Transcript RFSS: Lecture 15 Americium and Curium Chemistry • Readings: Am and Cm chemistry chapters  Nuclear properties  Production of isotopes  Separation and purification  Metallic state 

RFSS: Lecture 15 Americium and Curium
Chemistry
• Readings: Am and Cm
chemistry chapters
 Nuclear properties
 Production of isotopes
 Separation and
purification
 Metallic state
 Compounds
 Solution chemistry
 Coordination chemistry
15-1
Production of Am isotopes
•
•
•
•
•
Am produced in reactors from neutron irradiation of Pu
239Pu to 240Pu to 241Pu, then beta decay of 241Pu

241,243Am main isotopes of interest

Long half-lives

Produced in kilogram quantity

Chemical studies

Both isotopes produced in reactor
241Am

source for low energy gamma and alpha
 Alpha energy 5.44 MeV and 5.49 MeV

Smoke detectors

Neutron sources
 (a,n) on Be

Thickness gauging and density
242Cm production from thermal neutron capture

243Am

Irradiation of 242Pu, beta decay of 243Pu
Critical mass
242Am in solution

 23 g at 5 g/L
 Requires isotopic separation
15-2
Am solution chemistry
•
•
•
•
Oxidation states III-VI in solution

Am(III,V) stable in dilute acid

Am(V, VI) form dioxo cations
Am(II)

Unstable, unlike some lanthanides (Yb, Eu, Sm)
 Formed from pulse radiolysis
* Absorbance at 313 nm
* T1/2 of oxidation state 5E-6 seconds
Am(III)

Easy to prepare (metal dissolved in acid, AmO2
dissolution)
 Pink in mineral acids, yellow in HClO4
when Am is 0.1 M
7
5

F0 L6 at 503.2 nm (e=410 L mol cm-1)

Shifts in band position and molar absorbance
indicates changes in water or ligand coordination

9 to 11 inner sphere waters
 Based on fluorescence spectroscopy
* Lifetime related to coordination
 nH2O=(x/t)-y
 x=2.56E-7 s, y=1.43
 Measurement of fluorescence
lifetime in H2O and D2O
Am(IV)

Requires complexation to stabilize
 dissolving Am(OH)4 in NH4F
 Phosphoric or pyrophosphate (P2O74-)
solution with anodic oxidation
 Ag3PO4 and (NH4)4S2O8
 Carbonate solution with electrolytic
oxidation
15-3
Am solution chemistry
•
•
•
•
Am(V)

Oxidation of Am(III) in near neutral
solution
 Ozone, hypochlorate (ClO-),
peroxydisulfate
 Reduction of Am(VI) with bromide
5

I43G5; 513.7 nm; 45 L mol cm-1
5I 3I ; 716.7 nm; 60 L mol cm-1

4
7
Am(VI)

Oxidation of Am(III) with S2O82- or Ag2+
in dilute non-reducing acid (i.e., sulfuric)

Ce(IV) oxidizes IV to VI, but not III to VI
completely

2 M carbonate and ozone or oxidation at
1.3 V

996 nm; 100 L mol cm-1
 Smaller absorbance at 666 nm
Am(VII)

3-4 M NaOH, mM Am(VI) near 0 °C

Gamma2-irradiation 3 M NaOH with N2O
or S2O8 saturated solution
Am(VII)

Broad absorbance at 740 nm
15-4
Am solution chemistry
•
Am(III) luminescence
7F 5L at 503 nm

0
6
 Then conversion to
other excited state

Emission to 7FJ
5D 7F at 685 nm

1
1
5
7

D1 F2 at 836 nm

Lifetime for aquo ion is
20 ns
 155 ns in D2O

Emission and lifetime
changes with speciation
 Am triscarbonate
lifetime = 34.5 ns,
emission at 693 nm
•
•
•
•
•
Autoreduction
Formation of H2O2 and HO2 radicals from
radiation reduces Am to trivalent states

Difference
between 241Am and
243Am
Rate decreases with increase acid for
perchloric and sulfuric
Some disagreement role of Am
concentration

Concentration of Am total or
oxidation state
Rates of reduction dependent upon

Acid, acid concentration,

mechanism
 Am(VI) to Am(III) can go
stepwise

starting ion
 Am(V) slower than Am(VI)
15-5
Am solution chemistry
•
•
Disproportionation

