Chemistry · Chemical Bonding · NEET
Count the shared pairs (bonds) around the central atom, each bond gives 2 electrons. In BeCl2, Be forms 2 bonds = 4 electrons. In BF3, BCl3 and AlCl3, the central atom (B or Al) forms 3 bonds = 6 electrons. None of these reach 8, so they all have an incomplete octet. Simple rule: count the bonds and multiply by 2.
Beryllium is in Group 2, so it has only 2 valence electrons. It uses both to make 2 single bonds with 2 chlorine atoms. That gives 2 bond pairs = 4 electrons around Be. Be has no extra electrons to make more bonds, so it stops at 4. It stays stable even though its octet is not complete.
Boron is in Group 13 with 3 valence electrons. It makes 3 single bonds with 3 fluorine atoms, using all 3 of its electrons. That is 3 bond pairs = 6 electrons around boron. Boron has no more electrons and no lone pair, so it cannot reach 8. This is why BF3 is called electron deficient.
Electron deficient means the central atom has fewer than 8 electrons in its outer shell, so it is 'hungry' for more electrons. Because of this, molecules like BF3 and AlCl3 act as Lewis acids (electron pair acceptors). BF3 can accept a lone pair from NH3 to form a stable compound. For NEET, remember: incomplete octet = electron deficient = Lewis acid.
By its normal Lewis structure BF3 is electron deficient (only 6 electrons on B). Some books draw a B=F double bond by resonance to complete the octet, but NEET treats BF3 as electron deficient. In NEET 2021 the answer confirmed BF3 has 6 electrons around boron. So for exam purposes, count 6 electrons and call it incomplete octet.
The four classic examples are BeCl2 (4 electrons), BCl3, BF3 and AlCl3 (each 6 electrons). Also LiCl and BeH2 fit this idea. All have central atoms from Group 1, 2 or 13. These come up directly in 'count the species that do NOT follow octet' questions, so learn them by heart.
Incomplete octet means fewer than 8 electrons (BeCl2 = 4, BF3 = 6). Expanded octet means more than 8 electrons (PCl5 = 10, SF6 = 12). Both are exceptions to the octet rule but in opposite directions. In NEET 2023, a single question asked for species NOT having 8 electrons, and the answer mixed both types (AlCl3, BeCl2 short; PCl5 expanded).
Amongst the following, the total number of species NOT having eight electrons around the central atom in its outermost shell is: NH3, AlCl3, BeCl2, CCl4, PCl5
BF3 is planar and an electron-deficient compound. The hybridization and number of electrons around the central atom, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Even though the central atom has fewer than 8 electrons, molecules like BeCl2, BF3 and AlCl3 are real, stable compounds. They are exceptions to the octet rule but still exist normally.
Because the central atom is short of electrons, it can accept a lone pair of electrons from another molecule. This makes BF3 and AlCl3 strong Lewis acids used in many reactions.
No. Be in BeCl2 and B in BF3 have no lone pairs on the central atom. All their valence electrons are used in bonding, which is why the octet stays incomplete.
BeCl2 is sp hybridised (linear, 2 bonds). BF3, BCl3 and AlCl3 are sp2 hybridised (trigonal planar, 3 bonds). Both have no lone pair on the central atom.
Monomeric AlCl3 is electron deficient with 6 electrons around Al. In the solid or vapour it can dimerise to Al2Cl6 to complete the octet, but for NEET counting, treat single AlCl3 as incomplete octet (6 electrons).
