<P> The nonmetal binary fluorides are volatile compounds . They show a great difference between period 2 and other fluorides . For instance, period 2 elements elements fluorides never exceed the octet in their atoms . (Boron is an exception due to its specific position in the periodic table .) Lower - period elements, however, may form hypervalent molecules, such as phosphorus pentafluoride or sulfur hexafluoride . The reactivity of such species varies greatly--sulfur hexafluoride is inert, while chlorine trifluoride is extremely reactive--but there are some trends based on periodic table locations . </P> <P> Boron trifluoride is a planar molecule . It has only six electrons around the central boron atom (and thus an incomplete octet), but it readily accepts a Lewis base, forming adducts with lone - pair - containing molecules or ions such as ammonia or another fluoride ion which can donate two more electrons to complete the octet . Boron monofluoride is an unstable molecule with an unusual (higher than single) bond to fluorine . The bond order has been described as 1.4 (intermediate between a single and double bond). It is isoelectronic with N . </P> <P> Silicon tetrafluoride, similar to carbon tetrafluoride and germanium tetrafluoride, adopts a molecular tetrahedral structure . SiF is stable against heating or electric spark, but reacts with water (even moist air), metals, and alkalies, thus demonstrating weak acidic character . Reactions with organomagnesium compounds, alcohols, amines, and ammonia yield adduction compounds . Fluorosilicic acid, a derivative of SiF, is a strong acid in aqueous solution (the anhydrous form does not exist). </P> <P> Pnictogens (nitrogen's periodic table column) show very similar trends in reactivity and acidity of the highest fluorides (pentafluorides) and most common ones (trifluorides), with the said property increasing down the group: NF is stable against hydrolysis, PF hydrolyzes very slowly in moist air, while AsF completely hydrolyzes . SbF hydrolyzes only partially because of the increasing ionic character of the bond to fluorine . The compounds are weak Lewis bases, with NF again being an exception . The pentafluorides of phosphorus and arsenic are much more reactive than their trifluorides; antimony pentafluoride is such a strong acid that it holds the title of the strongest Lewis acid . Nitrogen is not known to form a pentafluoride, although the tetrafluoroammonium cation (NF) features nitrogen in the formal oxidation state of + 5 . Nitrogen monofluoride is a metastable species that has been observed in laser studies . It is isoelectronic with O and like BF, unusually, has a higher bond order than single - bonded fluorine . </P>

One sulfur atom for every two fluorine atoms