Chem 2
Ch # 23: Nuclear Chemistry.
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Radioactivity: An unstable nucleus spontaneously disintegrates or decays by
emitting radiation.
Nucleus contains protons and neutrons. They are composed of quarks.
Nucleus contains nucleons.
Atomic number: Z: Number of protons in a nucleus.
Atomic mass number: A: Number of protons + Number of neutrons.
Isotopes: Atoms having same atomic number and different atomic mass numbers.
Nuclide: A unique atom represented by:
Types of Radioactive Decay:
1. Alpha particle: Helium nucleus.
 Dense , positively charged particles.
 A decreases by 4, Z decreases by 2.
 Every element heavier than Pb and some lighter ones also.
2. Beta particle production: Mass number remains constant. An electron is released.
 Ex:
 Unstable nuclide creates an electron as it releases energy.
 A remains unchanged , Z increases by 1.
 Next higher atomic number is formed.
3. Gamma ray: High energy photon.
 Nucleus from excited state goes to ground state.
 They have no mass or charge, emission does not change A or Z.
4. Positron Production:
 Has same mass as e, but opposite charge.
 Antiparticle of an electron.
 Proton converted to a neutron.
 A remains unchanged, Z decreases by 1.
 Lower atomic number is formed.
5. Electron Capture:
 One of the inner orbital electrons is captured by nucleus.
 Nuclear proton is transferred into a neutron.
 A is unchanged, Z decreases by 1.
 Gamma rays are always produced in e capture.
Write balanced equations for the following:
1) C produces a positron.
2) Bi produces a beta particle.
3) Np produces an alpha particle.
4) Supply the missing particles:
Ga → Ge + ?
Pt → Os + ?
Bi →
Pb + ?
Cm + ? →
Am
Nuclear stability and the Mode of Decay :
 N/Z= 1, for lighter nuclei to be stable.
 In heavier nuclei N/Z ratio > 1.
 Very few stable nuclides exist with N/Z < 1 (H, He)
 Lighter nuclides are stable.
 N/Z ratio increases as Z increases.
 Z > 83 all nuclides are stable.
 Elements with even Z have larger number of stable nuclides.
Predicting the Mode of Decay
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Neutron-rich nuclides undergo β decay.
Neutron-poor nuclides undergo positron decay or electron capture.
Heavy nuclides undergo a decay.
The Kinetics of Radioactive Decay:
Rate of decay: Negative of the change in the number of nuclides per unit time.
rate = kN, K is the decay constant.
The rate of decay is proportional to the number of nuclides. This represents a first-order
process.
Decay rate (A) = -∆N/∆t
SI unit of decay is the becquerel (Bq) = 1d/s.
curie (Ci) =
number of nuclei disintegrating each second in 1g of radium-226 =
3.70x1010d/s
Large k means a short half-life and vice versa.
Half-Life:
...
the time required for the number of nuclides to reach half the original value
(N0/2).
ln(2 )
0.693

k
k
Technetium-99m(m=excited nuclear state to GS emits gamma particle) has a rate
t1/ 2 
4)
constant 1.16 x 10-1 /h. What is the half life of the nuclide Tc?
5) Halflife of Mo 99 is 67.0 h. How much of a 1.000mg sample of 99 Mo is left
after 335 h?
Detection and Uses of Radioactivity:
 GM counter: Probe filled with Ar gas which is ionized by moving particles.
 Scintillation counter: ZnS give off light when they are struck by high-energy
radiation.
Dating by Radioactivity:
 Radiocarbon dating/C-14 dating: Based on radioactivity of C which decays by
beta particle production.
 Reactions:
 C-14 can be used to date wood and cloth aircrafts.
 Half life of C is 5730 years.
 As a plant is cut C/ C ratio decreases.
 C has to be burnt to give CO2.
 A small amount can be used in mass spectrometer, C atoms are ionized and
accelerated through a magnetic field that deflects their path.
 Earth’s crust formed 4.3 bya.
6) The remains showed a C decay rate of 3.1 counts per min per gram of C.
Assuming that decay rate of C in freshly cut wood is 13.6 counts per min per gram
of C, calculate the age of remnants. The half life of C is 5730 years.
7) The charred bones of a sloth in a cave in Chile represent the earliest evidence of
human presence in the southern tip of South America. A sample of the bone has a
specific activity of 5.22 disintegrations per minute per gram of carbon (d/min*g). If the
ratio of 12C:14C in living organisms results in a specific activity of 15.3 d/min*g, how
old are the bones? (t1/2 of 14C = 5730 yr)
Particle accelerators and the Transuranium elements:
 Neutrons are used as projectiles, no charge, not repelled.
 Particle accelerators were used to give particles a higher K.E.
 Linear accelerator: Series of tubes of increasing lengths that through a source of
voltage change their polarities.
 Cyclotron: Uses electromagnet to give the particle a spiral path.
Effects of Nuclear Radiation of Matter:
1. Excitation:
 Radiation of relatively low energy interacts with an atom of a substance, which
absorbs some energy and reemits it.
 Radiation that causes excitation is called nonionizing radiation.
2. Ionization:
 Radiation collides with an atom to remove an electron.
 Called ionizing radiation.
 Cation-electron pairs result.
Effects of Ionizing Radiation:
1. Gray: (Gy) = 1 Joule of energy absorbed per kg of body tissue.
Rad(radiation-absorbed dose) =0.01 Gy
Rem (roentgen equvivalent for man) no of rems= no of radiations x RBE.
SI unit for dosage equvivalent is Sv- sievert . 1 rem= 0.01 sv.
2. Penetrating power of Emissions: penetrating power is inversely related to mass and
charge of the emission. Gamma rays are highly penetrating, β particles can penetrate
approx. 1cm. α particles are stopped by the skin.
3. Molecular interactions: Ionizing radiation causes free electrons, which produces free
radicals in water molecules. Lipids are attacked by radicals. This can lead to cell damage
as well as cancer. Genetic mutations can occur when bonds in the DNA of sperm and egg
cells are altered by free radicals.
4. People living near test centers, nuclear energy facilities, and waste disposal areas are
exposed to more radiation.
Applications of Radioisotopes:
 Tracer- emits nonionizing radiation that signals the presence of a substance.
 NAA(Neutron activation analysis) –to detect tracers of ammunition on a suspects
hand
 traces of hair in a victim of poisoning.
 Meaure friction in the engine.
 Used in detection of thyroid- patient drinks radiotracers of Na_I.
 Technetium-99 is also used for imaging heart, lungs and liver.
 Fe-59 used to detect hemoglobin in blood cells.
 PET is used for imaging brain and its functions.
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Thermodynamics Stability of the Nucleus:
When a system gains or loses energy it also gains or loses a quantity of mass.
E = mc2
E/c2= m E
m = mass defect 2  m
c
E = change in energy
If E =  (exothermic), mass is lost from the system.
Binding Energy: is the energy required to decompose the nucleus into its components.
Iron-56 is the most stable nucleus.
It has a binding energy per nucleon of 8.79 MeV
Nuclear Fission and Nuclear Fusion:
Fusion: Combining two light nuclei to form a heavier, more stable nucleus.
3
2 He
 1
1H 
4
2 He

0
1e
Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers.
1
0n

235
92 U

142
56 Ba

91
36 Kr
A self-sustaining fission process is called a chain reaction.
 301 n