Am(IV)
 In nitric and perchloric acid
 Second order with Am(IV)
* 2 Am(IV)Am(III) + Am(V)
* Am(IV) + Am(V)Am(III) +
Am(VI)
 Am(VI) increases with
sulfate

Am(V)
 3-8 M HClO4 and HCl
* 3 Am(V)
+4
H+Am(III)+2Am(VI)+2 H2O
 Solution can impact oxidation state
stability
Redox kinetics

Am(III) oxidation by peroxydisulfate
 Oxidation due to thermal
decomposition products
* SO4.-, HS2O8 Oxidation to Am(VI)
 Acid above 0.3 M limits oxidation
* Decomposition of S2O82 Induction period followed by reduction
 Rates dependent
upon temperature,
[HNO3], [S2O82-], and [Ag+2]
 In carbonate proceeds through Am(V)
* Rate to Am(V) is proportional to
oxidant
* Am(V) to Am(VI)
 Proportional to total Am
and oxidant
 Inversely proportional to
K2CO3
15-6
Am solution chemistry: Redox kinetics
• Am(VI) reduction

H2O2 in perchlorate is 1st order for peroxide and Am
 2 AmO22++H2O22 AmO2+ + 2 H++ O2

NpO2+
 1st order with Am(VI) and Np(V)
* k=2.45E4 L / mol s

Oxalic acid reduces to equal molar Am(III) and Am(V)
• Am(V) reduction

Reduced to Am(III) in NaOH solutions
 Slow reduction with dithionite (Na2S2O4), sulfite (SO32-),
or thiourea dioxide ((NH2)2CSO2)

Np(IV) and Np(V)
 In both acidic and carbonate conditions
* For Np(IV) reaction products either Np(V) or Np(VI)
 Depends upon initial relative concentration of
Am and Np
 U(IV) examined in carbonate
15-7
Am solution chemistry
•
•
Radiolysis

From alpha decay
 1 mg 241Am release 7E14 eV/s

Reduction of higher valent Am related
to dose and electrolyte concentration

In nitric acid formation of HNO2

In perchlorate numerous species
produced
 Cl2, ClO2, or ClComplexation chemistry

Primarily for Am(III)
 F->H2PO4->SCN->NO3->Cl>ClO4
Hard acid reactions
 Electrostatic interactions
* Inner sphere and outer
sphere
 Outer sphere for
weaker ligands

Stabilities similar to trivalent
lanthanides
 Some enhanced stability due to
participation of 5f electron in
bonding
15-8
Am solution
chemistry
•
•
Hydrolysis

Mono-, di-, and trihydroxide
species

Am(V) appears to have 2 species,
mono- and dihydroxide

Am hydrolysis (from CHESS
database)
 Am3++H2OAmOH2++H+:
log K =-6.402
 Am+3++2H2OAm(OH)2++
2H : log K =-14.11
3++3H OAm(OH) +3
 Am
3
+
H : log K2 =-25.72
Carbonate

Evaluated by spectroscopy

Includes mixed species
 Am hydroxide carbonate
species
 Based on solid phase analysis

Am(IV)
 Pentacarbonate studied (log
b=39.3)

Am(V) solubility examined
1mM Am3+;
1 mM Am, 1 mM carbonate
15-9
Am solution chemistry: Organics
• Number of complexes examined

Mainly for Am(III)
• Generally stability of complex
increases with coordination sites
• With aminopolycarboxylic acids,
complexation constant increases
with ligand coordination
• Natural organic acid

Number of measurements
conducted

Measured by spectroscopy and
ion exchange
• TPEN (N,N,N’,N’-tetrakis(2pyridylmethyl)ethyleneamine)

0.1 M NaClO4, complexation
constant for Am 2 orders
greater than Sm
15-10
Am solvent extraction
•
•
•
Tributylphosphate (TBP)

Am extracted from neutral or low acid solutions with high nitrate

Am(VI)
 Oxidation with (NH4)10P2W17O61 to stabilize Am(VI)
 100 % TBP from 1 M HNO3
* Separation factor 50 from Nd

Am separation from lanthanides
 1 M ammonium thiocyanate aqueous phase
Dibutyl butylphosphonate (DBBP)

Phosphonate functional group

Similar to TBP, stronger extractant of Am
Trialkylphophine oxide (TRPO)

