Plutonium (Pu)

Plutonium (Pu) is an actinide with atomic number 94, located in period 7 of the f-block between neptunium (Np) and americium (Am). It is distinctive for its multiple metallic allotropes, wide range of oxidation states (+3 to +7), and significant radiological and chemical reactivity.

Key isotopes include:

• \(^{239}\mathrm{Pu}\): fissile; long half-life (\(~2.41\times10^4\) years); central to reactor physics research and fuel studies.
• \(^{238}\mathrm{Pu}\): strong alpha emitter; used as a heat source in radioisotope thermoelectric generators (RTGs) for spacecraft.
• \(^{240}\mathrm{Pu}\): higher rate of spontaneous fission; influences neutron background in materials containing Pu.

In thermal reactors, \(^{238}\mathrm{U}\) can capture a neutron and, after two beta decays, become \(^{239}\mathrm{Pu}\):

This is a fertile \(\to\) fissile conversion pathway studied in nuclear fuel cycles.

\(^{239}\mathrm{Pu}\) can undergo fission with thermal (slow) neutrons, emitting energy and additional neutrons that may sustain a chain reaction. A simplified fission example is:

Common oxidation states are +3, +4, +5, +6, and +7 (rare). In water, linear dioxo cations called plutonyl form in higher states:

• \(\mathrm{PuO_2^+}\) (Pu(V))
• \(\mathrm{PuO_2^{2+}}\) (Pu(VI))

These species show rich redox chemistry and complexation with ligands (e.g., carbonates, nitrates, phosphates).

A commonly cited ground-state configuration is [Rn] 5f6 7s2 (with 6d/5f participation context-dependent). The involvement of 5f orbitals enables variable oxidation states, multiple bonding environments, and complex magnetic/structural behavior.

Important compounds include:

• \(\mathrm{PuO_2}\) (plutonium(IV) oxide): refractory solid; common matrix form.
• \(\mathrm{PuF_6}\) (plutonium(VI) fluoride): volatile hexafluoride analogous to \(\mathrm{UF_6}\) and \(\mathrm{NpF_6}\).
• \(\mathrm{PuCl_3}\) and \(\mathrm{PuCl_4}\): halides used in coordination chemistry studies.

Plutonium has six allotropes at ambient pressure (\(\alpha, \beta, \gamma, \delta, \delta', \epsilon\)), each with distinct crystal structures and densities. The \(\delta\)-phase can be stabilized at room temperature by alloying (e.g., with Ga), dramatically changing mechanical properties.

Yes. Many Pu isotopes are strong alpha emitters. Primary hazards include internal exposure (inhalation/ingestion of particulates) and chemical toxicity. Work with Pu typically requires licensed facilities, glove boxes or hot cells, HEPA-filtered ventilation, continuous contamination control, dosimetry, and strict waste management procedures.

Representative values (approx.):

• \(^{238}\mathrm{Pu}\): \(t_{1/2} \approx 87.7\) years; strong \(\alpha\) emitter (heat source).
• \(^{239}\mathrm{Pu}\): \(t_{1/2} \approx 2.41\times10^{4}\) years; \(\alpha\) decay.
• \(^{240}\mathrm{Pu}\): \(t_{1/2} \approx 6.56\times10^{3}\) years; \(\alpha\) decay; higher spontaneous fission rate.

MOX (Mixed Oxide) fuel is a ceramic blend of plutonium dioxide and uranium dioxide (typically \(\mathrm{PuO_2}\) + \(\mathrm{UO_2}\)). It allows recycling of plutonium into thermal reactors to generate energy, while requiring rigorous safeguards, materials control, and safety analyses.