Polonium is a chemical element with symbol Po and atomic number 84, discovered in 1898 by Marie Sklodowska-Curie and Pierre Curie. A rare and highly radioactive element with no stable isotopes, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Applications of polonium are few. They include heaters in space probes, antistatic devices, and sources of neutrons and alpha particles. Because of its position in the periodic table, polonium is sometimes classified as a metalloid. Other sources say that on the basis of its properties and behavior, it is "unambiguously a metal".
As is true of every element of higher atomic number, polonium's nuclear properties are front and center. It's atomic properties are secondary.
Thallium through polonium form a band in which elements transition from being mainly of chemical interest to being mainly of nuclear interest. Polonium exists solely as a radioactive member of all four actinide decay chains. In a band above N = 126, Po decays exclusively by alpha emission, turning decay chains back toward lower Z. These chains arrive in this band at the end of long series of alpha decays. They arrive neutron-rich. Polonium's alpha decays force actinide chains to oscillate back and forth until they arrive at lead or bismuth (chain almost extinct).
NUCLEAR PROPERTIES
The neutron shell closure at N = 126 exerts an obvious destabilizing effect on nuclides with N somewhat above 126 and Z >= 84. That effect is visible in Bi, where alpha decay appears between 214Bi and 211Bi, plus a weak branch at 210Bi. In general, the more neutron-rich an isotope of some element is, the longer its alpha-decay partial half-life. Any Bi isotope heavier than 209Bi should have a longer half-life than it does. Instead, alpha-decay half-lives range down to around 130 sec in 211Bi (N=128). There is a touch of instability which shows at 210Pb. Although its partial half-life against alpha decay is 11.7 million years, it should not alpha decay at all. This instability becomes dramatic at Po due to a second effect. Z = 82 is stabilized by a proton shell closure. Alpha decay of Po results in Pb, so is unusually likely to occur at all A. All observed isotopes between 219Po and 186Po are known to have an alpha-decay branch.
About 95 isotopes of Po have been predicted to exist, ranging from the neutron dripline near 270Po down to near 175Po, of which 37 have been observed (plus 29 isomers). 220Po and heavier isotopes all decay by beta emission, Their half-lives increase from near 0.001 sec at the neutron dripline to a peak of 9 min at 222Po.
Between 219Po and 217Pb, (negative) beta decay is also an active mode. Alpha decay is dominant except at 219Po, which has the longest half-life of any isotope between 219Po and 211Po. Beta decay disappears at 216Po and half-lives continue to fall down to 300 ns at 212Po (although 212mPo has very long half-life of 45 sec, which implies that its spin (+18) is so great that even alpha decay is inhibited). Half-life increases to 0.52 sec at 211Po, and to 25 sec in its high-spin isomer 211m1Po. At 210Po, alpha emission remains the only active decay mode, but half-life rises to 138.38 days, (210mPo is known, but has a nanosecond-scale half-life.)
From 209Po to 199Po positive beta decay dominates, with the branch ratio for alpha emission falling as low as 0.00003 in 208Po. This band also includes by far the longest-lived Po isotopes: t12(209Po) = 125 yr and t12(208Po) = 2.90 yr. Half-lives fall from there, reaching a few minutes by 199Po. Branch ratios of the ground state of 197Po are BR(b+) = 0.55 and BR(a) = 0.45 (although BR(a) of 197mPo is 0.84.) Other than that, alpha decay predominates between 198Po and 190Po and becomes the sole active decay mode between 189Po and 181Po. Half-lives fall to the microsecond range around 186Po and the nanosecond range by 183Po. Between 180Po and 176Po decay is predicted to occur by proton emission, with half-lives falling from millisecond (a questionable datum) to nanosecond range.
It is worth noting that the partial half-life against alpha decay is 27600 yrs at 209Po, a dramatic increase in stability over 210Po.
All bismuth isotopes 210Bi and heavier have a beta decay branch, which means 210Po and heavier Po isotopes can form via beta decay chains from initially neutron-rich material generated or expelled during a supernova or neutron star merger. No lighter isotopes can form in this way, There are two ways by which lighter isotopes can form in supernovae. First, the reaction e- + p --> n + neutrino (e-(p,n)nu produces neutrinos which can cause neutrino capture in recently or simultaneously formed r-process (rapid neutron capture) material lying outside the zone where neutronization (the reaction above) is occurring. Since only matter at near-nuclear density has high opacity to them, neutrino capture will produce only relatively small amounts of lighter nuclides. When a neutron star disintegrates, material ejected by centrifugal and tidal forces will be so neutron-rich that even light isotopes of a given Z will beta-decay. This mechanism is unimportant there. The second process which produces light isotopes of Po occurs because temperatures in both supernovae and disintegrating neutron stars exceeds 10 GK in times and places. That is equivalent to 8.62E-11 MeV/K or 8.62E-02 MeV/GK. 10 GK is equivalent to an average particle energy of 0.862 MeV, which means black-body photons are reaching the range which cause nuclides to eject particles. In neutron-rich nuclides, a neutron is usually the particle ejected, leading to a lighter isotope of the parent. As proton-richness increases, the ejected particle is more and more likely to be an alpha. In large nuclides, a captured gamma can promote that nuclide to a fission isomer, but that doesn't seem to be an issue at Po. It is likely that 209Po forms along with heavier isotopes, creating a fifth link between the actinides and stable band of elements, along with 209Bi, 208Pb, 207Pb, and 206Pb.
That link at 209Po lasts roughly 15000 years, long enough for it to be injected into a cloud core that is about to collapse, either as gas or in dust, but it becomes extinct long before substantial development of a stars-and-planets system.
Eight Po isotopes are reported to be present in small amounts; 215Po & 211Po from 235U; 218Po, 214Po, & 210Po from 238U; 213Po from 237Np; and 216Po & 212Po from 232Th. There are so many Po isotopes still present because all actinide decay chains have long sequences of alpha decays leading into the mass range 210 <= A <= 220. These arrive in the Po - Tl band of element very neutron-rich. They beta decay out to Po, which always alpha decays. Chains then remain in the Po - Tl band until reaching stable nuclides. Equilibrium concentrations of these elements can be computed readily since their half-lives and overall branch ratios from their chain heads are known. Concentration in earth and similar planets of seven of the eight is most easily expressed as mol/planet: [218Po] = 610 mol/pl, [216Po] = 0.15 mol/pl, [215Po] = 0.037 mol/pl, [214Po] = 0.00053 mol/pl, [213Po] = 4.8E-29 mol/pl, [212Po] = 2.0E-07 mol/pl, and [211Po] = 0.029 mol/pl, Note that such 213Po as forms is lost in the "noise" of nuclides produced in electromagnetic storms such as flares, infall hot spots, accretion discs, decretion discs, and polar jets. Only 210Po is best described by [210Po]/[238U] = 8.48E-11. In uranium ores, [210Po] can be present in quantities great enough to extract. (It is normally synthesized, though.)
ATOMIC PROPERTIES
Polonium is a p-block element located at Period 6, Group 16, and is a homolog of S, Se, and Te. Ionizing Po takes less energy than is required for smaller Group 16 elements. Polonium is generally considered a metal, rather than a metalloid.
USES
A delicious scoop of polonium ice cream. I scream, you scream, idiots all scream for polonium ice cream!
Polonium is used to make polonium ice cream, a glowing green ice cream tasting like vanilla. It was invented by Alexander Litvinenko.
(02-23-22)