Conceptions of the octet rule

A topic in science concepts

The octet rule is a heuristic (a rule of thumb) which is useful in elementary chemistry as a guide to identifying feasible chemical species.

Many atomic, molecular, and ionic species that are stable enough to be be significant in everyday life can be understood as meeting the octet rule.

The rule

The rule concerns the number of electrons in the outer shell of the species – suggesting that stable species usually have eight electrons in the outer shell (apart from elements in the first period where the number is usually two).

Applying the rule

Atoms

Most substances are not atomic, that is, they do not comprise of separate, discrete atoms. The exceptions are the noble gases which are usually found as atomic. Helium has two electrons in its outer (only) electronic shell. The others – neon, argon, krypton, xenon and radon – have eight electrons (sometimes called an octet of electrons) in the outer shell.

Ions

The most common ions of of many elements are those where the ion has an octet structure. So, group 1 elements form salts with the Na+, K+ Rb+ and Cs+ ions and groups 2 elements form salts with the Mg2+, Ca2+, Sr2+ and Ba2+ ions. Halides contain the F–, Cl–, Br– and I– ions. Oxides contains the O2- ion and nitrides the N3- ion. And so forth.

Molecules

Molecules contain covalent bonds between atoms, which can be understood as where a pair (or sometimes more than one pair) of electrons binds the atomic cores of two adjacent atoms together. (This is commonly described as electron pair 'sharing' although this metaphor is an anthropomorphism which can lead to learner confusion.)

Double counting bonding pairs

In most stable molecules the various atomic centres can be understood as meeting the octet rule provided one double counts. So, in the F2 molecule, where each F centre is surrounded by 6 non-bonding electrons, plus the bonding pair, each atomic centre is considered to have eight surrounding electrons even though (2 ╳ 8 = 16, and) there are only 14 outer shell electrons as the bonding electrons are considered to fully count for both atoms.

So, in oxygen, with a double bond, each atomic centre is surrounded by eight electrons (4 non-bonding, plus the two bonding pairs) and this fits ('obeys') the octet rule.

Exceptions to the octet rule

Whilst the octet rule works quite well in very elementary chemistry, there are a great many exceptions, where stable species have less or more than eight electrons in their outer shells (sometimes referred to 'expanding the octet'). Elements in period 3 such as sulphur, phosphorus and chlorine often form molecules where they are surrounded by more than eight electrons. So-called, 'electron deficient' compounds have atomic centres with less than eight electrons around them. Many transition elements commonly form stable salts with different ionic charges (e.g., Cu+ and Cu2+; Fe2+ and Fe3+). Compounds have been made of noble gas compounds with more than eight electrons around the noble gas element's atomic centre.

There are so many exceptions to the rule that it is of limited use in chemistry – except perhaps in very elementary classes.

Alternative conceptions of the octet rule

Having an octet of electrons is the same as having a full outer shell

Often learners conflate full shells and octets. This is the same thing in period 2. But not in other periods. For an atomic species of an element in period 3 to be said to have a full outer shell it would need 18 electrons. So S2- has an outer shell octet of electrons, but only S12- would have a full outer shell. And S12- is not a feasible chemical species – it could only be maintained under extreme conditions of a very strong electric field (using the kind of apparatus high energy physicists use to smash atoms and discover new subatomic particles).

Species with octets are stable

This is an easy mistake to make, as the octet rule indicates to learners many species which are unlikely to be stable (H3O, Na-, O3+, CH7, NH2, OF…and a vast number of others), and it is a simple logical error to think that if species that do not fit the octet rule are unstable, then those that ft the rule must all be stable.

Learners will commonly judge highly labile species, and even extremely non-viable ones (Cl11-) as stable if they fit the octet rule (Taber, 2009).

Read more about conceptions of chemical stability

Chemical bonds form to give octets

Chemical bonds form because of the forces between subatomic particles (and the restrictions imposed by quantum rules). But students commonly explain bond formation in anthropomorphic terms: that atoms want to, or need to, or are happy when they, have outer shell octets. Even high achieving, advanced students commonly express this kind of nonsense. Obviously, atoms neither know, nor care, about their electronic structures.

Chemical reactions occur to give species octets

Students often explain chemical reactions on the basis that the products consist of ions or molecules where the octet rule is met ('obeyed'). Yet those reactions occur between reactants where the octet rule is already met ('obeyed'). (Students often think that reactants start as discrete atoms: the assumption of initial atomicity.) This alternative conceptions is perhaps one of the most common and misleading ones in chemistry learning (Taber, 2024).

Read about the way the octet or full shell is misunderstood as an explanation for chemical processes

Ions with octets are more stable than the atoms

Learners often think that an ion with an outer shell octet must be more stable than the atom – so Na+ and Mg2+ respectfully will be incorrectly judged as more stable than Na and Mg. There are examples where this works (Cl– compared with Cl) but generally a neutral atom is more stable that a charged ion.

This is confusing for learners because in practice discrete ('naked') ions are not found in chemical systems. When the Na+ ion is solvated or part of a crystal lattice it is stabilised. But when comparing just the ion and the neutral atom: the ion is usually less stable.

Atoms will spontaneously ionise if this leads to an octet

Because students often assume an ion with an octet is inherently more stable than the atom, students will often think that, for example, a sodium atom will spontaneously eject an electron to become an ion. It would not – the negative electron would be attracted back by the positive ion formed.

Patterns in ionisation energies show how the octet is inherently stable.
  • The highest first ionisation enthalpy in period 2 is that of neon.
  • The highest first ionisation enthalpy in period 2 is that of argon.

Students will often suggest this is because these two atoms are especially stable having octets in the outer shell. Yet the ionisation energy values increase across the second and third period as the core charge increases, and neon fits the overall pattern for period 2, and argon fits the overall pattern for period 3 – so this can be explained without any reference to octets.

Species with octets cannot be ionised

They can! The amount of energy needed to ionise a species with an octet follows the same rules as any other species (according to factors such as core charge, distance of electron to be removed for nucleus…)

Read about the alternative conceptual framework learners often develop around the octet rule

Work cited:

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