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A non-metallic element of group 13, used...

A non-metallic element of group 13, used in making bullet prrof vests is extremely hard solid of black colour. It can exist in many allotroic forms and has unusally high melting point. Its trifluoride acts as Lewis acid towards ammonia. The element exhibits maximum covalency of four. Identify the element and write the reaction of tits triffuoride with ammonia. Explain why does the trifuoride act as Lewis acid.

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The only non-metallic element of group 13 is boron. It is an extremely hard substance and is used in making bullet proof vests. It exists in many allotropy forms and usually high melting point. Since B has only s and p-orbitals but no d-orbitals. The maximum covalency of boron is 4.
In trivalent state, the number of electrons around the central atom in a molecule will be six as in case of `BF_(3)`. Such electron deficient molecules have tendency to accept a pair of electron to achieve stable electronic configuration and behave as Lewis acid. `BF_(3)` easily accepts lone pair of electron from `NH_(3)`
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Boron, the first member of group 13 (p-block) is a typical non-metal. H exists in two allotropic forms. The compounds of boron are mainly electron deficient as well as Lewis acids. For example, all the trihalides of boron (BX_(3)) are Lewis acids. The expected order of acidic strength based on the electro negativity of halogen atoms is : BF_(3)gtBC l_(3)gtBBr_(3)gtBl_(3) . But the actual order is the reverse. This is explained with the help of the concept of back bonding (p pi- p pi) . Among the hydrides of boron. diborane has a bridged structure. Among the halides of the elements of group 13, the one which is most acidic is:

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