Q.A reaction between ammonia and boron trifluoride is given below:
:NH3 + BF3 → H3N:BF3
Identify the acid and base in this reaction. Which theory explains it? What is the hybridisation of B and N in the reactants?

Concept: Lewis acid–base theory and orbital hybridization

In this reaction, ammonia donates its lone pair of electrons to boron trifluoride, forming a coordinate covalent bond. The species that donates the electron pair is the Lewis base, while the species that accepts it is the Lewis acid.

NH₃ possesses a lone pair on nitrogen and acts as the Lewis base. BF₃ has an incomplete octet (only six electrons around boron) and accepts the lone pair, making it the Lewis acid. This electron-pair donor–acceptor interaction is explained by Lewis acid–base theory. …

The key to understanding this reaction lies in recognizing that not all acid-base reactions involve proton transfer. When ammonia and boron trifluoride combine, no $\ce{H+}$ changes hands. Instead, something more fundamental happens: a lone pair of electrons finds a new home.

Why Lewis Theory?

The Brønsted-Lowry definition (acids donate protons, bases accept them) cannot explain this reaction because boron trifluoride has no protons to give. Gilbert N. Lewis broadened the concept: an acid is any species that accepts an electron pair, while a base is any species that donates an electron pair. This definition encompasses proton-transfer reactions and extends far beyond them.

In $\ce{:NH3 + BF3 → H3N{:}BF3}$, the nitrogen atom in ammonia carries a lone pair of electrons. Boron in $\ce{BF3}$ has only six valence electrons around it—two short of an octet. The lone pair on nitrogen moves into the empty orbital on boron, forming a coordinate covalent bond (also called a dative bond, shown as $\ce{N→B}$).

Identifying Acid and Base

1. Ammonia ($\ce{NH3}$) is the Lewis base. Nitrogen has five valence electrons: three form bonds with hydrogen atoms, leaving one lone pair. This lone pair is available for donation. When ammonia donates this pair to boron, it acts as an electron-pair donor.

2. Boron trifluoride ($\ce{BF3}$) is the Lewis acid. Boron has three valence electrons, all used in bonding with three fluorine atoms. The resulting molecule has only six electrons around boron—an electron-deficient species. Boron accepts the lone pair from nitrogen to complete its octet, acting as an electron-pair acceptor.

Hybridisation in the Reactants

Understanding hybridisation requires examining the geometry and bonding in each molecule.

Nitrogen in $\ce{NH3}$

3. Nitrogen has the electronic configuration $1s^2 \, 2s^2 \, 2p^3$. To form three equivalent bonds with hydrogen and accommodate one lone pair, nitrogen undergoes $sp^3$ hybridisation.

4. Four $sp^3$ hybrid orbitals arrange themselves tetrahedrally. Three of these orbitals overlap with hydrogen $1s$ orbitals to form three $\ce{N-H}$ sigma bonds. The fourth $sp^3$ orbital holds the lone pair.

5. The molecular geometry is trigonal pyramidal (the electron geometry is tetrahedral, but we describe shape by atom positions only). The bond angle is approximately $107°$, slightly compressed from the ideal tetrahedral $109.5°$ because the lone pair repels bonding pairs more strongly.

Boron in $\ce{BF3}$

6. Boron has the electronic configuration $1s^2 \, 2s^2 \, 2p^1$. In $\ce{BF3}$, boron forms three bonds with fluorine atoms. To create three equivalent bonding orbitals, boron undergoes $sp^2$ hybridisation. …