Transcript 6.2 Atomic Nucleus Stability and Isotopes

Mass Spectrographs
and Isotopes
Isotopes
Isotopes are atoms of the same kind (same number of
protons – same atomic number) which differ in their atomic
masses (have different numbers of neutrons in their
nuclei).
Representing Isotopes : Symbols and Names
Symbols for isotopes are written with the atomic number (#
protons) at the lower left corner of the Atom’s symbol and
the atomic mass (# protons and neutrons) at the upper left
corner of the symbol. The name of the isotope is typically
written with the element’s name followed by its hyphenated
atomic mass.
Lithium-7
Einstein: Mass and Energy Relationship
Albert Einstein predicted that mass can be converted into
energy and energy can be converted into mass by the
relationship, E = mc2. E represents energy in joules, m
is mass in kg and c2 is the speed of light squared (9 x 1016
m2/s2). A small mass converts into much energy due to c2.
Verification of Einstein Theory
The truth of E = mc2 has been confirmed experimentally.
Energy particles of light have been observed to form
positrons (positive electrons) and electrons, while colliding
positrons and electrons annihilate each other forming light
photons. Positrons are rare in our universe.
Mass/Energy Conversion Relatively Rare on Earth
The conversion of energy into mass and mass into energy is
not a common occurrence on Earth since it happens only
in atomic fusion and fission and in a few other processes.
The vast majority of chemical processes involve no
conversion between mass and energy.
Forming Atomic Nuclei: Mass Defect
Experiments show that the mass of the protons and neutrons
bound together in a formed nucleus is always less than the
mass of the same number of protons and neutrons not yet
bound together. The mass that protons and neutrons lose
when they bind together into a nucleus is called the mass
defect.
Nuclear Formation: Energy Loss
When separate protons and neutrons bind together to form a
nucleus, the mass they lose is given off as energy by
Einstein’s relation E= mc2. Combined nuclei are more
stable than separate protons and neutrons because they
give off energy and therefore have a lower energy value.
Binding Energy
To separate the protons and neutrons in a nucleus, much
energy has to be used to pry them apart. The amount of
energy needed to separate nuclear particles is called the
binding energy of the nucleus. The binding energy is equal
to the energy produced from the mass defect (mass loss)
when separate nuclear particles bind together to form a
nucleus. The energy from the mass defect is obtained
from E = mc2.
A Sample Problem
Binding Energy Versus Atomic Number
As the atomic number of an atom increases, the binding
energy per nucleon (per nuclear particle - proton or
neutron) increases up to iron (Fe – atomic number 26).
After Fe, the binding energy per nucleon drops.
Atomic Fusion – Star Power
Gravity force pulling a nebula into a compact ball raises the
temperature (120,000,000 oC) of the nebula to a point where atomic
fusion begins. At fusion temperatures, positive nuclei are forced to
come close enough to each other to allow the strong nuclear force
(which only attracts within one nuclear radius) to pull the separate
nuclei together into a single nucleus, thereby releasing the energy
from the mass defect. The energy stars give off is from the fusion
processes occurring within the stars.
Nebula warmed
through gravity
collapse
Nebula
Fusion starts
and a star is
born
Atomic Fusion – Star Power
The energy stars give off is from the fusion processes
occurring within the stars, driven by extreme temperatures
which force nuclei close enough to be captured by the
strong nuclear force.
Stars Create New Elements
Fusion in stars creates larger and larger nuclei which sink to
the interior of the star due to their greater mass and
density. All the elements of nature (other than hydrogen)
are thought to have been formed by fusion processes in
stars.
Stability of Atomic Nuclei
As an atom’s nucleus gets
larger, the amount of
protons increases. This
causes the repulsive forces
to grow in a nucleus which
would tend to break apart a
nucleus. Adding more
neutrons to large nuclei
separates the repulsing
protons from each other,
stabilizing larger nuclei.
Neutrons Reduce Proton Repulsion
In larger nuclei, adding neutrons brings more stability since
repulsive proton forces are reduced by neutrons
separating protons from protons. (Z and N reversed on this
chart.)
Isotope Stability
and Decay Modes
As the number of nucleons
(protons and neutrons)
in a nucleus grows, more
neutrons are needed per
given number of protons
to make the nucleus
stable. Elements with an
excess of neutrons
undergo beta decay
while elements with a
neutron deficit undergo
positron decay. Heavier
nuclei with a neutron
deficit undergo alpha
decay.
