Actinides: Occurrence, Abundance & Preparation: 20 flashcards
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1. Which orbitals fill progressively across the actinide series?
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The 5f orbitals.
2. From which element does the name 'actinide' derive?
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Actinium (Ac).
3. What does the alternative term 'actinoid' literally mean?
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'Like actinium'.
4. Why are actinide chemical properties more variable than those of lanthanides?
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Because 5f, 6d, and 7s orbitals all participate in bonding.
5. Which four actinides occur naturally in appreciable amounts?
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Actinium (Ac), thorium (Th), protactinium (Pa), and uranium (U).
6. Which two actinides are the most abundant and widely distributed?
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Thorium and uranium.
7. What is the half-life of $^{232}$Th?
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About 14 billion years.
8. What percentage of natural uranium is $^{238}$U?
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About 99.28%.
9. Which natural uranium isotope is fissile, and at what abundance?
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$^{235}$U, at about 0.7%.
10. What are elements beyond uranium ($Z = 92$) called?
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Transuranium or transuranic elements.
11. How are transuranic elements produced?
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By nuclear-chemical methods — in nuclear reactors or particle accelerators.
12. In what form do actinides occur in the Earth's crust?
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Mainly as oxide and phosphate minerals.
13. What is the most important uranium ore, and what is its formula?
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Uraninite (pitchblende), $\text{UO}_2$ or $\text{U}_3\text{O}_8$.
14. What is carnotite's formula and appearance?
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$\text{K}_2\text{UO}_2(\text{VO}_4)_2\cdot 3\text{H}_2\text{O}$; a yellow uranium–vanadium ore.
15. Which mineral is a major source of thorium and has formula $\text{ThO}_2$?
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Thorianite.
16. Which phosphate mineral is a significant source of both lanthanides and thorium?
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Monazite.
17. What methods are used to prepare naturally occurring actinides from their minerals?
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Reduction, electrolysis, and thermal decomposition.
18. Where do trace amounts of neptunium and plutonium come from in nature?
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As byproducts of neutron capture and beta decay in uranium ores.
19. Which isotope of protactinium is found in uranium ores, and from what does it arise?
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$^{231}$Pa, as a decay product of $^{235}$U.
20. Which actinide isotope occurs in trace amounts in uranium ores as part of decay chains and has mass number 227?
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$^{227}$Ac (actinium-227).
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