Selenium dioxide is formed by the reaction of elemental selenium with oxygen:

  
    
      
        
          
            Se
            
              8
            
            
              
            
          
          +
          8
          
          
            O
            
              2
            
            
              
            
          
          ⟶
          8
          
          
            SeO
            
              2
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {Se8 + 8 O2 -> 8 SeO2}}}
  It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase.
Because of its similarity to tellurium, named for the Earth, Berzelius named the new element after the Moon.In 1873, Willoughby Smith found that the electrical resistance of grey selenium was dependent on the ambient light. This led to its use as a cell for sensing light. The first commercial products using selenium were developed by Werner Siemens in the mid-1870s. The selenium cell was used in the photophone developed by Alexander Graham Bell in 1879. Selenium transmits an electric current proportional to the amount of light falling on its surface. This phenomenon was used in the design of light meters and similar devices. Seleniums semiconductor properties found numerous other applications in electronics. The development of selenium rectifiers began during the early 1930s, and these replaced copper oxide rectifiers because they were more efficient. These lasted in commercial applications until the 1970s, following which they were replaced with less expensive and even more efficient silicon rectifiers. Selenium came to medical notice later because of its toxicity to industrial workers. Selenium was also recognized as an important veterinary toxin, which is seen in animals that have eaten high-selenium plants. In 1954, the first hints of specific biological functions of selenium were discovered in microorganisms by biochemist, Jane Pinsent. It was discovered to be essential for mammalian life in 1957. In the 1970s, it was shown to be present in two independent sets of enzymes. This was followed by the discovery of selenocysteine in proteins. During the 1980s, selenocysteine was shown to be encoded by the codon UGA.