130 | The History and Use of Our Earth’s Chemical Elements Only some of the compounds of molybdenum are toxic, particularly in the powder or mist
130 | The History and Use of Our Earth’s Chemical Elements Only some of the compounds of molybdenum are toxic, particularly in the powder or mist
form. Small traces of molybdenum are essential for plant and animal nutrition. TECHNETIUM
SYMBOL:Tc PERIOD:5 GROUP:7(VIIB) ATOMICNO:43 ATOMICMASS:98.9062amu VALENCE:4,6,and7 OXIDATIONSTATE:+4,+6,and
+7(also+2,+3,and+5) NATURALSTATE:Solid ORIGINOFNAME:Technetium’snamewasderivedfromtheGreekwordtechnetos, meaning“artificial.” ISOTOPES:Thereare47isotopes.Nonearestableandallareradioactive.Mostarepro- ducedartificiallyincyclotrons(particleaccelerators)andnuclearreactors.Theatomic massofitsisotopesrangesfromTc-85toTc-118.Mostoftechnetium’sradioactive isotopeshaveveryshorthalf-lives.Thetwonaturalradioisotopeswiththelongesthalf- lives—Tc-98=4.2×10 +6 yearsandTc-99=2.111×10 +5 years—areusedtoestablish technetium’satomicweight.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbital/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=13
s2,p6,d5
5-O=2
s2
Properties As the central member of the triad of metals in group 7, technetium (period 5) has similar
physical and chemical properties as its partners manganese (period 4) above it and rhenium (period 6) below it. The sizes of their atomic radii do not vary greatly: Mn = 127, Tc = 136, and Re = 137. Neither does their level of electronegativity vary significantly: Mn = 1.5, Tc =
1.9, and Re = 1.9. Technetium metal is grayish-silver and looks much like platinum. As with most transition elements, technetium in pure form is a noncorrosive metal. It requires only 55 ppm of tech- netium added to iron to transform the iron into a noncorroding alloy. Because of technetium’s radioactivity, its use as an alloy metal for iron is limited so as to not expose humans to unnec- essary radiation.
Technetium’s melting point is 2,172°C, its boiling point is 4,877°C, and its density is
11.50 g/cm 3 .
131 Characteristics
Guide to the Elements |
Technetium was the first element, not found on Earth, to be artificially produced by bom- barding molybdenum with deuterons. The major characteristic of technetium is that it is the only element within the 29 transi- tion metal-to-nonmetal elements that is artificially produced as a uranium-fission product in nuclear power plants. It is also the lightest (in atomic weight) of all elements with no stable isotopes. Since all of technetium’s isotopes emit harmful radiation, they are stored for some time before being processed by solvent extraction and ion-exchange techniques. The two long- lived radioactive isotopes, Tc-98 and Tc-99, are relatively safe to handle in a well-equipped laboratory.
Since all of technetium’s isotopes are produced artificially, the element’s atomic weight (atomic mass units) is determined by which isotopes are selected for the calculation.
AbundanceandSource Technetium is the 76th most abundant element, but it is so rare that it is not found as a
stable element on Earth. All of it is artificially produced. Even though natural technetium is so scarce that it is considered not to exist on Earth, it has been identified in the light spectrum from stars. Using a spectroscope that produces unique lines for each element, scientists are able to view several types of stars. The resulting spectrographs indicate that technetium exists in the stars and thus the universe, but not on Earth as a stable element.
It was the first new element to be produced artificially from another element experimental- ly in a laboratory. Today, all technetium is produced mostly in the nuclear reactors of electrical generation power plants. Molybdenum-98 is bombarded with neutrons, which then becomes molybdenum-99 when it captures a neutron. Since Mo-99 has a short half-life of about 66 hours, it decays into Tc-99 by beta decay.
History Mendeleev, who developed the periodic table, recognized a gap in group 7 of the transition
metals between manganese (Mn-55) and ruthenium (Re-186). Using his system for anticipat- ing unknown elements in his table, he used the word eka, which means “first” in Sanskrit, to name missing elements. Thus, element eka-manganese was suggested to fill the atomic num- ber 43 gap based on Mendeleev’s speculation that this missing element would have chemical and physical properties similar to manganese, located just above it in group 7. Some other examples of eka elements predicted by their placement on the periodic tables (some were accurate, others just close) were eka-aluminum, eka-gallium, eka-boron, eka-scandium, and eka-silicone. (Note: Mendeleev used atomic weights instead of atomic numbers for the ele- ments in his original periodic table.)
This system gave several scientists clues as to what to look for, but because there are no stable (nonradioactive) atoms of element 43 (eka-manganese) on Earth, they had to find new techniques for identifying element 43. Many scientists claimed to have discovered an element with atomic number 43 and even gave it names such as davyum, illmenium, lucium, and nip- ponium. None proved to be the correct element. About this time, it was known that Enrico Fermi (1901–1954) had changed one element to another by bombarding the element’s nuclei
with deuterons, the atomic nuclei of heavy hydrogen ( 2 H), which have 1 proton and 1 neu-
132 | The History and Use of Our Earth’s Chemical Elements tron. This was an example of artificial transmutation of one element to another element, a
technique long sought by ancient alchemists who unsuccessfully tried to turn lead into gold.
