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Selenium trioxide

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Melting point
118.4 °C

Molar mass
126.96 g/mol

3.44 g/cm³

Selenium trioxide Draw All Possible Resonance Structures For The Mol Cheggcom

white hygroscopic crystals

Selenium trioxide is the inorganic compound with the formula SeO3. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds.


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Selenium trioxide is difficult to prepare because it is unstable with respect to the dioxide:

2 SeO3 → 2 SeO2 + O2
Selenium trioxide Selenium trioxide O3Se ChemSpider

It has been generated in a number of ways despite the fact that the dioxide does not combust under normal conditions. One method entails dehydration of anhydrous selenic acid with phosphorus pentoxide at 150-160 °C. The reaction of liquid sulfur trioxide with potassium selenate

SO3 + K2SeO4 → K2SO4 + SeO3


In its chemistry SeO3 generally resembles sulfur trioxide, SO3, rather than tellurium trioxide, TeO3.

At 120 °C SeO3 reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentoxide:

SeO3 + SeO2 → Se2O5

It reacts with selenium tetrafluoride to form selenoyl fluoride, the selenium analogue of sulfuryl fluoride

SeO3 + SeF4 → SeO2F2

As with SO3 adducts are formed with Lewis bases such as pyridine, dioxane and ether.

Selenium trioxide SeO3 Lewis Dot Structure Selenium Trioxide Polar or Nonpolar

With lithium oxide and sodium oxide it reacts to form salts of SeVIO54− and SeVIO66−: With Li2O, it gives Li4SeO5, containing the trigonal pyramidal anion SeVIO54− with equatorial bonds, 170.6-171.9 pm; and longer axial Se-O bonds of 179.5 pm. With Na2O it gives Na4SeO5, containing the square pyramidal SeVIO54−, with Se-O bond lengths ranging from range 1.729 → 1.815 pm, and Na12(SeO4)3(SeO6), containing octahedral SeVIO66−. SeVIO66− is the conjugate base of the unknown orthoselenic acid (Se(OH)6).


Selenium trioxide O3Se

In the solid phase SeO3 consists of cyclic tetramers, with an 8 membered (Se-O)4 ring. Selenium atoms are 4-coordinate, bond lengths being Se-O bridging are 175 pm and 181pm, non-bridging 156 and 154 pm.

SeO3 in the gas phase consists of tetramers and monomeric SeO3 which is trigonal planar with an Se-O bond length of 168.78 pm.


Selenium trioxide Wikipedia

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