Nuclear Chemistry
v
In Chemical
Reactions: atoms are rearranged
to form different compounds. There is no change in the number or type of atoms
v In Nuclear Reactions: there are changes in the nucleus and
thus the type of atom can change. The study of x-rays lead to the discovery
of radioactivity.
v Transmutations:
when one element changes to an entirely different element as brought
about by a change in the nuclei
v Radioactivity or Radioactive decay:
radiation is produced while the nucleus of certain elements spontaneously
disintegrate to produce other elements
Nucleus
v Contains protons and neutrons packed together in a very
small space. Why don’t the protons
repel and fly apart?
v Strong Force: an attractive force that holds nucleus
together
v Neutrons have no charge and thus only the strong force acts
on them. Thus, neutrons act as glue to
hold nucleus together.
v As atomic number (and # of Protons) increases
proportionally more neutrons are needed for the atom to remain stable
Type of Radiation:
Rutherford’s experiments
v Alpha (á) Radiation: Alpha particles are similar to the nuclei
of helium-4 atoms (charge = +2, mass =4).
Their speed is 1/10 the speed of light, they can only travel a few
centimeters in the air, they can ionize gas particles in the air and they have
low penetrating ability and can be stopped by a single sheet of paper.
v Beta (b)
particles: a stream of
very high speed electrons (often near the speed of light). They have a mass of 0.00055 amu and a charge
o –1. They have greater penetrating power
than alpha particles but less ionizing ability. They can be stopped by a thin sheet of aluminum or by heavy
clothing.
v Gamma (g) rays:
are a type of electromagnetic radiation. They are similar to x-rays but have more penetrating power than
x-rays, á particles or b particles. It takes several centimeters of lead and even more iron to block
them. They can ionize atoms in flesh
and cause severe damage to cells. They
travel at the speed of light.
Natural Radioactivity
In nature most elements have more
than one isotope. Some isotopes are
unstable. Unstable Isotopes are called Radioisotopes
v They decay and emit radiation from their
nuclei
v Usually only certain isotopes are
radioactive: H-1 is stable, but H-2 and
H-3 are unstable
v After a series of radioactive decays, the
atoms become stable and stop decaying
v This process transmutes an atom into a new
element
v A sample that starts as pure uranium
gradually becomes contaminated with other elements that are produced as a
result of nucleur decay
Nuclear Equations
Visually represent nuclear decay
v á decay
v á particles have 2 protons and 2 neutrons
v Mass decreases by 4 and atomic # decreases by 2
Nuclear Equations
v b decay: converts a
neutron to a proton
v Mass is unchanged
v Atomic number increases by 1
Half-Life
v Half-Life: Time it
takes for half the atoms in a radioactive sample to decay
v Indium -115 has a half-life of 4.5 hours. If you start with a 12.0 mg sample, how much
Indium-115 is left after 13.5 hours?
4.5
hours: 6.0 mg
9 hours: 3.0
mg
13.5 hours: 1.5
mg
Half-Life
v A 7.500 mg sample of Chromium-55 is analyzed after 14.0
min. and found to contain 0.469 mg of Chromium-55. What is the half-life:
1 half
life: 3.750
mg
2 half
lives: 1.875 mg
3 half
lives: 0.9375 mg
4 half
lives: 0.469 mg
14.0 min = 4 half lives: Half life = 14.0/4 = 3.5 min
Radioactive Dating
v Uranium-238 (half life = 4.5 x 10 9
years)
v Useful way to date relics, ages of rocks,
etc. Based on the assumption that the
Earth is as old as the oldest rock
v It works by comparing the amount of U-238
(unstable) with the amount of Pb-206 (stable decay product)
v Carbon Dating: Half-life of C-14 is 5700 years.
This shorter span is useful for dating organic remains up to 60,000
years old.
v The ratio of C-14 to C-12 in the atmosphere
has remained essentially unchanged over time.
All living organisms take in Carbon atoms. Once they die, the C-14 atoms decay at a predictable rate. Thus, age can be calculated based on the
ratio of C-14 to C-12 atoms.
Artificial Radioactivity
v Nuclear Bombardement: a
normally stable element is bombarded with particles (such as alpha particles or
neutrons), some of the particles combine with the atom’s nucleus to form a new
(unstable) nucleus.
v Examples:
Particle Accelerators
Machines that accelerate alpha particles so that
they are moving fast enough to overcome forces of repulsion and collide with
the nucleus
v Cyclotron:
particles go around and gain energy
v Synchotron: focuses
beams with an electromagnet
v Linear accelerator:
Uses synchronized field of electric force
Radioactive Particles
v Positron: an
induced particle that occurs in the lab and is made up of a positive electron
v Antiparticles:
particles that are opposite in charge but have the same mass number
v Anti matter: a
collection of antiparticles
Biological Effects of Radiation
v Radiation from the sun can cause skin
damage. This amount of radiation is
measured in rems
v Somatic effects: individual cells are damaged
v Genetic effects: damage occurs that can be passed on to offspring
v Beneficial uses:
v Radioisotopes can be used as tracers when
they replace isotopes in a sample of tissue, geological piece, etc.
v Radiation can kill dividing cancer cells
Nuclear Fission:
Atom Splitting
v A large nucleus is split into 2 smaller
nuclei of approximately the same mass.
v Mass is not conserved. A very small amount of mass is converted
into a large amount of energy:
v E = mc2 c = the speed of light = 3 x 108
m/s2.
v only U-235 is useful in fission.
v Basis of a chain reaction: Three neutrons
released along with the creation of two smaller equal mass products with each
reaction. Each neutron causes three
more nuclear fission reactions which
causes more fission.
v The critical mass is the amount of
fissionable material that will support a self sustaining chain reaction.
Fission Reactors
v A device in which a fission reaction is
carried out at a controlled rate:
v Fuel is U-235
v control rods absorb neutrons and thus
regulate the speed of the nucleur chain reaction
v Uses:
heat produced is used to make steam for elecricity. This type of reactor is also used to power
submarines and surface vessels.
v Problems:
Radioactive waste and heat pollution (hot cooling water released into
lakes or rivers can kill fish and plants)
Fusion Reactors
v Two small nuclei join to form a large nucleus.
v As in fission, some mass is converted into a great deal of
energy
v Reactor is still in development. It is difficult to force two atoms together because of repulsion
of electrons
v Extremely high temperatures are used to strip electrons off
H atoms. “Sea” of bare nuclei that results is called a plasma.
v Thermonuclear device:
explosive device based on fussion (the hydrogen bomb)