Technetium was discovered in platinum ore shipped from Columbia through X-ray spectroscopy by Walter Noddack and Ida Tacke in Berlin. In 1937 Emilio Gino Segre (1905–1989) and Carlo Perrier (dates unknown) knew about Fermi’s work and decided that if technetium did not exist on Earth, they could make it using Fermi’s technique. They bom-
barded molybdenum ( 42 Mo) with deuterons in a cyclotron, which added a proton to each of molybdenum’s nuclei, and thus created technetium ( 43 Tc). It worked. Even though their sample was extremely small (10 -1 gram), it was enough to verify that a new element had been synthesized in the laboratory. They were given the privilege of naming the first artificially pro- duced element, just as other scientists had had for naming naturally occurring elements they had discovered. In 1939 they named it “technetium,” from the Greek word for “artificial.”
CommonUses Technetium is one of the few artificially produced elements that has practical industrial appli-
cations. One is that a very small amount (55-ppm) added to iron creates a corrosion-resistant alloy metal. This property is shared with many of the other transition metallic elements, but not with other artificially produced elements that have higher atomic numbers and are radioactive.
A radioisotope of technetium is widely used in nuclear medicine. The patient is injected with saline solution containing Tc-99 m (the superscript “m” means that the isotope is unstable and that its nuclei holds more energy than the regular Tc-99 nuclei into which it decays). This means that the Tc-99 m will start to emit energy and will finally decay and change to the regular nuclei of Tc-99 when injected into the patient. This energy is in the form of very penetrating gamma rays (a strong type of X-rays). The radioactive solution of Tc-99 m may be combined with other elements that are absorbed by certain organs of the human body being diagnosed or treated. For instance, adding tin to the solution targets the red blood cells, whereas phos- phorus in the solution concentrates the radioactive solution in heat muscles. The gamma rays are strong enough to expose an X-ray film that depicts the internal image of the organ under examination. This procedure is safe because Tc-99 m has a half-life of only 6.015 hours, and the Tc-99 has a half-life of over 200,000 years. However, the radioactivity will be harmless in less than a day because the body rapidly eliminates the residual radioactive solution.
Technetium is also used as an alloy metal to produce super-strong magnets that are super- cooled to near absolute zero to improve their efficiency. Powerful magnets are used in imaging equipment and possibly in future magnetic driven trains. Its radioactivity makes it useful as a tracer in the production of metals and tracing flowing fluids in pipelines.
ExamplesofCompounds Technetium- 99 m is produced in commercial quantities in nuclear reactors by bombarding
molybdenum with large numbers of neutrons. A simplified version of the radioactive decay reaction follows:
99 MoO 2- →β→ 99m TcO 1-
4 4 →γ→ 99 TcO 1- 4
It might be noted that the oxidation states for technetium can easily change from one to the other. The oxidation states of Tc(III), Tc(IV), and Tc(V) for various compounds can be adjusted to target different organs when used for medical diagnostics and treatments.
133 Only a few compounds of technetium have been made. Some examples follow:
Guide to the Elements |
Technetium(IV) dioxide: Tc 4+ + 2O 2- → TcO 2 . Technetium(VI) chloride: Tc 6+ + 6Cl 1- → TcCl 6 . Tectnetium(VII) septoxide: 2Tc 7+ + 7O 2- → Tc 2 O 7 .
It might be noted that there are several forms of technetium oxides. Their formula depends
on the oxidation state of Tc ions; an example is NH 4 TcO 4 .
Hazards The hazards of technetium are the same as for all radioactive elements. Excessive exposure
to radiation can cause many kinds of tissue damage—from sunburn to radiation poisoning to death.
RUTHENIUM SYMBOL:Ru PERIOD:5 GROUP:8(VIII) ATOMICNO:44
ATOMICMASS:101.07amu VALENCE:3 OXIDATIONSTATE:+3(also+4,+5,+6,and +8) NATURALSTATE:Solid ORIGINOFNAME:“Ruthenium”isderivedfromtheLatinwordRutheniameaning“Russia,” whereitisfoundintheUralMountains. ISOTOPES:Thereare37isotopesforruthenium,ranginginatomicmassnumbersfrom 87to120.Sevenofthesearestableisotopes.Theatomicmassesandpercentageof contributiontothenaturaloccurrenceoftheelementonEarthareasfollows:Ru-96= 5.54%,Ru-98=1.87%,Ru-99=12.76%,Ru-100=12.60%,Ru-101=17.06%,Ru- 102=31.55%,andRu-104=18.62%.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=15
s2,p6,d7
5-O=1
s1