316 | The History and Use of Our Earth’s Chemical Elements Hazards
316 | The History and Use of Our Earth’s Chemical Elements Hazards
All compounds as well as metallic uranium are radioactive—some more so than others. The main hazard from radioactive isotopes is radiation poisoning. Of course, another potential hazard is using fissionable isotopes of uranium and plutonium for other than peaceful pur- poses, but such purposes involve political decisions, not science.
NEPTUNIUM SYMBOL:Np PERIOD:7 SERIESNAME:Actinide ATOMICNO:93
ATOMICMASS:237.0482amu VALENCE:3,4,5,and6 OXIDATIONSTATES:+3,+4, +5,and+6 NATURALSTATE:Solid
ORIGINOFNAME:NamedfortheplanetNeptune. ISOTOPES:Thereareatotalof23isotopesofneptunium.Nonearestable.Allareradioac-
tivewithhalf-livesrangingfromtwomicrosecondsto2.144×10 +6 yearsfortheisotope Np-237,whichspontaneouslyfissionsthroughalphadecay.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=22
s2,p6,d10,f4
6-P=9
s2,p6,d1
7-Q=2
s2
Properties The chemistry of neptunium ( 93 Np) is somewhat similar to that of uranium ( 92 U) and plu-
tonium ( 94 Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four; the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptu- nium decay series proceeds as follows, starting with the isotope plutonium-241: Pu-241→ Am-241→Np-237→Pa-233→U-233→Th-229→Ra-225→Ac-225→Fr-221→At-217→Bi- 213→Ti-209→Pb-209→Bi-209.
Neptunium is a silvery-white radioactive, heavy metal. Its melting point is 644°C, its boil-
ing point is 3,902°C, and its density is 20.25g/cm 3 .
317 Characteristics
Guide to the Elements |
Neptunium is the first of the subseries of the actinide series known as the transuranic ele- ments —those heavy, synthetic (man-made) radioactive elements that have an atomic number greater than uranium in the actinide series of the periodic table. An interesting fact is that neptunium was artificially synthesized before small traces of it were discovered in nature. More is produced by scientists every year than exists in nature.
Neptunium has an affinity for combining with nonmetals (as do all transuranic elements) such as oxygen, the halogens, sulfur, and carbon.
AbundanceandSource At one time, neptunium’s entire existence was synthesized by man. Sometime later, in the
mid-twentieth century, it was discovered that a very small amount is naturally produced in uranium ore through the actions of neutrons produced by the decay of uranium in the ore pitchblende. Even so, a great deal more neptunium is artificially produced every year than ever did or does exist in nature. Neptunium is recovered as a by-product of the commercial produc- tion of plutonium in nuclear reactors. It can also be synthesized by bombarding uranium-238 with neutrons, resulting in the production of neptunium-239, an isotope of neptunium with
a half-life of 2.3565 days. History In 1940 Edwin Mattison McMillan (1907–1991) and a former graduate student, Philip
Hauge Abelson (1913–1995), jointly experimented at the physics laboratory of the University of California, Berkeley, by bombarding uranium oxide with high-speed neutrons in a cyclo- tron. Their experiment indicated that a new element exhibited oxidation states of +4 and +6 and that it exhibited chemical and physical properties similar to uranium. They prepared their samples of neptunium-239 by bombarding uranium-238 with high-energy neutrons, which resulted in uranium-239 that, in turn, decayed by beta radiation, resulting in neptunium-239. The reaction follows: U-238 + (neutrons, gamma rays) → U-239 (decays by beta radiation) →
Np-239. Their discovery was confirmed and their data was published in 1940. Because 93 Np’s atomic number followed 92 U, McMillan decided to name the new element “neptunium” for the planet Neptune, which was next in line following Uranus in the solar system. McMillan’s and Abelson’s work was interrupted during World War II and was later continued by Arthur
C. Wahl and Joseph W. Kennedy, who were able to determine the physical reactions that resulted in neptunium.
CommonUses The most important radioactive isotope of neptunium is Neptunium-237, with a half-life
of 2.144×10 +6 years, or about 2.1 million years, and decays into protactinium-233 through alpha decay. Neptunium’s most important use is in nuclear research and for instruments designed to detect neutrons.
ExamplesofCompounds Some examples of halogen compounds of neptunium that are formed by its ions with
oxidation states of +3, +4, +5, and +6 follow:
318 | The History and Use of Our Earth’s Chemical Elements
Neptunium (III) fluoride: Np 1- + 3F → NpF
Neptunium (IV) chloride: Np 1- + 4Cl → NpCl
Neptunium (V) iodide: Np 1- + 5I → NpI
Neptunium (VI) fluoride: Np 1- + 6F → NpF
Neptunium also forms compounds with oxygen. An example follows:
2 . Neptunium dioxide is a powder used to form metal targets that are to be radiated by plutonium.