Increase in basicity of P=O functional group from TBP to DPPB to
TRPO

Am and Cm extraction from 1-2 M HNO3

30 % TRPO in kerosene
 Am, Cm, tetravalent Np and Pu, hexavalent U extracted
* Actinides stripped with 5.5 M HNO3 (Am fraction)
 TRPO with C6-C8 alkyl group
15-11
Am solvent extraction
•
•
Bis(2-ethylhexyl)phosphoric acid (HDEHP)

Has been used to Am separation

Part of TALSPEAK
 Extracts lanthanides stronger that actinides
 TALSPEAK components
HDEHP
* Bis(2-ethyl-hexyl)phosphoric acid (HDEHP)
* HNO3
* DTPA
* Lactic acid
Carbamoylphosphine oxide (CMPO)

Synthesized by Horwitz
 Based on DHDECMP extractions
* Recognized functional group, simplified ligand synthesis
* Purified by cation exchange

Part of TRUEX
 TRUEX (fission products)
* 0.01 to 7 M HNO3
* 1.4 M TBP
* 0.2 M Diphenyl-N,N-dibutylcarbamoyl phosphine oxide (CMPO)
* 0.5 M Oxalic acid
* 1.5 M Lactic acid
* 0.05 M DTPA
15-12
CMPO
Am solvent extraction
• Tertiary amine salt

Low acid, high nitrate or chloride solution
 (R3NH)2Am(NO3)5
• Quaternary ammonium salts (Aliquat 336)

Low acid, high salt solutions
 Extraction sequence of Cm<Cf<Am<Es

Studies at ANL for process separation of Am
• Amide extractants

(R1,R2)N-C(O)-CR3H-C(O)-N(R1R2)
 Diamide extractant
 Basis of DIAMEX process

N,N’-dimethyl-N,N’-dibutyl-2-tetradecyl-malonamide
(DMDBTDMA)
 DIAMEX with ligand in dodecane with 3-4 M HNO3
* Selective extraction over Nd
15-13
•
•
•
•
Am/Ln solvent extraction
Extraction reaction

Am3++2(HA)2AmA3HA+3
H+
 Release of protons upon
complexation requires
pH adjustment to achieve
extraction
* Maintain pH greater
than 3
Cyanex 301 stable in acid

HCl, H2SO4, HNO3
 Below 2 M
Irradiation produces acids and
phosphorus compounds

Problematic extractions when
dosed 104 to 105 gray
New dithiophosphinic acid less
sensitive to acid concentration

R2PSSH; R=C6H5, ClC6H4,
FC6H4, CH3C6H4
 Only synergistic
extractions with, TBP,
TOPO, or
tributylphosphine oxide
 Aqueous phase 0.1-1 M
HNO3
 Increased radiation
resistance
Distribution ratios of Am(III ) and Ln(III ) in 1.0 M
Cyanex 301‐heptane (16 mol% of Cyanex 301
neutralized before extraction contacts)
15-14
•
•
•
•
Ion exchange separation Am
from
Cm
LiCl with ion exchange achieves separation from
lanthanide
Separation of tracer level Am and Cm has been
performed with displacement complexing
chromatography

DTPA and nitrilotriacetic acid in presence of
Cd and Zn as competing cations

displacement complexing chromatography
method is not suitable for large scale
Ion exchange has been used to separate trace levels of
Cm from Am

Am, Cm, and lanthanides sorbed to a cation
exchange resin at pH 2
 Separation of Cm from Am was
performed with 0.01 % ethylenediaminetetramethylphosphonic acid at pH 3.4 in
0.1 M NaNO3
 separation factor of 1.4
Separation of gram scale quantities of Am and Cm by
cation and anion exchange

use of a-hydroxylisobutyrate or
diethylenetriaminepentaacetic acid as an
eluting agent or a variation of eluant
composition by addition of methanol to nitric
acid
 best separations were achieved under
high pressure conditions
* separation factors greater than 400
Distribution
coefficients of actinides and
lanthanides into Dowex 1 8 resin
15-15
from 10 M LiCl
Extraction chromatography
• Mobile liquid phase and stationary liquid phase

