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Chapter 4
Atomic Structure
Warm – Up
We have not discussed this material, 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.
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. Rutherfords
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.
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
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!
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!
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.
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, NO charge!
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!
Atomic Model
Protons = Electrons
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?
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 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
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.
K
Potassium
Three isotopes = 39
K
19
Percent Composition: 93.26%
K
19
40
0.01%
K
41
19
6.73%
Calculate the Atomic Mass of K
1. Use % composition and convert to relative
abundance (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
Amu = (R.A.)x(Mass) + ((R.A.)x(Mass))…
• What element is this?
Isotope Mass of Isotope
Percent abundance
6X
6.015 amu
7.59%
7X
7.016 amu
92.41%
Find the atomic mass
What element is this? (Use the Periodic 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.
• 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
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.
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.
– Change in the atom’s nucleus.
• 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 – (remember the gold foil
experiment?!?!) made up of POSITIVE “alpha
particles”.
• 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
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
Penetrating Power of Radiation
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.
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 we need more neutrons to
stabilize the nucleus!!!
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!
Decay Practice
4He +
238Pu
decay
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
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!!
• 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.)
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.
Half Life Calculations
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-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 = 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 10 mg
sample of cobalt-60 is left over after 10.54
years? After four half lives?
N = N0 (½)n
N = 10 mg (½)10.54/5.27
N = 10 mg (½)4
Half – Life Calculations
• Do the problem intuitively…
Think about how many half lives have passed
and just do the division
Two half lives (10 mg/2)/2 = 2.5 mg
Four half lives 10 mg/2/2/2/2 = 0.625 mg
Warm - Up!
• 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 197 years. How
much remains of a 2.0 g sample after 10 years?
• Strontium has a half life of 29 years. How long
will it take for a 56 g sample to decay to 1.75 g?
Nuclear Reactions
• Induced Transmutation – FORCE an element to
change its identity by bombarding it with
radioactive particles!
4He + 14
N
2
7
17O
8
+ 11 H
• Particle accelerators move particles at
extremely high speeds to overcome repulsive
forces.
Nuclear Fission
• Splitting of a nucleus into fragments - LARGE
release of energy!
• Large atoms want to be smaller for stability
(Atomic # larger than 60 = large atoms)
Kr - 92
neutron
U-235  U-236
Ba - 141
Neutrons Perpetuate Fission
Critical Mass
• Fissionable material must have sufficient mass to
split
– Not massive enough – subcritical – no chain reaction
– Extremely massive – violent nuclear reaction
Violent Nuclear
Reactions
Subcritical masses that get
together to form supercritical
mass.
Equal to 20,000 tons of TNT
Nuclear Reactors
Reactor core controlled by Cd or B to absorb neutrons
Assignment : Look up Chernobyl Nuclear Power Plant!
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 waste
• What happens to all the radioactive waste
accumulated at a power plant?
• Treated with advanced technologies so the
material doesn’t deteriorate
• Sealed and buried underground
• 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.