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Lesson: Chapter 10 — Chemical Bonds

10.2. Covalent bonds 8 of 8

When the octet does not hold

Every molecule so far has ended with eight electrons around each atom. That is the usual outcome, but it is not a law, and two familiar compounds break it in opposite directions.

Short of eight: aluminium chloride

Aluminium is 2, 8, 3, so it brings three valence electrons. Three chlorine atoms each share a pair with it, giving three bonds.

Aluminium chloride: an aluminium atom joined by a single bond to each of three chlorine atoms, leaving only six electrons around the aluminium.

Count around the aluminium atom and there are three bonding pairs — six electrons, not eight. Its octet is incomplete. Count around any chlorine atom and the octet is complete, as usual.

Beyond eight: phosphorus pentachloride

Phosphorus is 2, 8, 5, so it brings five valence electrons and can share a pair with each of five chlorine atoms.

Phosphorus pentachloride: a phosphorus atom joined by a single bond to each of five chlorine atoms, giving ten electrons around the phosphorus.

Five bonding pairs is ten electrons around the phosphorus atom. Here the octet has been exceeded. Each chlorine atom, again, finishes at exactly eight.

Molecule Electrons on the central atom Octet
CH4 8 Complete
AlCl3 6 Incomplete
PCl5 10 Exceeded

A guide, not a rule

The octet describes what usually happens, and it will predict most structures correctly. But a diagram is not wrong merely because the central atom ends with six or ten. Check what the atoms actually have available before insisting on eight.