period Flashcards
Characteristics of actincides
Partially filled 5f shell
* 2-4 valence electrons in atomic 6d and 7s states
* Metals have conduction band structure more
complicated than transition metals or the rare earth metals because of the unusual nature of
their atomic structure.
physical properties
There are no stable isotopes of these elements.
* Actinides have a strong electro-positivity.
* Metals tarnish quickly in the air. All are pyrophoric.
* Actinides are metals that are extremely dense and
have different structures.
chemical properties
Actinides show variable oxidation states because of
the smaller energy gap between 5f, 6d, and 7s
orbitals.
* Other oxidation states are possible despite the fact
that 3+ is the most stable oxidation state.
shielding
The ionic radius of
actinides decreases
regularly along the series.
* Decrease in ionic radius of
actinides is called actinide
contraction.
* Due to the poor screening
effect of the inner
electrons with increasing
proton number so the
outer electrons are not so
well shielded from the
effects of nuclear charge
light An and overlap
Dominant role f shell electrons can have on the properties illustrated by looking at the band overlaps.
* Multiple bands always form when an atom has more than one valence electron.
* Illustrates how the overlap of s, p, d, and f states allows them to hybridize with each other.
* Narrow f band dominates at the Fermi energy and therefore has many more states at that energ
Electron density in an atom and radial characteristics
Valence electrons are the ones that ultimately
dictate chemical behavior.
* The 4f electron density lies close to nucleus and
do not participate in bonding.
* The 6d, 7s, 7p electrons are less affected by
nuclear charge and are delocalized and
participate in bonding.
* The 5f electron density extends much further
from nucleus (greater radial extension).
* Net effect is 5f electrons of actinides can more
readily participate in bonding than 4f electrons
n Plutonium Allotropes
Six allotropic phases of
Pu exhibit anomalous
thermal and volume expansion
Volume increase due to
partial delocalization of
5f electrons in delta
phase
why is knowing plutonium oxidation important?
Important for many reasons:
* Processing and purification of
pure plutonium metal
* Managing our nation’s nuclear
wastes
* Predicting its behavior in the
environment
* Predicting the effects of aging
& safety of nuclear weapon
why is Pu at crossroads?
Unusual electronic properties
of plutonium, stemming from
the dominant role of its
narrow 5f band.
* Stands at the crossover
between materials whose
broad-band electrons form
superconducting ground
states and magnetic materials
beta decay
Beta decay is a bitwise process toward
stability for an actinide.
Involves a down quark converting to and
up quark
Conversion of a neutron to a proton with
emission of an electron/positron
Fissionable material -
isotopes capable of
undergoing nuclear fission after neutron
capture.
*Fertile material
consists of isotopes that are
not fissionable by thermal neutrons but can be
converted into fissile isotopes (after neutron
absorption and subsequent nuclear decay).
Typical fertile materials:
*Fissile material
consists of fissionable
isotopes capable of undergoing nuclear
fission only after capturing a thermal
neutron.
critical energy
Ecrit large for light nuclei Z < 90.
* Ecrit for heavy nuclei with Z > 90,
the critical energy is about 4 to
6 MeV for A-even nuclei and
generally is much lower for Aodd nuclei.
odd nuclear
Odd-mass nuclei (A-odd) do not exhibit the same degree of nucleon pairing, and their binding energy per nucleon
may be somewhat lower, leading to a lower fission threshold energy.
nuclear binding energy
At the nuclear level, the nuclear
binding energy is the energy
required to disassemble* a
nucleus of an atom into its
component parts.
