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Ammonia and Boron trifluoride are an example of:
A.A pair of isomers
B.An electron donor-acceptor pair
C.An acid-alkali pair
D.A pair of allotropes

Last updated date: 18th Jul 2024
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Hint: To answer this question we must have the knowledge of electron pair acceptor and donor molecules. Any element will accept electrons if its octet is incomplete and the one with excess of electron will lose electrons.

Complete step by step solution:
Let us study the given pair of species,
Ammonia is a well known base to us, ammonia has one nitrogen atom as a central atom and 3 hydrogen atoms are bound to it via a single bong and the geometry is trigonal bipyramidal. Since the atomic number of nitrogen is 7 and it has 5 electrons in its valence shell, but it is bonded to only 3 hydrogen atoms. So it has one lone pair left, which nitrogen can donate and hence ammonia is an electron donor.
While talking about boron trifluoride, boron is the central atom and 4 fluorine atoms are attached to it. Boron belongs to the group 13 element; its atomic number is 5 and is attached with 3 fluorine atoms. So no extra electrons are left with boron, even after bonding with fluorine its valence shell only contains 6 electrons, 3 of its own and 3 via bonding. Hence, it is an electron deficient species and can accept electrons
So the above example is a perfect example of an electron donor-acceptor pair.

Note: The ammonia and boron trifluoride cannot be isomers because to be isomers they must have the same molecular formula. The allotropes are the different forms of the same elements. Ammonia is a base and boron trifluoride is an alkali-acid pair not acid-alkali pair.