nuclear chemistry chapter 22 notes. the nucleus nucleons – the particles found in the nucleus of...
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Nuclear Chemistry
Chapter 22 Notes
The Nucleus• Nucleons – the particles found
in the nucleus of an atom; protons and neutrons.
• In nuclear chemistry, the atoms are called nuclides. Each one is identified by its name and mass number.– Radium-288– Ra288
88
Mass Defect• For a helium atom, we have 2 protons, 2
neutrons and 2 electrons. – 2 protons: 2*1.007276 amu = 2.014552 amu– 2 neutrons: 2*1.008665 amu = 2.017330
amu– 2 electrons: 2*0.0005486 amu = 0.001097
amu
• The sum of these is 4.0032979 amu.
Mass Defect• We’d expect an atom of helium-4 to
have a mass of 4.032979 amu.• It actually has a mass of 4.00260
amu, which is 0.03038 amu less than expected.
• This difference is known as the mass defect.
• Where does this “lost” mass go? Isn’t mass a conserved quantity?
Nuclear Binding Energy
• We can use E=mc2 to calculate the amount of energy “held” by the mass that was lost.– Convert the mass to kilograms first
• m = 5.0446x10-29 kg
– E = (5.0446x10-29 kg)*(3.00x108 m/s)2
= 4.54x10-12 J
• The nucleus of each atom is held together by this nuclear binding energy.
Nucleons and Nuclear Stability
• What makes a nucleus stable?– It doesn’t undergo nuclear reactions.– It has enough neutrons to overcome the
repulsion of the protons.– It lies on the band of stability.
• Band of Stability: the stable nuclei cluster over a range of neutron-proton ratios
Band of Stability
Nuclear Reactions• Nuclear reaction – a
reaction that affects the nucleus of an atom
• Transmutation – a change in the identity of a nucleus as a result of a change in the number of its protons
Radioactive Decay• Radioactive decay –
the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation or both
Radioactive Decay• Nuclear radiation – particles or
electromagnetic emitted from the nucleus during radioactive decay
• Radioactive nuclide – an unstable nucleus that undergoes radioactive decay
Alpha Particles• Contain two protons and two neutrons• Very low penetrating power – stopped
by a piece of paper• Written as: αor Heor α 4
242
Beta Particles• High speed electrons• Medium penetrating
power – stopped by thin metal foil
• Released when a neutron decays into a proton and an electron
• Written asβor eor β 0
1-01-
Gamma Rays• High speed radiation
without mass or charge
• Strong penetrating power – partially stopped by thick lead and/or thick concrete
• Causes tissue damage
• Represented byγ
Nuclear Reaction Equations
• Shows the changes occurring within the nuclei
• The types of atoms will change, but two things must be constant:– The sum of the mass number– The sum of the atomic number
Nuclear Reaction Equations
HeThU 42
23490
23892
Types of Radioactive Decay
• Alpha Decay• Beta Decay• Positron Emission• Electron Capture
Alpha Decay• Releasing alpha particles – very
heavy nuclei getting rid of mass to increase stability
• Alpha decay of radium-226
226 88 Ra + 4
2 He222 86 Rn
Beta Decay• Elements above the band of stability
are unstable because they have too many neutrons– One neutron is converted into a proton
and an electron– The electron is released from the nucleus
• Beta decay of carbon-14
14 6 C +
0-1 β14
7 N
Positron Emission
15 8 O + 0
+1 β ?157 N? ?
• Elements below the band of stability are unstable because they have too many protons– One proton is converted into a neutron by releasing
a positron– A positron is a beta particle with a positive charge– The positron is released from the nucleus
• Positron emission of oxygen-15
Electron Capture• Another way of stabilizing a nucleus
with too many protons• Capturing an electron from the inner
orbitals– The electron combines with a proton to
make another neutron
• Electron capture of rubidium-81
8137 Rb + 0
-1 e 8136 Kr
Gamma Emission• Gamma rays are released when
nucleons drop to a lower energy level– Similar to the light released when
electrons drop to a lower energy level
• Released whenever another type of radioactive decay occurs
Half-Life• Half-life, t½: the time required for half
the atoms in a radioactive material to decay
• Half-life is an indication of the stability of a nuclide– Short half-life means very unstable
(microseconds)– Long half-life means very stable (billons
of years)
Half-Life Equation
2
1t
t
o 2
1mm
mass
Original mass
Time
Half-life
Example 1• The half-life of polonium-210 is 138.4
days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?
