4
92
2
U
He
+
234
Th
90
+
2
Mixed Practice
149
61
Pm
247
96
247
95
209
82
0
4
He +
2
Cm
Am
Pb
+
-1
0
-1
4
+
He +
2
149
62
Sm
243
94
247
96
205
80
Pu
Cm
Hg
Penetrating Power of Radiation
Warm up
• Complete the nuclear reactions:
204
81
144
60
Tl
Nd
0
-1
4
+
He +
2
204
82
140
58
Pb
Ce
• Identify the TYPE of decay.
• Why is a gamma ray more penetrating than an
a particle?
Warm Up
Complete these reactions and indicate the TYPE
of radioactive process:
241Pu
94
 241
95 Am + ____
214 Bi
 210
81 Tl + ____
85 Rb
85 Kr + ____
 36
83
37
Why is a gamma ray more penetrating than an a
particle?
Penetrating Power
Least
Alpha particles most mass and charge.
Isotopic mass
4He2+
2
Beta particles less mass (only the mass of an
electron) and a neg charge.
Most
Gamma rays have no mass and no charge.
Give reactions for:
• Carbon-14 undergoes beta decay
• Americium-245 undergoes alpha decay.
• An alpha decay produces lead-205
• Curium-247 is the product of a beta decay.
Warm Up
• Polonium-209 undergoes alpha decay
• Beta decay produces mercury-203
• Cerium-143 undergoes an alpha decay
followed by a beta decay.
B
A
Write a balanced nuclear equation for the
decay shown on the right. Identify A and B
212Bi
4He +
208Tl
+
83
81
2
208Tl A
81
208Pb B
82
Bismuth -212
Today’s Agenda
• Question for today: Why do some atoms
undergo decay and how fast do they decay?
• Nuclear decay ratio
• Half life calculations
• HW: Ch 24 problems 34-58 evens Due
Wednesday
• TEST 4/24 FRIDAY!
In the Nucleus
• Radioactive decay – transmutation
– Atomic # is altered = identity of element changed
Nucleons
+
+
+
Strong nuclear force between all nucleons.
Repulsive force between 2 protons (electrostatic).
Neutron attraction have to overcome the repulsive forces –
as atomic # increases more neutrons needed for stabilization!!
All nuclei with more than 82 protons are radioactive!
Low atomic #’s have a 1:1
neutron to proton ratio
4He
2
High atomic #’s are
stabilized by a 1.5:1 ratio
200Hg
80
If atom is not in band (belt)
of stability it undergoes
radioactive decay to get
there!
Half Life
• Time required for one half of the nuclei to
decay into its products.
• Strontium-90 half life is 29 years.
If you had 10 g now, in 29 years you would have
5g.
Work together to solve…
Kr is used in indicator lights of appliances. Kr-85
has a half life of 11 years. If a refrigerator light
contains 2.0 mg of Kr-85, after 33 years, how
much is left?
Half Life Formula
N = N0 (½)n
N – remaining amount of element
N0 – initial amount of element
n – number of half lives that have passed
Kr-85 has a half life of 11 years. Kr is used in indicator
lights of appliances. If a refrigerator light contains 2.0
mg of Kr-85, after 33 years, how much is left?
N=?
N0 = 2 mg
n = 33 years/11 years (years that have passed/half life)
Kr is used in indicator lights of appliances. Kr-85
has a half life of 11 years. If a refrigerator light
contains 2.0 mg of Kr-85, after 33 years, how
much is left?
N = 2.0 mg (½)(33/11)
N = 2.0 mg (½)3
N = 2.0 mg (⅛)
N = 0.25 mg left after 33 years
Half Life Practice
• The half life of Ra-222 is 3.8 days. How much
is left of a 10 mg sample after 15.2 days?
N = N0 (½)n
N = 10mg (½)(15.2/3.8)
N = 10mg (½)4
N = 10mg (1/16)
N = 0.625mg
Half Life Practice
Bandages can be sterilized by exposure to
gamma radiation from cobalt-60, which has a
half life of 5.27 years. How much of a sample of
cobalt-60 did we start with if after 10.54 years
we have 0.75 mg?
