Atomic number 84

Polonium

Po
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84 electrons in 6 shells: 2, 8, 18, 32, 18, 6.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Metalloid
Period: 6
Group: 16
Block: p
Atomic mass: 208.98243 Da (longest-lived isotope)
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴
Phase at 25 °C: solid
Melting point: 254 °C
Boiling point: 962 °C
Density: 9.32 g/cm³
Electronegativity (Pauling): 2.0
Discovered: 1898, Marie Sklodowska Curie

Neighbours in the table

Polonium was the first element Marie Sklodowska Curie discovered, in 1898, while hunting the source of the radioactivity in pitchblende mined at Joachimsthal in Bohemia, Los Alamos records. The unrefined ore was more radioactive than the uranium taken out of it, so she reasoned, in Jefferson Lab’s account, that it held at least one more radioactive element. Her electroscope showed the newcomer separating with bismuth. It is named after Poland, Curie’s native country, and was also called radium F. Winning even tiny amounts of polonium and radium meant refining several tons of pitchblende.

Polonium belongs to group 16, which Jefferson Lab labels the chalcogens. Its configuration, [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴, ends with four electrons in the 6p subshell, and its six shells carry 2, 8, 18, 32, 18 and 6 electrons. No isotope of polonium is stable. PubChem gives the mass of its longest-lived isotope, polonium-209, 208.98243 Da.

Two allotropes, distinct solid forms of the same element, are known. Polonium-210 is volatile: Los Alamos reports half of a sample vaporising in air within 45 hours at 55 °C. A strong source glows blue, caused by the surrounding gas being excited.

Where it is found

Los Alamos calls polonium a very rare natural element. A ton of uranium ore holds only about 100 micrograms, and its abundance is about 0.2 per cent of radium’s. Jefferson Lab estimates 2 × 10⁻¹⁰ milligrams per kilogram of the crust and 1.5 × 10⁻¹⁴ milligrams per litre of seawater.

Natural polonium turns up as links in the uranium decay chain. The Hazardous Substances Data Bank (HSDB), in PubChem’s record for radon, has radon-222 emitting an alpha particle to become polonium-218. Polonium-214, another of radon’s short-lived daughters, the isotopes its decay produces, decays to lead-210, which has a half-life of about 20 years. Lead-210 then decays to bismuth-210 and on to polonium-210.

Scarcity is why polonium is usually made in a nuclear reactor, Jefferson Lab explains. Neutrons striking bismuth-209 turn it into bismuth-210. That isotope, gone by half in 5 days, becomes polonium-210 by beta decay. Los Alamos dates this route to 1934. Polonium-209 and polonium-208 come instead from cyclotrons, a kind of particle accelerator, firing alpha particles or protons at lead or bismuth, an expensive process.

Why it matters

Polonium-210’s own decay turns it into a heater. Nearly every alpha particle it emits halts within the solid itself or its casing and hands over its energy there. For that reason, Los Alamos says, polonium drew interest as a light source of heat to generate thermoelectric power, electricity made directly from heat, aboard satellites. Jefferson Lab puts the output of one gram of polonium-210 at 140 watts. Its decay energy alone takes a small capsule of it above 500 °C, Los Alamos notes. Jefferson Lab describes it as being considered as a heat source for spacecraft power.

Closer to the ground, polonium removes static electricity from machinery. It clears the static that builds up as paper is rolled, plastic sheet is made and synthetic fibres are spun, according to the chemists’ international union (IUPAC). Brushes carrying it lift dust from photographic films and camera lenses. Los Alamos stresses that the polonium in them is carefully sealed and controlled, and adds that beta sources are more common in such work and less dangerous.

Mixed or alloyed with beryllium, polonium also makes a neutron source, Los Alamos notes. When alpha particles from polonium or radium strike beryllium, the beryllium gives off neutrons, according to Los Alamos.

Properties

Properties of polonium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	84
Atomic mass	208.98243 Da	Da	PubChem, longest-lived isotope
Melting point	254 °C	°C	Los Alamos: 254 °C
Boiling point	962 °C	°C	Los Alamos: 962 °C
Density	9.32 g/cm³	g/cm³
Electronegativity (Pauling)	2.0
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁴
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Polonium is an alpha emitter, and its hazard is ionising radiation. Haz-Map’s radionuclide summary in PubChem names polonium-210 as the isotope of most concern, the spleen as its critical organ and its internal toxicity as very high. Los Alamos calls polonium-210 very dangerous. Anyone who works with polonium will find its reference sheet in the PubChem Laboratory Chemical Safety Summary.

Questions

What is the half-life of polonium-210?

Polonium-210 lasts 138.376 days in the International Atomic Energy Agency's (IAEA) evaluation and 138.39 days according to Los Alamos and Jefferson Lab, all by alpha emission in IAEA's data. Polonium-209 survives longest: 124 years in IAEA's figure and 102 in Jefferson Lab's. Jefferson Lab has it decaying to lead-205 by alpha emission; IAEA gives alpha emission for 99.546 per cent of decays and another decay mode for 0.454 per cent.

Is polonium a metal or a metalloid?

PubChem's periodic table files polonium among the metalloids, while Jefferson Lab classifies it as a metal and Los Alamos calls polonium-210 a low-melting, fairly volatile metal. Its Pauling electronegativity, an atom's tendency to attract a bonding pair of electrons, is 2, below the figure of about 2.2 that Chem1 associates with electropositive, metallic character. Its configuration ends in 6p⁴, and it sits in group 16, which Jefferson Lab labels the chalcogens.

What are the oxidation states of polonium?

Jefferson Lab gives +4 and +2, while Los Alamos lists 6, 5, 4, 2 and −1. Polonium dissolves readily in dilute acids but only slightly in alkali, Los Alamos notes, and its salts of organic acids char rapidly. PubChem lists its electron affinity, the energy released when a neutral atom takes on an extra electron, as both 1.9 eV and 1.32 eV.

Why is polonium so dangerous?

Polonium-210 emits alpha particles, and Los Alamos traces the harm to the complete absorption of each particle's energy into tissue. Polonium also reaches the lungs by way of radon. Radon's short-lived daughters, the isotopes its decay produces, polonium-218 and polonium-214 among them, attach to the first surface they meet, the Hazardous Substances Data Bank (HSDB) says, and are then breathed in and largely kept in the respiratory tract.