Apply results from solvent extraction
 HDEHP, Aliquat 336, CMPO
* Basis for Eichrom resins
* Limited use for solutions with fluoride, oxalate, or
phosphate
 DIPEX resin (Eichrom)
* Bis-2-ethylhexylmethanediphosphonic acid on inert support
* Lipophilic molecule
 Extraction of 3+, 4+, and 6+ actinides
* Strongly binds metal ions
 Need to remove organics from support

Variation of support
 Silica for covalent bonding
 Functional organics on coated ferromagnetic particles
* Magnetic separation after sorption
15-16
Am separation and purification
• Precipitation method

Formation of insoluble Am species
 AmF3, K8Am2(SO4)7 , Am2(C2O4)3, K3AmO2(CO3)2
* Am(V) carbonate useful for separation from Cm
* Am from lanthanides by oxalate precipitation
 Slow hydrolysis of dimethyloxalate
 Oxalate precipitate enriched in Am
 50 % lanthanide rejection, 4 % Am

Oxidation of Am(VI) by K2S2O8 and precipitation of Cm(III)
• Pyrochemical process

Am from Pu
 O2 in molten salt, PuO2 forms and precipitates
 Partitioning of Am between liquid Bi or Al and molten
salts
* Kd of 2 for Al system
 Separation of Am from PuF4 in salt by addition of OF2
* Formation of PuF6, volatility separation
15-17
Am metal and alloys
•
•
•
Preparation of Am metal

Reduction of AmF3 with Ba or Li

Reduction of AmO2 with La

Bomb reduction of AmF3 with Ca

Decomposition of Pt5Am
 1550 °C at 10-6 torr

La or Th reduction of AmO2 with
distillation of Am
Metal properties

Ductile, non-magnetic

Double hexagonal closed packed
(dhcp) and fcc

Evidence of three phase between room
temperature and melting point at 1170
°C
 Alpha phase up to 658 °C
 Beta phase from 793 °C to 1004
°C
 Gamma above 1050 °C

Some debate in literature
 Evidence of dhcp to fcc at 771 °C

Interests in metal properties due to 5f
electron behavior
 Delocalization under pressure
 Different crystal structures
* Conversion of dhcp to fcc
 Discrepancies between different
experiments and theory
Alloys investigated with 23 different elements

Phase diagrams available for Np, Pu,
and U alloys
15-18
Am compounds: Oxides and Hydroxides
•
•
•
•
•
•
AmO, Am2O3, AmO2

Non-stoichiometric phases between
Am2O3 and AmO2
AmO lattice parameters varied in experiments

4.95 Å and 5.045 Å

Difficulty in stabilizing divalent Am
Am2O3

Prepared in H2 at 600 °C

Oxidizes in air

Phase transitions with temperature
 bcc to monoclinic between 460 °C
and 650 °C
 Monoclinic to hexagonal between
800 °C and 900 °C
AmO2

Heating Am hydroxides, carbonates,
oxalates, or nitrates in air or O2 from 600
°C to 800 °C

fcc lattice
 Expands due to radiation damage
Higher oxidation states can be stabilized

Cs2AmO4 and Ba3AmO6
Am hydroxide

Isostructural with Nd hydroxides

Crystalline Am(OH)3 can be formed, but
becomes amorphous due to radiation
damage
 Complete
degradation in 5 months
for 241Am hydroxide

Am(OH)3+3H+,Am3++3H2O
 logK=15.2 for crystalline
 Log K=17.0 for amorphous
15-19
Am organic compounds
•
•
From precipitation (oxalates) or solution evaporation
Includes non-aqueous chemistry

AmI3 with K2C8H8 in THF
 Yields KAm(C8H8)2

Am halides with molten Be(C5H5) forms Am(C5H5)3
 Purified by fractional sublimation
 Characterized by IR and absorption spectra
15-20
Am coordination chemistry
•
•
Little known about Am coordination chemistry

46 compounds examined

XRD and compared to isostructural lanthanide compounds

Structural differences due to presence of oxo groups in oxidized Am
Halides

Coordination numbers 7-9, 11

Coordination include water
 AmCl2(H2O)6+
* Outer sphere Cl may be present
15-21
Am coordination chemistry
• Oxides
 Isostructural with Pu oxides
 AmO may not be correct
 Am(V)=O bond distance of 1.935 Å
 Am2O3 has distorted Oh symmetry with Am-O
bond distances of 2.774 Å, 2.678 Å, and 1.984
15-22
Am coordination chemistry
•
•
Cyclopentadienyl (CP) ligands