N = N0 (½)n
0.75 mg = N0 (½)26.35/5.27
0.75 mg = N0
(½)5
Half – Life Calculations
• Do the problem intuitively…
Think about how many half lives have passed and
just do the math!
26.35 years/5.27 years in half life = five half lives
Multiply 0.75 mg by 2 five times
0.75 mg x 2 x 2 x 2 x 2 x 2 = 24 mg
Warm Up!
• Write the equations for decay:
– krypton-81 undergoes alpha decay
– beta decay of uranium-238
– thorium-230 undergoes alpha decay followed by
two beta decays
• If 100 g of carbon-14 decays until only 12.5 g
of carbon is left after 11,460 years, what is the
half-life of carbon-14?.
Today’s Agenda
• QOTD: What is fission and how do we use it
for energy?
• Nuclear Reactions
• Fission vs Fusion
• Review!
• Homework due tomorrow. Test Friday.
Review from Yesterday!
• Use your notes to quietly answer the following:
• What keeps the nucleons together?
• What causes radioactive decay?
• Do the following isotopes decay?
Nitrogen-15
Carbon-14
Phosphorous-30
Solve!
• Americium-241 has a half life of 430 years. How
much of a 15 mg sample is left after 2150 years?
• A radioisotope has a half life of 17 years. If 0.25 g
remains today, how much did you have 85 years
ago?
• Strontium has a half life of 29 years. How long
will it take for a 56 g sample to decay to 0.875 g?
Nuclear Reactions
• Induced Transmutation – FORCE an element to
change its identity by bombarding it with
radioactive particles!
1n + 14
N
0
7

15N
7
 24He + 11
B
5
• Particle accelerators move particles at
extremely high speeds to overcome repulsive
forces.
Nuclear Fission
• Force a nucleus to split into fragments –
resulting in a LARGE release of energy!
Kr - 92
Particle
accelerator
neutron
235
236
1
n
+
U
92
92 U
0
Ba - 141
What products are formed?
Neutrons Perpetuate Fission
Critical Mass
• Fissionable material must have critical mass
– Not massive enough – subcritical – no chain reaction
– Extremely massive – supercritical - violent nuclear
reaction
Critical Mass
Violent Nuclear
Reactions
Subcritical masses that get
together to form supercritical
mass.
Equal to 20,000 tons of TNT
Nuclear Reactors
Control
Rods
Reactor core controlled by Cd or B to absorb neutrons
Warm Up!
• Describe the process that occurs during a nuclear
chain reaction and explain how to monitor a chain
reaction in a nuclear reactor.
True/False
• Great amounts of energy can be liberated from small
amounts of matter in a nuclear reaction.
• The amount of U-235 in a nuclear reactor should
always be kept subcritical.
• Nuclear power plants do not contribute to air
pollution.
• Nuclear power use is dangerous because plants are
commonly are out of control.
Today’s Agenda
• QOTD: How do you analyze half life data?
• Homework questions
• Half-life lab!
• HMWK – Review sheet due Friday. Start
tonight for questions tomorrow! Test Friday!
Warm Up!
• Curium-248 has a half life of 5.2 minutes. If
you have a 6.4 mg sample after 41.6 minutes,
how much did you have at the start?
• Curium has three isotopes, Cm-248 (15%),
Cm-247 (74%), and Cm-246 (?%). Find the
percent abundance for Cm-246 and the
average atomic mass of Cm.
Half life practice
• The half life of Braintrium-128 is 48 minutes. If
we have a 0.3 g sample left after 8 hours, how
much did we have to start with?
• How much time has passed if our 0.3 g sample
of Btr-128 has decayed to .0188 g?
Nuclear Reactions
• What is the isotopic symbol for an element
with 99 protons, 153 neutrons, and 97
electrons?
• If that element undergoes an alpha decay
followed by 2 beta decays, what is the nuclear
reaction?
Half Life Lab
• Statement of problem: Given a sample of
M&M’s, collect decay data, graph data using
excel, and fit with an exponential function.
• Background info:
What is a half life? What happens to a sample as
each half life passes?
Sketch a graph that you expect.
Nuclear Fusion
• Bind low atomic mass (less than 60 because
60 is ideal) to form more stable atom.
