A polonide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

of the

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

element

polonium Polonium is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character ...

with any element less

electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

than polonium. Polonides are usually prepared by a direct reaction between the elements at temperatures of around 300–400 °C... They are amongst the most chemically stable compounds of polonium,. and can be divided into two broad groups: *ionic polonides, which appear to contain the Po²⁻ anion; *intermetallic polonides, in which the bonding is more complex. Some polonides are intermediate between these two cases and others are

non-stoichiometric In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); m ...

compounds.

Alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

s containing polonium are also classed as polonides. As polonium is immediately below

tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

in the periodic table, there are many chemical and structural similarities between polonides and tellurides.

Naturally occurring polonides

Lead polonide (PbPo) occurs naturally, as

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

is produced in the

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...

of polonium.

Ionic polonides

The polonides of the most

electropositive Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

metals show classic ionic structural types, and can be considered to contain the Po²⁻ anion. With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater covalency in the bonding.

Magnesium polonide Magnesium polonide (MgPo) is a salt of magnesium and polonium. It is a polonide, a set of very chemically stable compounds of polonium. Preparation Magnesium polonide can be produced by heating a mixture of elemental magnesium and polonium at 300� ...

is unusual as it is not isostructural with magnesium telluride: MgTe has a

wurtzite structure In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crysta ...

, although a

nickeline Nickeline or niccolite is a mineral consisting primarily of nickel arsenide (NiAs). The naturally-occurring mineral contains roughly 43.9% nickel and 56.1% arsenic by mass, but composition of the mineral may vary slightly. Small quantities of ...

-type phase has also been reported. The

effective radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic spherical symmetry or is at least circularly symmetric as viewed in the plane o ...

of the polonide ion (Po²⁻) can be calculated from the Shannon (1976) ionic radii of the cations:. 216 pm for 4-coordination, 223 pm for 6-coordination, 225 pm for 8-coordination. The effect of the

lanthanide contraction The lanthanide contraction is the greater-than-expected decrease in atomic radii/ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected atomic radii ...

is clear, in that the 6-coordinate telluride ion (Te²⁻) has an ionic radius of 221 pm. The lanthanides also form sesquipolonides of formula Ln₂Po₃, which can be considered to be ionic compounds..

Intermetallic polonides

The

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...

s form very stable polonides of formula LnPo with the halite (NaCl) structure: as the +2 oxidation state is disfavoured for most lanthanides, these are probably best described as intermetallic compounds rather than charge-separated ionic species. These compounds are stable to at least 1600 °C (the melting point of thulium polonide, TmPo, is 2200 °C), in contrast the ionic polonides (including the lanthanide sesquipolonides Ln₂Po₃) which decompose at around 600 °C. The thermal stability and non-volatility of these compounds (polonium metal boils at 962 °C) is important for their use in polonium-based heat sources. Mercury and lead also form 1:1 polonides. Platinum forms a compound formulated as PtPo₂, while nickel forms a continuous series of phases NiPo_''x'' (''x'' = 1–2). Gold also forms solid solutions with polonium over a wide range of compositions,. while bismuth and polonium are completely miscible. No reaction is observed between polonium and aluminium, carbon, iron, molybdenum, tantalum or tungsten.

References

{{Polonium compounds * Polonides