Chemistry · Ch 5 — Chemical Bonding
Limitation of octet rule
Limitation of octet rule
Despite its usefulness, the octet rule fails to hold — or fails to explain a molecule's real behaviour — in three distinct ways. First, INCOMPLETE OCTET: some stable covalent molecules have a central atom with fewer than eight valence electrons around it, e.g. Li in LiCl (only 2), Be in BeCl2 (only 4), and B in BF3 (only 6) — these elements simply do not have enough valence electrons to reach a full octet even after bonding, yet the resulting molecules are perfectly stable. Second, EXPANDED OCTET: some molecules have a central atom with MORE than eight electrons around it, made possible by that atom's access to low-lying d orbitals that can accommodate the extra shared pairs — e.g. SF6 (12 electrons around S), PCl5 (10 around P), and H2SO4 (12 around S); by contrast, SCl2 is a case where sulfur DOES obey the ordinary octet rule (only 8 electrons around S), showing that an expanded octet is a property of the specific compound formed, not an inherent trait of the element. Third, ODD-ELECTRON MOLECULES: species such as NO and NO2 have an odd TOTAL number of valence electrons, so however the structure is drawn, at least one atom is necessarily left with a single unpaired electron rather than a complete octet of paired electrons. Beyond these three classes of exception, the octet rule ALSO cannot explain a molecule's actual observed shape/geom …
What this figure shows. Examples where the central atom is stable with fewer than eight valence electrons: LiCl (Li has only 2 electrons around it), BeCl2 (Be has 4 electrons around it — 2 single bonds, no lone pairs, since Be has only 2 valence electrons to share), and BF3 (B has 6 electrons around it — 3 single B–F bonds and no lone pair on B, since boron has only 3 vale …
What this figure shows. Examples where the central atom exceeds eight electrons because it has accessible d-orbitals to house the extra shared pairs: SF6 (12 electrons around sulfur — 6 S–F single bonds, an octahedral arrangement), PCl5 (10 electrons around phosphorus — 5 P–Cl single bonds, a trigonal-bipyramidal arrangement), and H2SO4 (12 electrons around sulfur). By contrast, sulfur dichloride SCl2 is given as a case where sulfur DOES obey the simple octet rule — only 8 electrons around S (2 single S–Cl bonds plus 2 lone pairs) — showing that expanded octets are a property of the specific compound, no …
Worked out. Nitric oxide (NO) and nitrogen dioxide (NO2) are examples of molecules that cannot obey the octet rule at all, because both N and O (in NO) — or the molecule as a whole (in NO2) — end up with an odd total number of valence electrons, so at least one atom is necessarily left with an unpaired (odd) electron rather than a complete set of four electron pairs. NO is drawn N=O with one unpaired electron; NO2 is drawn O=N–O (with a formal + charge shown on N in the source), again with one elec …