Technetium (Tc)

Technetium (Tc, Z = 43) lacks a combination of protons and neutrons that yields a nucleus with a sufficiently low decay probability. All known isotopes lie outside the "valley of stability" and undergo radioactive decay (e.g., \(\beta^-\), isomeric transition). Nuclear shell effects near Z = 43 do not provide a magic-number configuration, so no isotope is energetically stable against decay.

Pertechnetate is the tetraoxo anion \(\mathrm{TcO_4^-}\), with technetium in the +7 oxidation state. It is the most common aqueous form of Tc and is highly soluble and relatively inert under oxidizing conditions. In medicine, \(^{99\mathrm{m}}\mathrm{Tc}\) is eluted as \(\mathrm{Na^{+}TcO_4^{-}}\) from a molybdenum generator and used to prepare radiopharmaceuticals for imaging.

Most hospitals obtain \(^{99\mathrm{m}}\mathrm{Tc}\) from a \(^{99}\mathrm{Mo} \rightarrow {}^{99\mathrm{m}}\mathrm{Tc}\) generator. \(^{99}\mathrm{Mo}\) (half-life \(\approx 66\) h) decays to \(^{99\mathrm{m}}\mathrm{Tc}\), which is periodically "milked" (eluted) as pertechnetate. The metastable \(^{99\mathrm{m}}\mathrm{Tc}\) (half-life \(\approx 6.0\) h) emits gamma photons (~140 keV) ideal for diagnostic imaging and then decays to \(^{99}\mathrm{Tc}\).

Technetium exhibits several oxidation states:

• +7: \(\mathrm{TcO_4^-}\) (colorless to pale), strong oxidant.
• +6/+5: oxo-complexes such as \(\mathrm{TcO_3^{+}}\), often yellow–orange to red.
• +4: \(\mathrm{TcO_2}\) and related complexes, typically brown/black; more reduced and less soluble.

Colors vary with ligands (e.g., carbonyls \(\mathrm{Tc(CO)_6^{+}}\) are often pale) and coordination geometry.

All three belong to Group 7 (Mn, Tc, Re) and form oxo species with high oxidation states and similar structural motifs (e.g., tetraoxo anions \(\mathrm{MnO_4^-}\), \(\mathrm{TcO_4^-}\), \(\mathrm{ReO_4^-}\)). Trends:

• Oxidation stability increases down the group (Re > Tc > Mn).
• \(\mathrm{TcO_4^-}\) is a weaker oxidant than \(\mathrm{MnO_4^-}\) but more akin to \(\mathrm{ReO_4^-}\).
• Carbonyl chemistry is well developed for Tc and Re (e.g., \(\mathrm{Tc(CO)_6^{+}}\)).

Technetium is primarily artificial but does occur in trace amounts in nature. It has been detected in uranium ores where it is produced by spontaneous fission and in certain stars (identified spectroscopically). However, all macroscopic quantities used in labs and hospitals are produced in nuclear reactors or accelerators.

The dominant application is nuclear medicine using \(^{99\mathrm{m}}\mathrm{Tc}\) radiopharmaceuticals for SPECT imaging of bones, heart, lungs, and other organs. Historically, minute amounts of Tc compounds (e.g., \(\mathrm{KTcO_4}\)) were used as corrosion inhibitors in carbon steel, but this use is now rare due to radioactivity concerns and regulatory limits.

Because all isotopes are radioactive, follow ALARA principles:

• Time: minimize exposure time.
• Distance: maximize distance from sources; use tongs/shields.
• Shielding: appropriate gamma shielding for \(^{99\mathrm{m}}\mathrm{Tc}\) (lead or tungsten).

Use closed systems, wear dosimeters, and follow waste rules. In medicine, the short half-life (\(~6\) h) and photon energy enable imaging with relatively low patient dose.

\(\mathrm{TcO_4^-}\) is an anion that is weakly sorbing and highly soluble under oxidizing, neutral to alkaline conditions, making it mobile in groundwater. Immobilization strategies include:

• Reducing Tc(VII) to Tc(IV) using reductants (e.g., \(\mathrm{Fe^{2+}}\), sulfides), forming sparingly soluble \(\mathrm{TcO_2\cdot nH_2O}\).
• Encapsulation in engineered barriers and using redox-active minerals to maintain reducing conditions.

The ground-state configuration is \([\mathrm{Kr}]\,4d^5\,5s^2\). While many transition elements favor half-filled subshells via \(ns^1\) promotions, Tc commonly appears as \([\mathrm{Kr}]\,4d^6\,5s^1\) in some references due to near-degenerate \(4d\) and \(5s\) energies; however, the accepted configuration is \([\mathrm{Kr}]\,4d^5\,5s^2\). Its chemistry reflects flexible \(d\)-electron participation across oxidation states.