Radioactivity
Nuclear Chemistry
Discovery of RadioactivityWilhelm Roentgen discovered x-rays in
1895.Henri Becquerel discovered that
uranium salts produced similar emissions.
Marie and Pierre Curie isolated the source on the emissions in Becquerel's sample—uranium.
Marie Curie named the process of emitting rays—radiation.
Changes in the Nucleus
A change in the nucleus of an atom is called a nuclear reaction.
These reactions are associated with radioactivity – the emission of radiation in the form of rays and particles.
Nuclear reactions occur because of nuclei that are unstable.
Unstable isotopes are referred to as radioisotopes.
When is a nucleus stable?
The strong nuclear force holds the protons and neutrons together in the nucleus of an atom.
The electrostatic charge of the protons is repulsive, but the presence of the neutrons adds a stronger attractive force.
To be stable, a nucleus must have from 1 to 1.5 neutrons for each proton. Atoms with more protons need the higher number of neutrons.
TransmutationThe changes in the nucleus can sometimes
transform an element into a different element The result = change in atomic number
An element is considered radioactive because it releases
Energyduring transmutation
Artificial TransmutationChange in nucleus occurs when
bombarded by an alpha particle or neutron
Natural TransmutationChange in nucleus occurs spontaneouslyThe ONLY reactant is the atom undergoing
transmutation
Ca-44 + n Ca-4510
U-238 Th-234 +
Stability of NucleiMost nuclei are stable ex. C-12As atoms increase in size the ratio of
neutrons to protons increases the larger the ratio the more unstable the atom ex. C-14
All nuclei with atomic numbers > 83 are unstable and radioactive
Unstable nuclei will spontaneously decay in order to form a
stable nucleus
Types of RadiationName Symbol Mass
(amu)Charge Description
Alpha 4 2+ a helium nucleus
Beta 1/1840 1- electrons
Gamma 0 0 short wavelength electromagnetic waves
Radioactive Shielding
Alpha Particles
or Hehelium nucleus
HeaviestTravels slowest & least amount of distance
Poor penetrating power MOST DAMAGING
42
Alpha DecayA type of radioactive decay where
an atomic nucleus emits (releases) an alpha particle (2 p+ and 2 no).
The atom reduces its atomic number by 2 and its mass number by 4. This transforms the atom into a different element.
238 92U
234 90Th
4 2He
Alpha Decay
U238 92
He
Th
42
234 90
Alpha Particle is one productmass and charge
must be conserved
Beta Particles
ß-
Electrons
travel near the speed of lightmoderate penetrating power
can be stopped by aluminum
e 0
-1
Beta DecayA type of radioactive decay where
a beta particle (electron or positron) is emitted.
This causes a neutron to be turned into a proton.
Beta Decay
Gamma Radiation
No mass no charge
similar to x-rays
Fastest type of radiationStrongest penetrating powernot very dangerous
Where did they come from?Uranium DecayAtoms of U-238 go through a series of
decays to make Thorium Th Radon Rn Radium Ra Lead Pb
Example of natural decay
ß ß
ß ß
ß ß
Radioactive Decay RatesOver time, radioisotopes will decay
into stable atoms. The time this takes is measured in half-lives.
A half-life is the time it takes one-half of radioisotope to turn into stable atoms.
Radioactive Decay Rates (cont’d)
Example: 20 g of the strontium-90 will decay into 10 g over 29 years.
Amount remaining = (Initial amount)(½)t/T
t = total time T = half-life
0 1 2 3 4Number of half-lives
Rad
iois
otop
e re
mai
ning
(%
)
100
50
25
12.5
Initial amountof radioisotope
t1/2
t1/2
t1/2
After 1 half-life
After 2 half-lives
After 3 half-lives
Half-life of Radiation