Am(C5H5)3
 Isostructural with Pu(III) species
* Not pyrophoric
 Absorbance on films examined
* Evaluated 2.8 % relative bond covalency
* Indicates highly ionic bonding for species
* Data used for calculations and discussion of 5f and 6d
orbitals in interactions
Bis-cyclooctatetraenyl Am(III) KAm(C8H8)2

In THF with 2 coordinating solvent ligands

Decomposes in water, burns in air

XRD shows compound to be isostructural with Pu and Np
compounds
 From laser ablation mass spectra studies, examination of
molecular products
 Differences observed when compared to Pu and Np
compounds
 Am 5f electrons too inert to form sigma bonds with
organic, do not participate
15-23
Curium: Nuclear properties
• Isotopes from mass 237 to
251
• 242Cm, t1/2=163 d
 122 W/g
 Grams of oxide glows
 Low flux of 241Am
target decrease fission
of 242Am, increase yield
of 242Cm
• 244Cm, t1/2=18.1 a
 2.8 W/g
• 248Cm, t1/2= 3.48E5 a
 8.39% SF yield
 Limits quantities to 1020 mg
 Target for production
of transactinide
elements
15-24
Cm Production
• From successive neutron capture of higher Pu isotopes
242Pu+n243Pu (b-, 4.95 h)243Am+n244Am (b-, 10.1 h)244Cm


Favors production of 244,246,248Cm
 Isotopes above 244Cm to 247Cm are not isotopically pure
 Pure 248Cm available from alpha decay of 252Cf
• Large campaign to product Cm from kilos of Pu
• 244Cm separation

Dissolve target in HNO3 and remove Pu by solvent extraction

Am/Cm chlorides extracted with tertiary amines from 11 M LiCl
in weak acid
 Back extracted into 7 M HCl

Am oxidation and precipitation of Am(V) carbonate
• Other methods for Cm purification included NaOH, HDEHP, and
EDTA

Discussed for Am
15-25
Cm aqueous chemistry
•
•
•
•
•
Trivalent Cm
242Cm at 1g/L will boil
9 coordinating H2O from fluorescence

Decreases above 5 M HCl

7 waters at 11 M HCl

In HNO3 steady decrease from 0 to 13 M
 5 waters at 13 M
 Stronger complexation with NO3Inorganic complexes similar to data for Am

Many constants determined by TRLFS
Hydrolysis constants (Cm3++H2OCmOH2++H+)

K11=1.2E-6

Evaluated under different ionic strength
15-26
Cm atomic and spectroscopic data
•
•
•
Cm(III) absorbance

Weak absorption in near-violet
region

Solution absorbance shifted 20-30 Å
compared to solid
 Reduction of intensity in solid
due to high symmetry
* f-f transitions are symmetry
forbidden

Spin-orbit coupling acts to reduce
transition energies when compared
to lanthanides
Cm(IV) absorbance

Prepared from dissolution of CmF4
 CmF3 under strong fluorination
conditions
5f7 has enhanced stability

Half filled orbital
 Large oxidation potential for
IIIIV
 Cm(IV) is metastable
15-27
Absorption and fluorescence process of Cm3+
Optical Spectra
Fluorescence Process
30
Wavenumber (10
3
-1
cm )
H
G
F
Emissionless
Relaxation
20
A
7/2
Excitation
10
Fluorescence
Emission
15-28
0
Z
7/2
Cm fluorescence
• Fluoresce from 595-613
nm
 Attributed to
6D 8S
7/2
7/2
transition
 Energy dependent
upon coordination
environment
Speciation
Hydration
complexation
constants
15-29
Cm separation and purification:
Similar to Am
•
•
•
Solvent extraction

Organic phosphates
 Function of ligand structure
* Mixed with 6 to 8 carbon chain better than TBP

HDEHP
 From HNO3 and LiCl

CMPO
 Oxidation state based removal with different stripping agent

Extraction of Cm from carbonate and hydroxide solutions, need to keep
metal ions in solution
 Organics with quaternary ammonium bases, primary amines,
alkylpyrocatechols, b-diketones, phenols
Ion exchange

