Download Nuclear Reactions Radioactive Decay The stability of an isotope

Nuclear Reactions
Radioactive Decay
The stability of an isotope depends on the ratio of protons and neutrons in the
nucleus. If the ratio does not lie in the curve of stability (dots on the graph), the
nucleus can undergo spontaneous radioactive decay. There are several different
forms of decay, and an atom will usually go through a many decays on its way to
stability.
Radioactive Half-Life
Every radioactive isotope decays at a different rate. The time it takes for half of the sample to decay is known
as the half-life.
e.g. Carbon-14
Half-life (T½) of carbon-14 = 5730 years.
Starting with 1.0 g of carbon-14, in 5730 years, 0.50 g will be remaining.
In 11460 years 0.25 g will remain.
In 22920 years 0.0625 g will remain. Etc…
Mathematically:
mass remaining = initial mass * (½)n
where n = time passed (t) ÷ half-life (t1/2)
OR:
Decay Types
alpha decay (4α)
2 – the spontaneous emission of a helium nucleus from the atom
226
e.g.
Ra → 222Rn + 4He
These particles are easily blocked by a sheet of paper, but readily damage
chromosomes if inhaled or ingested.
beta decay (β-) - a neutron is converted into a proton and a beta particle (an electron
created within the nucleus but otherwise indistinguishable from an orbital
electron)
14
0
e.g.
C → 14
-1 N + β
gamma decay (0γ)
– a gamma particle is a high energy photon which is emitted when
0
the nucleus is in an excited state. Gamma rays are dangerous because it is
hard to block them, but this also makes them useful for medical diagnostics.
Radioactive Emissions
Mass Defect
If you were to take any atom and separate it into its subatomic components (protons, neutrons, electrons) and
weigh them, the sum of the masses of the components will be higher than when they were all together as an
atom. This mass difference is called the mass defect.
Violation of the law of conservation of mass?
No! Due to Einstein’s famous equation, E=mc2, we know that the missing mass changed into energy to keep
the atom together. The law of conservation of mass and the law of conservation of energy are the same
conservation law. Mass and energy are still conserved, as they can be converted into each other.
e.g. The mass of a proton is 1.0073 amu (atomic mass units) and a neutron is 1.0087 amu. This means
that a helium nucleus (with 2 protons and 2 neutrons) should have a mass of 4.032 amu, but its actual
mass is 4.0015 amu.
E = mc2
E = energy (J), m = mass (kg), c = speed of light = 3.0 x 108 m/s
For a large sample of helium that has a mass defect of 0.0305 g,
E = (0.000 030 5 kg)( 3.0 x 108 m/s)2
= 2.75 x 1012 J
was released when creating the atom!
Mass Difference
Atoms that are more stable require less energy to keep them together. By making atoms that are more stable
from atoms that are less stable, the difference in energy can be released and used! The energy difference
between the less stable atoms and the more stable atoms that are created is seen as a difference in mass, as
according to E=mc2.
Nuclear Fission
By taking a large, unstable, atom and breaking it apart into two (or more) more stable atoms, the mass deficit
(difference) will be positive and energy will be released. This is generally achieved when a neutron strikes an
unstable nucleus, causing it to split apart.
e.g. unstable Uranium-235
235
U + 1n → 92Kr + 141Ba + 3 1n + energy
When U-235 is struck by a neutron, it produces 3 more neutrons which then can react with other uranium
atoms creating a chain reaction that can produce massive amounts of energy in under a microsecond. This
process is called Nuclear Fission, and is the principle being Nuclear Power.
Canadian CANDU reactors use natural uranium (mostly U-238) but can also run on other radioactive
materials, including plutonium taken from old nuclear weapons, and used fuel rods from other nuclear plant
designs. CANDU reactors use heavy water (D2O – where D = 2H) to slow down neutrons so they can be more
easily absorbed by the radioactive isotopes to initiate nuclear fission.
238
U + 1n → 239U → 239Np + β- → 239Pu + β- → 235U + 4He
Nuclear Fusion
By taking two (or more) smaller atoms and combining them into a large and more stable atom, the mass
deficit (difference) will positive and energy will be released. This is called nuclear fusion. Nuclear fusion
creates a larger difference in mass than nuclear fission, thus releases even more energy. Nuclear fusion is the
process by which our sun (and all other stars) create their energy.
e.g. the sun
The sun consists of a series of layers where different nuclear fusion reactions take place. In the sun’s
core, hydrogen atoms are combined to create helium atoms.
1
H + 1H → 2H + β +
2
H + 1H → 3He
3
He + 3He → 4He + 2 1H
In other layers helium combine to form beryllium, beryllium combine with helium to form carbon and
carbon combine with helium to form oxygen
Iron is the most stable atom, so nuclear fission only
releases energy for atoms larger than iron. Similarly,
nuclear fusion can only occur up to element 26
(iron), but after this point, the amount of energy
required to fuse the nuclei in stars is too large to be
sustainable. This stops the reactions, in turn
stopping the outward pressure from the gases
formed, leading to a collapse of the star and finally a
supernova. In a supernova, the pressure of the
collapsing star generates the energy needed to
produce all of the elements higher than iron.
Example: fusion