Selenium reacts with fluorine to form selenium hexafluoride:

  
    
      
        
          
            Se
            
              8
            
            
              
            
          
          +
          24
          
          
            F
            
              2
            
            
              
            
          
          ⟶
          8
          
          
            SeF
            
              6
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {Se8 + 24 F2 -> 8 SeF6}}}
  In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary irritant.
It dissolves in water to form selenous acid, H2SeO3.