Anion exchange with HCl, LiCl, and HNO3
 Includes aqueous/alcohol mixtures
 Formation of CmCl4- at 14 M LiCl
* From fluorescence spectroscopy
Precipitation

Separation from higher valent Am
 10 g/L solution in base
 Precipitation of K5AmO2(CO3)3 at 85 °C
 Precipitation of Cm with hydroxide, oxalate, or fluoride
15-30
Cm metallic state
•
•
•
Preparation of Cm metal

CmF3 reduction with Ba or Li
 Dry, O2 free, and above 1600 K

Reduction of CmO2 with Mg-Zn alloy
in MgF2/MgCl2
Melting point 1345 °C

Higher than lighter actinides Np-Am

Similar to Gd (1312 °C)
Two states

Double hexagonal close-packed (dhcp)
 Neutron diffraction down to 5 K
 No structure change

fcc at higher temperature
•
•
•
•
XRD studies on 248Cm
Magnetic susceptibility studies

Antiferrimagnetic transition near 65 K
 200 K for fcc phase
Metal susceptible to corrosion due to self heating

Formation of oxide on surface
Alloys

Cm-Pu phase diagram studied

Noble metal compounds
 CmO2 and H2 heated to 1500 K in
Pt, Ir, or Rh
* Pt5Cm, Pt2Cm, Ir2Cm, Pd3Cm,
Rh3Cm
15-31
Cm oxide compounds
• Cm2O3

Thermal decomposition of CmO2 at 600 °C and 10-4 torr

Mn2O3 type cubic lattice
 Transforms to hexagonal structure due to radiation
damage
 Monoclinic at 800 °C
• CmO2

Heating in air, thermal treatment of Cm loaded resin, heating
Cm2O3 at 600 °C under O2, heating of Cm oxalate

Shown to form in O2 as low as 400 °C
 Evidence of CmO1.95 at lower temperature

fcc structure

Magnetic data indicates paramagnetic moment attributed to
Cm(III)
 Need to re-evaluate electronic ground state in oxides
• Oxides

Similar to oxides of Pu, Pr, and Tb
 Basis of phase diagram

BaCmO3 and Cm2CuO4
 Based on high T superconductors
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 Cm compounds do not conduct
Cm compounds
•
•
•
•
Cm(OH)3

From aqueous solution, crystallized by aging in water

Same structure as La(OH)3; hexagonal
Cm2(C2O4)3.10H2O

From aqueous solution

Stepwise dehydration when heated under He
 Anhydrous at 280 °C
 Converts to carbonate above 360 °C
* TGA analysis showed release of water (starting at 145 °C)
 Converts to Cm2O3 around 500 °C
Cm(NO3)3

Evaporation of Cm in nitric acid

From TGA, decomposition same under O2 and He
 Dehydration up 180 °C, melting at 400 °C

Final product CmO2

Oxidation of Cm during decomposition
Organometallics

Studies hampered by radiolytic properties of Cm

Some compounds similar to Am
 Cm(C5H5)3 form CmCl3 and Be(C5H5)2
 Weak covalency of compound
 Strong fluorescence
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Review
• Production and purification of Am and Cm isotopes
 Suitable reactions
 Basis of separations from other actinides
• Formation of Am and Cm metallic state and properties
 Number of phases, melting points
• Compounds
 Range of compounds, limitations on data
• Solution chemistry
 Oxidation states
• Coordination chemistry
 Organic chemistry reactions
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Questions
• What is the longest lived isotope of Am?
• Which Am isotope has the highest neutron induced
fission cross section?
• What are 3 ligands used in the separation of Am?
 What are the solution conditions?
• What column methods are useful for separating Am
from the lanthanides?
• Which compounds can be made by elemental reactions
with Am?
• What Am coordination compounds have been
produced?
• What is the absorbance spectra of Am for the different
oxidation states?
• How can Am be detected?
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Questions
• Which Cm isotopes are available for chemical
studies?
• Describe the fluorescence process for Cm
 What is a good excitation wavelength?
• What methods can be use to separate Cm from
Am?
• How many states does Cm metal have? What
is its melting point?
• What are the binary oxides of Cm? Which will
form upon heating in normal atmosphere?
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Pop Quiz
• How can high valent oxidation states of Am be
made?
• Why does Cm have fewer accessible oxidation
states than Am?
• Respond to lecture in blog
• Provide pop quiz answers
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