• Combination of nuclei called fusion.
• How the sun works…
411H
+
2
energy +
4He
2
• Need very high energy to initiate and sustain.
Nuclear Energy vs. Fossil Fuel
• Why is nuclear energy considered cleaner
than fossil fuels with respect to greenhouse
gases?
• Write the balanced nuclear fission reaction
below:
2 neutrons (10n) are absorbed by a uranium-235
nucleus, which then undergoes alpha decay.
Decay Practice
1
decay
2
4He +
238Pu
2
94
234U
92
Thorium-229 is used to increase the lifetime of fluorescent bulbs.
What type of decay occurs when thorium-229 decays to form radium-225?
Write out the nuclear equation.
4He +
229Th
2
90
225Ra
88
3
B
A
Write a balanced nuclear equation for the
decay shown on the right. Identify A and B
212Bi
4He +
208Tl
+
83
81
2
208Tl A
81
208Pb B
82
Bismuth -212
Warm Up
Isotope
Percent Abundance
Mass (amu)
X-50
4.35
49.946
X-52
83.79
51.941
X-53
9.50
52.941
X-54
2.36
53.939
Find the atomic mass and identify the element.
How many electrons, and protons does this element have?
How many electrons protons and neutrons does 52X2+ have? Is it in cationic
or anionic form?
Warm – Up!
• What is an isotope?
• What is a radioactive isotope?
• What does it mean for something to be
radioactive?
Warm Up!
• What are the three types of radioactive
particles?
• What are the charges on those particles?
• Complete the following nuclear reaction
(Remember that = e-)
137
Cs
55
+
137
Ba
56
Warm – Up!!
• What happens to the atomic mass number
and the atomic number of a radioisotope
when it undergoes alpha emission?
• High speed electrons emitted by an unstable
nucleus are ________ particles.
• What isotope of what element is produced if
krypton-81 undergoes beta decay?
Write out the nuclear reaction.
Warm – Up!!
• What is the band of stability and how does it
relate to the proton to neutron ratio?
• How does the neutron to proton ratio change
when polonium-210 decays into lead-206? What
type of decay does polonium-210 undergo?
(Low atomic # elements are happy with a 1:1
ratio of neutrons to protons. Heavier elements
need a 1.5:1 ratio and all elements above 82 are
radioactive.)
Warm Up
1. Silver has two naturally occurring isotopes,
Ag-107 and Ag-109. The atomic mass of Ag is
107.868 amu. Calculate the percent
abundances of the two isotopes.
2. 240Am + 2 01n _______  0 + ______
95
-1
3. The half life of polonium-218 is 3 min. If you
start with 26 g, how long will it be before
only 2 g remains?
Agenda
• Review
• Homework
• Worksheet
Lab for today:
Rutherford’s Marble Experiment?
• Copy into lab notebook
• Statement of Problem: Given 10 marbles of
approximately equal diameter, use a system of
collision and probability to indirectly measure
the average diameter of the marbles.
Rutherford and the Nucleus
Nucleus is positively charged to deflect alpha
particles and to balance electron charge.
Warm – Up!
• What experiment determined the mass and
charge of an electron?
• Dalton concluded that the atom was the
smallest particle of matter. Was he correct?
• What did the gold foil experiment prove?
Today’s Agenda
• Question for today: What does radioactive
mean and what makes certain atoms
radioactive?
• Isotope calcs
• Radioactive particles
• Decay practice
• Homework set 2 due date TBD. Don’t wait until the last
minute!! Final background paper due Friday!
Today’s Agenda
• Question for today: What are the ways that
large elements decay? How harmful are these
particles?
• Alpha, beta, gamma decay, positron emission
and electron capture
• Nuclear forces
• Band of stability
• Quiz tomorrow! Final paper due tomorrow!
• Homework set due TUES. Test Ch 4&24 Thurs.
Other types of decay…
• Nuclei with low neutron to proton ratio
undergo positron emission and electron
capture.
• Positron – particle with same mass as an
electron but opposite charge (e+ or ).
Positron emission = p  n + e+
Electron capture = p + e-  n
both processes decrease # protons to increase
stability
Positron Emission and e- Capture