Example 1• The half-life of polonium-210 is 138.4
days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?
2
1t
t
o 2
1mm
Example 1• The half-life of polonium-210 is 138.4
days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?
days4.138
days2.415
2
1mg0.2m
Example 1• The half-life of polonium-210 is 138.4
days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?
mg25.0m
Example 2• The half-life of uranium-238 is
4.46x109 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?
Example 2• The half-life of uranium-238 is 4.46x109
years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?
2
1t
t
o 2
1mm
Example 2• The half-life of uranium-238 is 4.46x109
years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?
years
years
gm9
10
1046.4
1000.1
2
100.4
Example 2• The half-life of uranium-238 is 4.46x109
years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?
g845.0m
Decay Series• Decay series – a series of radioactive
reactions that begins with a radioactive nuclide and ends with a stable one
• Parent nuclide – the heaviest particle in a decay series
• Daughter nuclides – all of the nuclides produced in a decay series
• Page 710 shows the decay of uranium-238 into lead-206
Artificial Transmutations
• Artificial transmutations – causing a nucleus to undergo a radioactive reaction– Hit the nucleus with high speed particles– The particles go into the nucleus,
causing it to become unstable– The unstable nucleus undergoes
radioactive decay to return to stability
Fermilab (Illinois)4 miles in diameter!
*photo from Google Map
Transuranium Elements
• Elements with atomic numbers higher than 92 (past uranium)– Very unstable – they decay within
microseconds– Only found in laboratories now– Were probably found in the very
beginning of the universe, but they all decayed a long, long time ago
Radiation Exposure• The energy in nuclear radiation can
be transferred to atoms in your cells, causing damage to your body.
• The amount of radiation is measured in units called rems
• Being exposed to a higher amount of radiation = more damage, therefore you’d be more likely to get radiation poisoning or cancer
Radiation Detection• Film badges – use exposure to
approximate radiation exposure• Geiger-Mϋller counters – count the
ions made by radiation to measure its strength
• Scintillation counters – measure the light made by radiation to measure its strength
Applications of Nuclear Radiation
• Radioactive dating – use the concentration of radioactive nuclides to approximate the age of an object.
• Radioactive tracers – use radioactive atoms in medical procedures to follow their pathway through the body
• In agriculture, radioactive atoms can be used to test the effectiveness of fertilizers.
Nuclear Waste• Nuclear waste – radioactive products
of nuclear reactions• It must be contained to protect
humans from the radiation– Storage: store on-site if the half-life is
short (ponds or dry casks)– Disposal: put the waste somewhere
isolated, with no intentions of ever touching it again (Yucca Mountain)
Nuclear Fission• Splitting of nucleus
into fragments• Involves heavy
atoms breaking down into lighter atoms
• Fat man and little boy were fission bombs
Nuclear fission of uranium-235
10 n 235
92 U
10 n
+
9236 Kr 141
56 Ba+ +
Notice change from heavy atom to lighter atoms
3
Fission Chain Reaction• Multiple fission reactions; initiated by
new neutrons released• LOTS OF ENERGY PRODUCED• Example: atomic bomb, nuclear
power plant
Nuclear Power Plant• Generates steam by
nuclear fission; steams drives turbines to produce electricity– Pros: less fuel, fewer
pollutants– Cons: radioactive waste,
more catastrophic plant accidents
Nuclear Fusion• Combining two or more small nuclei
into one stable nucleus
• Example: the Sun (or stars)
hydrogen
Helium +
ENERGYFUSION
Hydrogen Bomb
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