The Incomplete Octet



 

Learning Objective

  • Describe the ways that B, Al, Li, and H deviate from the octet rule

Key Points

    • The octet rule states that atoms with an atomic number below 20 tend to combine so that they each have eight electrons in their valence shells, which gives them the same electronic configuration as a noble gas.
    • The two elements that most commonly fail to complete an octet are boron and aluminum; they both readily form compounds in which they have six valence electrons, rather than the usual eight predicted by the octet rule.
    • While molecules exist that contain atoms with fewer than eight valence electrons, these compounds are often reactive and can react to form species with eight valence electrons. For example, BF3 will readily bind a fluoride anion to form the BF4 anion, in which boron follows the octet rule.

Terms

  • valence electronsThe electrons in the outermost (valence) principal energy level of an atom that can participate in the formation of chemical bonds with other atoms.
  • atomic numberThe number, equal to the number of protons in an atom, that determines its chemical properties. Symbol: Z.
  • octet ruleAtoms gain, lose, or share electrons with other atoms in order to fill their valence level with eight electrons.

The Octet Rule and Its Exceptions

The octet rule states that atoms below atomic number 20 tend to combine so that they each have eight electrons in their valence shells, which gives them the same electronic configuration as a noble gas. The rule is applicable to the main-group elements, especially carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium and magnesium.

Valence electrons can be counted using a Lewis electron dot diagram. In carbon dioxide, for example, each oxygen shares four electrons with the central carbon. These four electrons are counted in both the carbon octet and the oxygen octet because they are shared.

Carbon dioxideA Lewis dot diagram for carbon dioxide.

Hydrogen and Lithium

However, many atoms below atomic number 20 often form compounds that do not follow the octet rule. For example, with the duet rule of the first principal energy level, the noble gas helium, He, has two electrons in its outer level. Since there is no 1p subshell, 1s is followed immediately by 2s, and thus level 1 can only have at most two valence electrons. Hydrogen only needs one additional electron to attain this stable configuration, through either covalent sharing of electrons or by becoming the hydride ion (:H), while lithium needs to lose one by combining ionically with other elements. This leads to hydrogen and lithium both having two electrons in their valence shell—the same electronic configuration as helium—when they form molecules by bonding to other elements.

Boron and Aluminum

There are also a variety of molecules in which there are too few electrons to provide an octet for every atom. Boron and aluminum, from Group III (or 13), display different bonding behavior than previously discussed. These atoms each have three valence electrons, so we would predict that these atoms want to bond covalently in order to gain 5 electrons (through sharing) to fulfill the octet rule. However, compounds in which boron or aluminum atoms form five bonds are never observed, so we must conclude that simple predictions based on the octet rule are not reliable for Group III.

Consider boron trifluoride (BF3). The bonding is relatively simple to model with a Lewis structure if we allow each valence level electron in the boron atom to be shared in a covalent bond with each fluorine atom. In this compound, the boron atom only has six valence shell electrons, but the octet rule is satisfied by the fluorine atoms.

Lewis structure of boron trifluorideEach pair of dots represents a pair of electrons. When placed between two atoms, the electrons are in a bond. A bond can be drawn as a line between two atoms, which also indicates two electrons. Notice that the central boron atom has only 6 electrons in the final Lewis diagram/structure of this molecule.

We might conclude from this one example that boron atoms obey a sextet rule. However, boron will form a stable ion with hydrogen, BH4, in which the boron atom does have a complete octet. In addition, BF3 will react with ammonia (NH3), to form a stable compound, NH3BF3, for which a Lewis structure can be drawn that shows boron with a complete octet.

Boron trifluoride-ammonia complexThis covalent compound (NH3BF3) shows that boron can have an octet of electrons in its valence level.

Compounds of aluminum follow similar trends. Aluminum trichloride (AlCl3), aluminum hydride (AlH3), and aluminum hydroxide (Al(OH)3) indicate a valence of three for aluminum, with six valence electrons in the bonded molecule. However, the stability of aluminum hydride ions (AlH4) indicates that Al can also support an octet of valence shell electrons.

Although the octet rule can still be of some utility in understanding the chemistry of boron and aluminum, the compounds of these elements are harder to predict than for other elements.