Neptunium (IV) didioxide: Np 2- + 2O → NpO
Hazards All isotopes of neptunium are highly radioactive and are hazardous and thus need to be
carefully used in controlled laboratory settings. These isotopes as well as neptunium’s com- pounds are radioactive poisons.
PLUTONIUM SYMBOL:Pu PERIOD:7 SERIESNAME:Actinide ATOMICNO:94
ATOMICMASS:239.11amu VALENCE:3,4,5,and6 OXIDATIONSTATE:+3,+4,+5, and+6 NATURALSTATE:Solid ORIGINOFNAME:NamedfortheplanetPluto. ISOTOPES:Thereareatotalof24isotopesofplutonium.Allofthemareunstableand
radioactive.Theirhalf-livesrangefrom28nanosecondsto8.00×10 +7 years.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=24
s2,p6,d10,f6
6-P=8
s2,p6
7-Q=2
s2
Properties All isotopes of plutonium are radioactive. The two isotopes that have found the most uses
are Pu-238 and Pu-239. Pu-238 is produced by bombarding U-238 with deuterons in a cyclotron, creating neptunium-238 and two free neutrons. Np-238 has a half-life of about two days, and through beta decay it transmutates into plutonium-238. There are six allotropic metallic crystal forms of plutonium. They all have differing chemical and physical properties. The alpha (α) allotrope is the only one that exists at normal room temperatures and pressures. The alpha allotrope of metallic plutonium is a silvery color that becomes yellowish as it oxi- dizes in air. All the other allotropic forms exist at high temperatures.
319 The most stable isotope of plutonium is Pu-244, with a half-life of 8.00×10 +7 years (about
Guide to the Elements |
82,000,000 years). Being radioactive, Pu-244 can decay in two different ways. One way involves alpha decay, resulting in the formation of the isotope uranium-240, and the other is through spontaneous fission.
The melting point of plutonium is 640°C, its boiling point is 3,232°C, and its density is
over 19 times that of the same volume of water (19.84g/cm 3 ).
Characteristics
A single kilogram of radioactive metallic plutonium-238 produces as much as 22 million kilowatt-hours of heat energy. Larger amounts of Pu-238 produce more heat. However, Pu-238 is not fissionable, and thus it cannot sustain a chain reaction. However, plutonium-239 is fissionable, and a 10-pound ball can reach a critical mass sufficient to sustain a fission chain reaction, resulting in an explosion, releasing the equivalent of over 20,000 tons of TNT. This 10-pound ball of Pu-239 is only about one-third the size of fissionable uranium-235 required to reach a critical mass. This makes plutonium-239 the preferred fissionable material for nuclear weapons and some nuclear reactors that produce electricity.
Plutonium has some peculiar qualities. In its molten state it will corrode any container in which it is stored. It has an unusual ability to combine with almost all the other elements listed on the periodic table, and it can change its density by as much as 25% according to its environment. Small pieces of it can spontaneously ignite at temperatures as low as 150°C.
A modern atomic bomb has three main parts: the primary, the secondary, and a radiation container. The primary is formed as a small ball of plutonium called a “pit,” which is used for the “core” of an atomic (nuclear) bomb. This pit is surrounded by the secondary component, which is a chemical explosive material, which is then surrounded by a shell of uranium. When the chemical explosive is triggered, it implodes, compressing the plutonium core, which results in an increase in the pit’s density. This forces the nuclei of the atoms closer together, producing “free” neutrons. This compression causes the plutonium nuclei to fission in a chain reaction that results in the release of tremendous energy. This reaction also causes the surrounding uranium to fission, which releases more nuclear energy—enough to wipe out New York City or Washington, D.C.
The first nuclear bomb that was tested was called “Trinity” and weighed 10,000 pounds. Today’s bombs weigh about 250 pounds and are about the size of an average suitcase.
AbundanceandSource Plutonium exists in trace amounts in nature. Most of it isotopes are radioactive and man-
made or produced by the natural decay of uranium. Plutonium-239 is produced in nuclear reactors by bombarding uranium-238 with deuterons (nuclei of deuterium, or heavy hydro- gen). The transmutation process is as follows: 238 U + deuterons→ 2 nuclei + 239 Np + β→ decays to→ 238 Pu + β-.
There is more than an adequate supply of plutonium-239 in the world because it is a “waste” product of the generation of electricity in nuclear power plants. One of the objec- tions to developing more nuclear reactors is the dilemma of either eliminating or storing all the excess plutonium. In addition, there is always the risk of terrorists’ obtaining a supply of Pu-239 to make nuclear weapons.