Polonium – facts and properties
- Symbol
- Po
- Atomic number
- 84
- Atomic mass
- 209 u
- Period
- 6
- Group
- 16
- Category
- metalloid
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴
- Electronegativity
- 2
- Atomic radius
- 135 pm
- Valence electrons
- 6
- State at standard conditions
- solid
- Melting point
- 254 °C
- Boiling point
- 962 °C
- Density
- 9.32 g/cm³
- Discovered
- 1898
Polonium is a silvery-white, highly radioactive element with atomic number 84. Marie and Pierre Curie discovered it in 1898 in pitchblende and named it after Marie's homeland, Poland. It has no stable isotope and occurs only in traces. The most important isotope, polonium-210, is hardly dangerous from outside but so toxic inside the body that it ranks among the most dangerous substances known.
Position in the periodic table
Occurrence and extraction
Polonium occurs in the Earth's crust only in traces, because all of its isotopes are radioactive and decay quickly. The most common one, polonium-210, has a half-life of about 138 days. It is constantly formed anew in the decay series of uranium-238. From the noble gas radon, lead-210, bismuth-210 and finally polonium-210 arise through several steps, and the last decays to stable lead-206. Uranium ores therefore contain far less than a milligram of polonium per tonne.
Decay products of radon also settle on plant leaves. In this way polonium-210 reaches tobacco leaves and, in smoking, the lungs, where it adds to the radiation dose.
Marie Curie and her husband Pierre studied pitchblende, a uranium ore that radiated more strongly than its uranium content explained. In July 1898 they found a first new radiating element in it and called it polonium. The name recalls Poland, which was then divided between Russia, Prussia and Austria. Radium followed a few months later. For both discoveries Marie Curie received the 1911 Nobel Prize in Chemistry.
Properties and reactions
Polonium is in period 6 and group 16, below tellurium, with bismuth to its left and astatine to its right. It has 6 valence electrons, its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴, and its electronegativity is 2. Its atomic mass is given as 209 u, the mass number of the longest-lived isotope, polonium-209. Polonium melts at 254 °C and boils at 962 °C, and its density is 9.32 g/cm³.
Polonium is placed between the metals and the semimetals. It resembles tellurium chemically and appears mainly in the oxidation states plus two and plus four. Its crystal structure is special: the atoms sit in a simple cubic lattice, which no other element shows under normal conditions.
Polonium-210 emits almost only alpha radiation. Alpha particles travel just a few centimetres in air and are stopped by a sheet of paper or the outer layer of skin, so the radiation is hardly dangerous from outside. Because the particles release their energy over a very short distance, a sample heats itself strongly, and the air around it can glow blue.
If the substance enters the body, it hands its energy directly to the surrounding tissue. Liver, kidneys and bone marrow are mainly affected. Even tiny amounts, far too small to see, can be fatal. The result is acute radiation sickness with hair loss and organ failure.
In November 2006 the former Russian intelligence officer Alexander Litvinenko died in London of poisoning with polonium-210. A British inquiry judged his death a murder in 2016.
Research and synthesis
Polonium-210 is not won from ores, because too little of it is present. It is produced artificially in nuclear reactors when bismuth is irradiated with neutrons. This happens only in a few specialised plants and in tiny amounts.
There is no everyday use. Polonium-210 is still found in some antistatic brushes and ionisers for photographic film and precision instruments: the alpha radiation makes the air conductive, so that electric charge drains away. Together with beryllium it served as a neutron source, because alpha particles knock neutrons out of beryllium. It was also used as a heat source, for example in the Soviet Lunokhod moon rovers. As neutron and heat sources it has been replaced by other materials, because its output drops quickly with the short half-life.
In research, polonium-210 and lead-210 together help to date sediment layers from the last hundred years or so. Scientists also study how the substance spreads through the environment, since seafood takes it up.
Quick question
What is the atomic number of polonium (Po)?
FAQ
Why is polonium so toxic?
Polonium-210 is a strong alpha emitter. Outside the body, skin already stops the particles. Inside the body, the whole energy strikes the tissue right next to it, and even tiny amounts can severely damage organs.
Why is it called polonium?
Marie Curie named the element after her homeland, Poland, in Latin Polonia. It was the first of two new elements that the Curies found in pitchblende in 1898. The second was radium.
Do cigarettes contain polonium?
Yes. Tobacco leaves take up decay products of radon from the air, among them lead-210 and polonium-210. When people smoke, the polonium reaches the lungs and raises the radiation dose.