Beta Decay Processes
Beta minus decay converts a neutron into a proton, releasing
an electron and an electron antineutrino.
Beta plus decay converts a proton into a neutron, releasing a
positron (positive electron) and a neutrino.
Beta Minus Animation
Nucleus undergoing beta decay:
Neutron changing into proton
inside the nucleus (particles
shot out of the nucleus)
Neutrons and Protons: Made up of Quarks
Neutrons and protons are particles called baryons. A baryon
is made up of 3 particles called quarks. There are six kinds
of quarks, each having a fractional charge in relation to the
charge of a proton. The most common baryons are
neutrons and protons. Protons are made up of two up and
one down quark (+2/3 + 2/3 – 1/3 = +1) while neutrons are
made up of two down and one up quark (-1/3 -1/3 + 2/3 = 0).
Beta Decay: Changing A Quark
In beta decay, the weak nuclear force mediates a change of
one quark into another. In beta plus decay, an up quark is
converted into a down quark while in beta minus decay, a
down quark is converted into an up quark.
Stability Obtained by Beta- and Beta+ Decay
Elements with too many neutrons/protons tend to decay by
beta minus while elements with too few neutrons/protons
tend to decay by beta plus or positron decay.
Beta
C-14
- Decay
N-14
C-13
C-12
Stable
Isotope
Increasing
neutron number
B-11
C-11
Beta + decay
Increasing proton number
Positron Electron Annihilation
A positive electron (positron) is attracted to an electron and
the two particles annihilate into two photons of gamma ray
energy. Positrons are more rare in our universe which is
dominated by electrons, protons and neutrons. Any
positron generated by decay is quickly annihilated by the
relative excess of electrons.
Alpha Decay Process
In alpha decay, an atom releases an alpha particle (helium
nucleus made up of 2 protons and 2 neutrons). This
process produces a new element that is closer to the
stable isotopes in the previous graphs.
Instability of Very Large Nuclei
Very large nuclei like in uranium atoms tend to be unstable due to
two factors. They have increasing repulsive forces in their
nuclei due to their large numbers of protons and their binding
force per nucleon (proton or neutron) is declining. Uranium
nuclei have just enough binding energy to keep the repulsive
forces from breaking the nucleus apart.
Upsetting the Balance of a Uranium Nucleus
Adding one neutron to a uranium nucleus reduces the
binding energy per nucleon so that now the binding force
is less than the repulsive forces per nucleon. This causes
the nucleus to break apart – atomic fission.
Energy From Fission and Fusion
The energy released from fission and fusion comes from
mass loss. Fission produces less energy than fusion.
Comparing Fusion and Fission
Lighter atomic nuclei can fuse if high temperatures force
them within a distance of a radius, allowing the strong
nuclear force to bind them together. Heavier atomic nuclei
can fission (split) if a particle is added that reduces the
binding energy per nucleon and/or increases the repulsive
force. Fission produces less energy than fusion. Both
processes produce energy by converting energy from
mass by E=mc2.
Separating Isotopes
The mass spectrograph is an instrument that separates
mixtures of isotopes or other charged particles. It was
invented and worked on by Francis Aston from 1925-1937.
Aston won a Nobel Prize for discovering isotopes with it.
How A Mass Spectrograph Works
An element sample is injected and heated to a vapour. An
electron source gives the gaseous atoms a charge. The
charged ions are accelerated by an electric field and then
are passed into a magnetic field. The magnetic field
separates the moving ions by their mass into a series of
bands.
The Spectrograph’s Separating Principle
Moving balls in a wind are deflected on the basis of their
mass with lighter balls deflecting more. Likewise in a
magnetic field, moving particles (each having its own
magnetic field repelling the outside magnetic field) are
deflected more if they are lighter.
External Magnetic Field
wind
Less
massive,
more
deflection
Less
massive,
more
deflection
Moving
Objects
Moving
Ions
Isotopic Separation by a Magnetic Field
The heavier isotopes deflect the least while the lighter
isotopes deflect the most.
Determining Isotope Abundance
Mass Spectrograph detectors record the number of isotopes
hitting the detector as separate bands. When photographic
film is used, a larger band width represents a larger
fraction of that isotope. The % abundance is the isotopic
band width divided by the sum of all the band widths.
Isotope bands for magnesium
Mg-24
78.99 %
Mg-25
10.00 %
Mg-26
11.01%
Another Representation of Isotopic Abundance
Sensors can display intensities of separated isotopes as a
percent of the total intensity to give a reading of relative
abundance of each isotope.
End of Presentation
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