Why is BF3 a planar molecule, while NH3 is not?

To understand why BF3 is a planar molecule (meaning that all its atoms lie in the same plane), while NH3 is not, we need to consider the molecular geometry and the bonding arrangements of the atoms involved.

In BF3 (boron trifluoride), boron is the central atom bonded to three fluorine atoms. Boron has an incomplete valence shell and forms three covalent bonds, one with each fluorine atom. The electron configuration of boron is 1s²2s²2p¹, and by forming three covalent bonds, it achieves a stable arrangement with a full valence shell (like a noble gas configuration). The three bonds form a trigonal planar arrangement around the central boron atom, with 120° bond angles between them. As a result, BF3 is a planar molecule.

On the other hand, in NH3 (ammonia), nitrogen is the central atom bonded to three hydrogen atoms. Nitrogen has a valence electron configuration of 1s²2s²2p³ and needs three more electrons to complete its valence shell. It forms three covalent bonds with hydrogen atoms, resulting in a total of 4 electron density regions around the nitrogen atom. According to the valence shell electron pair repulsion (VSEPR) theory, electron pairs (bonds or lone pairs) repel each other to minimize repulsive forces. In this case, the repulsion between the three bonding pairs and an unshared pair of electrons pushes the hydrogen atoms down, giving the molecule a pyramidal shape. The bond angles in NH3 are approximately 107°.

In summary, the difference in molecular geometry between BF3 and NH3 is due to the electron arrangement around the central atoms and the need to minimize electron pair repulsion. BF3 has a trigonal planar geometry since it forms three bonds and has no lone pairs, while NH3 has a pyramidal shape because it forms three bonds and has one lone pair.

BF3 consists of three B-F bonds and since B is in group III, it has no unshared pair of electrons. It does not have an octet of electrons. Count the regions of high electron density. It is 3 and 3 gives a trigonal planar structure. NH3, on the other hand, has a complete octet; therefore, it has a unshared pair of electrons as well as the three pairs shared with the three H atoms. Thus NH3 has an unshared pair of electrons plus the three N-H bonds to make four regions of high electron density which makes the electronic structure tetrahedral.